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<h2 class="chapter-name chapter-name-short">Protocols</h2>
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<p class="para">
<a href="#TP40014097-CH25-XID_346">
Protocol Syntax
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<p class="para">
<a href="#TP40014097-CH25-XID_347">
Property Requirements
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Method Requirements
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Mutating Method Requirements
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Protocols as Types
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Delegation
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Adding Protocol Conformance with an Extension
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Collections of Protocol Types
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Protocol Inheritance
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Protocol Composition
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Checking for Protocol Conformance
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Optional Protocol Requirements
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<section class="section">
<p class="para">
A <em>protocol</em> defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol doesn’t actually provide an implementation for any of these requirements—it only describes what an implementation will look like. The protocol can then be <em>adopted</em> by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to <em>conform</em> to that protocol.
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<p class="para">
Protocols can require that conforming types have specific instance properties, instance methods, type methods, operators, and subscripts.
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<h3 class="section-name" tabindex="0">Protocol Syntax</h3>
<p class="para">
You define protocols in a very similar way to classes, structures, and enumerations:
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<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">SomeProtocol</span> {</code></li>
<li><code class="code-voice"> <span class="c">// protocol definition goes here</span></code></li>
<li><code class="code-voice">}</code></li>
</ul>
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</section><p class="para">
Custom types state that they adopt a particular protocol by placing the protocol’s name after the type’s name, separated by a colon, as part of their definition. Multiple protocols can be listed, and are separated by commas:
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<span class="caption"></span>
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<ul class="code-lines">
<li><code class="code-voice"><span class="kt">struct</span> <span class="vc">SomeStructure</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="c">// structure definition goes here</span></code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
If a class has a superclass, list the superclass name before any protocols it adopts, followed by a comma:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">SomeClass</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="c">// class definition goes here</span></code></li>
<li><code class="code-voice">}</code></li>
</ul>
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</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_347"></a>
<h3 class="section-name" tabindex="0">Property Requirements</h3>
<p class="para">
A protocol can require any conforming type to provide an instance property or type property with a particular name and type. The protocol doesn’t specify whether the property should be a stored property or a computed property—it only specifies the required property name and type. The protocol also specifies whether each property must be gettable or gettable <em>and</em> settable.
</p><p class="para">
If a protocol requires a property to be gettable and settable, that property requirement cannot be fulfilled by a constant stored property or a read-only computed property. If the protocol only requires a property to be gettable, the requirement can be satisfied by any kind of property, and it is valid for it also to be settable if this is useful for your own code.
</p><p class="para">
Property requirements are always declared as variable properties, prefixed with the <code class="code-voice">var</code> keyword. Gettable and settable properties are indicated by writing <code class="code-voice">{ get set }</code> after their type declaration, and gettable properties are indicated by writing <code class="code-voice">{ get }</code>.
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">SomeProtocol</span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">mustBeSettable</span>: <span class="n"></span> { <span class="kt">get</span> <span class="kt">set</span> }</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">doesNotNeedToBeSettable</span>: <span class="n"></span> { <span class="kt">get</span> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
Always prefix type property requirements with the <code class="code-voice">class</code> keyword when you define them in a protocol. This is true even though type property requirements are prefixed with the <code class="code-voice">static</code> keyword when implemented by a structure or enumeration:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">AnotherProtocol</span> {</code></li>
<li><code class="code-voice"> <span class="kt">class</span> <span class="kt">var</span> <span class="vc">someTypeProperty</span>: <span class="n"></span> { <span class="kt">get</span> <span class="kt">set</span> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
Here’s an example of a protocol with a single instance property requirement:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">FullyNamed</span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">fullName</span>: <span class="n"></span> { <span class="kt">get</span> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
The <code class="code-voice">FullyNamed</code> protocol defines any kind of thing that has a fully-qualified name. It doesn’t specify what <em>kind</em> of thing it must be—it only specifies that the thing must be able to provide a full name for itself. It specifies this requirement by stating that any <code class="code-voice">FullyNamed</code> type must have a gettable instance property called <code class="code-voice">fullName</code>, which is of type <code class="code-voice">String</code>.
</p><p class="para">
Here’s an example of a simple structure that adopts and conforms to the <code class="code-voice">FullyNamed</code> protocol:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">struct</span> <span class="vc">Person</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">fullName</span>: <span class="n"></span></code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">john</span> = <span class="vc">Person</span>(<span class="vc">fullName</span>: <span class="s">"John Appleseed"</span>)</code></li>
<li><code class="code-voice"><span class="c">// john.fullName is "John Appleseed"</span></code></li>
</ul>
</div>
</section><p class="para">
This example defines a structure called <code class="code-voice">Person</code>, which represents a specific named person. It states that it adopts the <code class="code-voice">FullyNamed</code> protocol as part of the first line of its definition.
</p><p class="para">
Each instance of <code class="code-voice">Person</code> has a single stored property called <code class="code-voice">fullName</code>, which is of type <code class="code-voice">String</code>. This matches the single requirement of the <code class="code-voice">FullyNamed</code> protocol, and means that <code class="code-voice">Person</code> has correctly conformed to the protocol. (Swift reports an error at compile-time if a protocol requirement is not fulfilled.)
</p><p class="para">
Here’s a more complex class, which also adopts and conforms to the <code class="code-voice">FullyNamed</code> protocol:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">Starship</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">prefix</span>: <span class="n"></span>?</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">name</span>: <span class="n"></span></code></li>
<li><code class="code-voice"> <span class="kt">init</span>(<span class="vc">name</span>: <span class="n"></span>? = <span class="vc">nil</span>) {</code></li>
<li><code class="code-voice"> <span class="kt">self</span>.<span class="vc">name</span> = <span class="vc">name</span></code></li>
<li><code class="code-voice"> <span class="kt">self</span>.<span class="vc">prefix</span> = <span class="vc">prefix</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">fullName</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">return</span> (<span class="vc">prefix</span> ? <span class="vc">prefix</span>! + <span class="s">" "</span> : <span class="s">""</span>) + <span class="vc">name</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">ncc1701</span> = <span class="vc">Starship</span>(<span class="vc">name</span>: <span class="s">"Enterprise"</span>, <span class="vc">prefix</span>: <span class="s">"USS"</span>)</code></li>
<li><code class="code-voice"><span class="c">// ncc1701.fullName is "USS Enterprise"</span></code></li>
</ul>
</div>
</section><p class="para">
This class implements the <code class="code-voice">fullName</code> property requirement as a computed read-only property for a starship. Each <code class="code-voice">Starship</code> class instance stores a mandatory <code class="code-voice">name</code> and an optional <code class="code-voice">prefix</code>. The <code class="code-voice">fullName</code> property uses the <code class="code-voice">prefix</code> value if it exists, and prepends it to the beginning of <code class="code-voice">name</code> to create a full name for the starship.
</p>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_348"></a>
<h3 class="section-name" tabindex="0">Method Requirements</h3>
<p class="para">
Protocols can require specific instance methods and type methods to be implemented by conforming types. These methods are written as part of the protocol’s definition in exactly the same way as for normal instance and type methods, but without curly braces or a method body. Variadic parameters are allowed, subject to the same rules as for normal methods.
</p><div class="note">
<a name="TP40014097-CH25-XID_349"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">Protocols use the same syntax as normal methods, but are not allowed to specify default values for method parameters.
</p>
</aside>
</div><p class="para">
As with type property requirements, you always prefix type method requirements with the <code class="code-voice">class</code> keyword when they are defined in a protocol. This is true even though type method requirements are prefixed with the <code class="code-voice">static</code> keyword when implemented by a structure or enumeration:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">SomeProtocol</span> {</code></li>
<li><code class="code-voice"> <span class="kt">class</span> <span class="kt">func</span> <span class="vc">someTypeMethod</span>()</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
The following example defines a protocol with a single instance method requirement:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">RandomNumberGenerator</span> {</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">random</span>() -> <span class="n"></span></code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
This protocol, <code class="code-voice">RandomNumberGenerator</code>, requires any conforming type to have an instance method called <code class="code-voice">random</code>, which returns a <code class="code-voice">Double</code> value whenever it is called. (Although it is not specified as part of the protocol, it is assumed that this value will be a number between <code class="code-voice">0.0</code> and <code class="code-voice">1.0</code> inclusive.)
</p><p class="para">
The <code class="code-voice">RandomNumberGenerator</code> protocol does not make any assumptions about how each random number will be generated—it simply requires the generator to provide a standard way to generate a new random number.
</p><p class="para">
Here’s an implementation of a class that adopts and conforms to the <code class="code-voice">RandomNumberGenerator</code> protocol. This class implements a pseudorandom number generator algorithm known as a <em>linear congruential generator</em>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">LinearCongruentialGenerator</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">lastRandom</span> = <span class="m">42.0</span></code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">m</span> = <span class="m">139968.0</span></code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">a</span> = <span class="m">3877.0</span></code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">c</span> = <span class="m">29573.0</span></code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">random</span>() -> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="vc">lastRandom</span> = ((<span class="vc">lastRandom</span> * <span class="vc">a</span> + <span class="vc">c</span>) % <span class="vc">m</span>)</code></li>
<li><code class="code-voice"> <span class="kt">return</span> <span class="vc">lastRandom</span> / <span class="vc">m</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">generator</span> = <span class="vc">LinearCongruentialGenerator</span>()</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"Here's a random number: </span>\(<span class="vc">generator</span>.<span class="vc">random</span>())<span class="s">"</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "Here's a random number: 0.37464991998171"</span></code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"And another one: </span>\(<span class="vc">generator</span>.<span class="vc">random</span>())<span class="s">"</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "And another one: 0.729023776863283"</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_350"></a>
<h3 class="section-name" tabindex="0">Mutating Method Requirements</h3>
<p class="para">
It is sometimes necessary for a method to modify (or <em>mutate</em>) the instance it belongs to. For instance methods on value types (that is, structures and enumerations) you place the <code class="code-voice">mutating</code> keyword before a method’s <code class="code-voice">func</code> keyword to indicate that the method is allowed to modify the instance it belongs to and/or any properties of that instance. This process is described in <span class="x-name"><a href="Methods.html#TP40014097-CH15-XID_305" data-id="TP40014097-CH15-XID_305">Modifying Value Types from Within Instance Methods</a></span>.
</p><p class="para">
If you define a protocol instance method requirement that is intended to mutate instances of any type that adopts the protocol, mark the method with the <code class="code-voice">mutating</code> keyword as part of the protocol’s definition. This enables structures and enumerations to adopt the protocol and satisfy that method requirement.
</p><div class="note">
<a name="TP40014097-CH25-XID_351"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">If you mark a protocol instance method requirement as <code class="code-voice">mutating</code>, you do not need to write the <code class="code-voice">mutating</code> keyword when writing an implementation of that method for a class. The <code class="code-voice">mutating</code> keyword is only used by structures and enumerations.
</p>
</aside>
</div><p class="para">
The example below defines a protocol called <code class="code-voice">Togglable</code>, which defines a single instance method requirement called <code class="code-voice">toggle</code>. As its name suggests, the <code class="code-voice">toggle</code> method is intended to toggle or invert the state of any conforming type, typically by modifying a property of that type.
</p><p class="para">
The <code class="code-voice">toggle</code> method is marked with the <code class="code-voice">mutating</code> keyword as part of the <code class="code-voice">Togglable</code> protocol definition, to indicate that the method is expected to mutate the state of a conforming instance when it is called:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">Togglable</span> {</code></li>
<li><code class="code-voice"> <span class="kt">mutating</span> <span class="kt">func</span> <span class="vc">toggle</span>()</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
If you implement the <code class="code-voice">Togglable</code> protocol for a structure or enumeration, that structure or enumeration can conform to the protocol by providing an implementation of the <code class="code-voice">toggle</code> method that is also marked as <code class="code-voice">mutating</code>.
</p><p class="para">
The example below defines an enumeration called <code class="code-voice">OnOffSwitch</code>. This enumeration toggles between two states, indicated by the enumeration cases <code class="code-voice">On</code> and <code class="code-voice">Off</code>. The enumeration’s <code class="code-voice">toggle</code> implementation is marked as <code class="code-voice">mutating</code>, to match the <code class="code-voice">Togglable</code> protocol’s requirements:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">enum</span> <span class="vc">OnOffSwitch</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">case</span> <span class="vc">Off</span>, <span class="vc">On</span></code></li>
<li><code class="code-voice"> <span class="kt">mutating</span> <span class="kt">func</span> <span class="vc">toggle</span>() {</code></li>
<li><code class="code-voice"> <span class="kt">switch</span> <span class="kt">self</span> {</code></li>
<li><code class="code-voice"> <span class="kt">case</span> <span class="vc">Off</span>:</code></li>
<li><code class="code-voice"> <span class="kt">self</span> = <span class="vc">On</span></code></li>
<li><code class="code-voice"> <span class="kt">case</span> <span class="vc">On</span>:</code></li>
<li><code class="code-voice"> <span class="kt">self</span> = <span class="vc">Off</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">lightSwitch</span> = <span class="vc">OnOffSwitch</span>.<span class="vc">Off</span></code></li>
<li><code class="code-voice"><span class="vc">lightSwitch</span>.<span class="vc">toggle</span>()</code></li>
<li><code class="code-voice"><span class="c">// lightSwitch is now equal to .On</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_352"></a>
<h3 class="section-name" tabindex="0">Protocols as Types</h3>
<p class="para">
Protocols do not actually implement any functionality themselves. Nonetheless, any protocol you create will become a fully-fledged type for use in your code.
</p><p class="para">
Because it is a type, you can use a protocol in many places where other types are allowed, including:
</p><ul class="list-bullet">
<li class="item"><p class="para">
As a parameter type or return type in a function, method, or initializer
</p>
</li><li class="item"><p class="para">
As the type of a constant, variable, or property
</p>
</li><li class="item"><p class="para">
As the type of items in an array, dictionary, or other container
</p>
</li>
</ul><div class="note">
<a name="TP40014097-CH25-XID_353"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">Because protocols are types, begin their names with a capital letter (such as <code class="code-voice">FullyNamed</code> and <code class="code-voice">RandomNumberGenerator</code>) to match the names of other types in Swift (such as <code class="code-voice">Int</code>, <code class="code-voice">String</code>, and <code class="code-voice">Double</code>).
</p>
</aside>
</div><p class="para">
Here’s an example of a protocol used as a type:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">Dice</span> {</code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">sides</span>: <span class="n"></span></code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">generator</span>: <span class="n"></span></code></li>
<li><code class="code-voice"> <span class="kt">init</span>(<span class="vc">sides</span>: <span class="n"></span>) {</code></li>
<li><code class="code-voice"> <span class="kt">self</span>.<span class="vc">sides</span> = <span class="vc">sides</span></code></li>
<li><code class="code-voice"> <span class="kt">self</span>.<span class="vc">generator</span> = <span class="vc">generator</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">roll</span>() -> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">return</span> <span class="vc">Int</span>(<span class="vc">generator</span>.<span class="vc">random</span>() * <span class="vc">Double</span>(<span class="vc">sides</span>)) + <span class="m">1</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
This example defines a new class called <code class="code-voice">Dice</code>, which represents an <em>n</em>-sided dice for use in a board game. <code class="code-voice">Dice</code> instances have an integer property called <code class="code-voice">sides</code>, which represents how many sides they have, and a property called <code class="code-voice">generator</code>, which provides a random number generator from which to create dice roll values.
</p><p class="para">
The <code class="code-voice">generator</code> property is of type <code class="code-voice">RandomNumberGenerator</code>. Therefore, you can set it to an instance of <em>any</em> type that adopts the <code class="code-voice">RandomNumberGenerator</code> protocol. Nothing else is required of the instance you assign to this property, except that the instance must adopt the <code class="code-voice">RandomNumberGenerator</code> protocol.
</p><p class="para">
<code class="code-voice">Dice</code> also has an initializer, to set up its initial state. This initializer has a parameter called <code class="code-voice">generator</code>, which is also of type <code class="code-voice">RandomNumberGenerator</code>. You can pass a value of any conforming type in to this parameter when initializing a new <code class="code-voice">Dice</code> instance.
</p><p class="para">
<code class="code-voice">Dice</code> provides one instance method, <code class="code-voice">roll</code>, which returns an integer value between 1 and the number of sides on the dice. This method calls the generator’s <code class="code-voice">random</code> method to create a new random number between <code class="code-voice">0.0</code> and <code class="code-voice">1.0</code>, and uses this random number to create a dice roll value within the correct range. Because <code class="code-voice">generator</code> is known to adopt <code class="code-voice">RandomNumberGenerator</code>, it is guaranteed to have a <code class="code-voice">random</code> method to call.
</p><p class="para">
Here’s how the <code class="code-voice">Dice</code> class can be used to create a six-sided dice with a <code class="code-voice">LinearCongruentialGenerator</code> instance as its random number generator:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">d6</span> = <span class="vc">Dice</span>(<span class="vc">sides</span>: <span class="m">6</span>, <span class="vc">generator</span>: <span class="vc">LinearCongruentialGenerator</span>())</code></li>
<li><code class="code-voice"><span class="kt">for</span> <span class="kt">_</span> <span class="kt">in</span> <span class="m">1</span>...<span class="m">5</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"Random dice roll is </span>\(<span class="vc">d6</span>.<span class="vc">roll</span>())<span class="s">"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// Random dice roll is 3</span></code></li>
<li><code class="code-voice"><span class="c">// Random dice roll is 5</span></code></li>
<li><code class="code-voice"><span class="c">// Random dice roll is 4</span></code></li>
<li><code class="code-voice"><span class="c">// Random dice roll is 5</span></code></li>
<li><code class="code-voice"><span class="c">// Random dice roll is 4</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_354"></a>
<h3 class="section-name" tabindex="0">Delegation</h3>
<p class="para">
<em>Delegation</em> is a design pattern that enables a class or structure to hand off (or <em>delegate</em>) some of its responsibilities to an instance of another type. This design pattern is implemented by defining a protocol that encapsulates the delegated responsibilities, such that a conforming type (known as a delegate) is guaranteed to provide the functionality that has been delegated. Delegation can be used to respond to a particular action, or to retrieve data from an external source without needing to know the underlying type of that source.
</p><p class="para">
The example below defines two protocols for use with dice-based board games:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">DiceGame</span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">dice</span>: <span class="n"></span> { <span class="kt">get</span> }</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">play</span>()</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">DiceGameDelegate</span> {</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">gameDidStart</span>(<span class="vc">game</span>: <span class="n"></span>)</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">game</span>(<span class="vc">game</span>: <span class="n"></span>)</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">gameDidEnd</span>(<span class="vc">game</span>: <span class="n"></span>)</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
The <code class="code-voice">DiceGame</code> protocol is a protocol that can be adopted by any game that involves dice. The <code class="code-voice">DiceGameDelegate</code> protocol can be adopted by any type to track the progress of a <code class="code-voice">DiceGame</code>.
</p><p class="para">
Here’s a version of the <em>Snakes and Ladders</em> game originally introduced in <span class="x-name"><a href="Control Flow.html#TP40014097-CH9-XID_153" data-id="TP40014097-CH9-XID_153">Control Flow</a></span>. This version is adapted to use a <code class="code-voice">Dice</code> instance for its dice-rolls; to adopt the <code class="code-voice">DiceGame</code> protocol; and to notify a <code class="code-voice">DiceGameDelegate</code> about its progress:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">SnakesAndLadders</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">finalSquare</span> = <span class="m">25</span></code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">dice</span> = <span class="vc">Dice</span>(<span class="vc">sides</span>: <span class="m">6</span>, <span class="vc">generator</span>: <span class="vc">LinearCongruentialGenerator</span>())</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">square</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">board</span>: <span class="n"></span>[]</code></li>
<li><code class="code-voice"> <span class="kt">init</span>() {</code></li>
<li><code class="code-voice"> <span class="vc">board</span> = <span class="vc">Int</span>[](<span class="vc">count</span>: <span class="vc">finalSquare</span> + <span class="m">1</span>, <span class="vc">repeatedValue</span>: <span class="m">0</span>)</code></li>
<li><code class="code-voice"> <span class="vc">board</span>[<span class="m">03</span>] = +<span class="m">08</span>; <span class="vc">board</span>[<span class="m">06</span>] = +<span class="m">11</span>; <span class="vc">board</span>[<span class="m">09</span>] = +<span class="m">09</span>; <span class="vc">board</span>[<span class="m">10</span>] = +<span class="m">02</span></code></li>
<li><code class="code-voice"> <span class="vc">board</span>[<span class="m">14</span>] = -<span class="m">10</span>; <span class="vc">board</span>[<span class="m">19</span>] = -<span class="m">11</span>; <span class="vc">board</span>[<span class="m">22</span>] = -<span class="m">02</span>; <span class="vc">board</span>[<span class="m">24</span>] = -<span class="m">08</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">delegate</span>: <span class="n"></span>?</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">play</span>() {</code></li>
<li><code class="code-voice"> <span class="vc">square</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"> <span class="vc">delegate</span>?.<span class="vc">gameDidStart</span>(<span class="kt">self</span>)</code></li>
<li><code class="code-voice"> <span class="vc">gameLoop</span>: <span class="kt">while</span> <span class="vc">square</span> != <span class="vc">finalSquare</span> {</code></li>
<li><code class="code-voice"> <span class="kt">let</span> <span class="vc">diceRoll</span> = <span class="vc">dice</span>.<span class="vc">roll</span>()</code></li>
<li><code class="code-voice"> <span class="vc">delegate</span>?.<span class="vc">game</span>(<span class="kt">self</span>, <span class="vc">didStartNewTurnWithDiceRoll</span>: <span class="vc">diceRoll</span>)</code></li>
<li><code class="code-voice"> <span class="kt">switch</span> <span class="vc">square</span> + <span class="vc">diceRoll</span> {</code></li>
<li><code class="code-voice"> <span class="kt">case</span> <span class="vc">finalSquare</span>:</code></li>
<li><code class="code-voice"> <span class="kt">break</span> <span class="vc">gameLoop</span></code></li>
<li><code class="code-voice"> <span class="kt">case</span> <span class="kt">let</span> <span class="vc">newSquare</span> <span class="kt">where</span> <span class="vc">newSquare</span> > <span class="vc">finalSquare</span>:</code></li>
<li><code class="code-voice"> <span class="kt">continue</span> <span class="vc">gameLoop</span></code></li>
<li><code class="code-voice"> <span class="kt">default</span>:</code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">diceRoll</span></code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">board</span>[<span class="vc">square</span>]</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="vc">delegate</span>?.<span class="vc">gameDidEnd</span>(<span class="kt">self</span>)</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
For a description of the <em>Snakes and Ladders</em> gameplay, see the <span class="x-name"><a href="Control Flow.html#TP40014097-CH9-XID_174" data-id="TP40014097-CH9-XID_174">Break</a></span> section of the <span class="x-name"><a href="Control Flow.html#TP40014097-CH9-XID_153" data-id="TP40014097-CH9-XID_153">Control Flow</a></span> chapter.
</p><p class="para">
This version of the game is wrapped up as a class called <code class="code-voice">SnakesAndLadders</code>, which adopts the <code class="code-voice">DiceGame</code> protocol. It provides a gettable <code class="code-voice">dice</code> property and a <code class="code-voice">play</code> method in order to conform to the protocol. (The <code class="code-voice">dice</code> property is declared as a constant property because it does not need to change after initialization, and the protocol only requires that it is gettable.)
</p><p class="para">
The <em>Snakes and Ladders</em> game board setup takes place within the class’s <code class="code-voice">init()</code> initializer. All game logic is moved into the protocol’s <code class="code-voice">play</code> method, which uses the protocol’s required <code class="code-voice">dice</code> property to provide its dice roll values.
</p><p class="para">
Note that the <code class="code-voice">delegate</code> property is defined as an <em>optional</em> <code class="code-voice">DiceGameDelegate</code>, because a delegate isn’t required in order to play the game. Because it is of an optional type, the <code class="code-voice">delegate</code> property is automatically set to an initial value of <code class="code-voice">nil</code>. Thereafter, the game instantiator has the option to set the property to a suitable delegate.
</p><p class="para">
<code class="code-voice">DiceGameDelegate</code> provides three methods for tracking the progress of a game. These three methods have been incorporated into the game logic within the <code class="code-voice">play</code> method above, and are called when a new game starts, a new turn begins, or the game ends.
</p><p class="para">
Because the <code class="code-voice">delegate</code> property is an <em>optional</em> <code class="code-voice">DiceGameDelegate</code>, the <code class="code-voice">play</code> method uses optional chaining each time it calls a method on the delegate. If the <code class="code-voice">delegate</code> property is nil, these delegate calls fail gracefully and without error. If the <code class="code-voice">delegate</code> property is non-nil, the delegate methods are called, and are passed the <code class="code-voice">SnakesAndLadders</code> instance as a parameter.
</p><p class="para">
This next example shows a class called <code class="code-voice">DiceGameTracker</code>, which adopts the <code class="code-voice">DiceGameDelegate</code> protocol:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">class</span> <span class="vc">DiceGameTracker</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">numberOfTurns</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">gameDidStart</span>(<span class="vc">game</span>: <span class="n"></span>) {</code></li>
<li><code class="code-voice"> <span class="vc">numberOfTurns</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"> <span class="kt">if</span> <span class="vc">game</span> <span class="kt">is</span> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"Started a new game of Snakes and Ladders"</span>)</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"The game is using a </span>\(<span class="vc">game</span>.<span class="vc">dice</span>.<span class="vc">sides</span>)<span class="s">-sided dice"</span>)</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">game</span>(<span class="vc">game</span>: <span class="n"></span>) {</code></li>
<li><code class="code-voice"> ++<span class="vc">numberOfTurns</span></code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"Rolled a </span>\(<span class="vc">diceRoll</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">gameDidEnd</span>(<span class="vc">game</span>: <span class="n"></span>) {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"The game lasted for </span>\(<span class="vc">numberOfTurns</span>)<span class="s"> turns"</span>)</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
<code class="code-voice">DiceGameTracker</code> implements all three methods required by <code class="code-voice">DiceGameDelegate</code>. It uses these methods to keep track of the number of turns a game has taken. It resets a <code class="code-voice">numberOfTurns</code> property to zero when the game starts; increments it each time a new turn begins; and prints out the total number of turns once the game has ended.
</p><p class="para">
The implementation of <code class="code-voice">gameDidStart</code> shown above uses the <code class="code-voice">game</code> parameter to print some introductory information about the game that is about to be played. The <code class="code-voice">game</code> parameter has a type of <code class="code-voice">DiceGame</code>, not <code class="code-voice">SnakesAndLadders</code>, and so <code class="code-voice">gameDidStart</code> can access and use only methods and properties that are implemented as part of the <code class="code-voice">DiceGame</code> protocol. However, the method is still able to use type casting to query the type of the underlying instance. In this example, it checks whether <code class="code-voice">game</code> is actually an instance of <code class="code-voice">SnakesAndLadders</code> behind the scenes, and prints an appropriate message if so.
</p><p class="para">
<code class="code-voice">gameDidStart</code> also accesses the <code class="code-voice">dice</code> property of the passed <code class="code-voice">game</code> parameter. Because <code class="code-voice">game</code> is known to conform to the <code class="code-voice">DiceGame</code> protocol, it is guaranteed to have a <code class="code-voice">dice</code> property, and so the <code class="code-voice">gameDidStart</code> method is able to access and print the dice’s <code class="code-voice">sides</code> property, regardless of what kind of game is being played.
</p><p class="para">
Here’s how <code class="code-voice">DiceGameTracker</code> looks in action:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">tracker</span> = <span class="vc">DiceGameTracker</span>()</code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">game</span> = <span class="vc">SnakesAndLadders</span>()</code></li>
<li><code class="code-voice"><span class="vc">game</span>.<span class="vc">delegate</span> = <span class="vc">tracker</span></code></li>
<li><code class="code-voice"><span class="vc">game</span>.<span class="vc">play</span>()</code></li>
<li><code class="code-voice"><span class="c">// Started a new game of Snakes and Ladders</span></code></li>
<li><code class="code-voice"><span class="c">// The game is using a 6-sided dice</span></code></li>
<li><code class="code-voice"><span class="c">// Rolled a 3</span></code></li>
<li><code class="code-voice"><span class="c">// Rolled a 5</span></code></li>
<li><code class="code-voice"><span class="c">// Rolled a 4</span></code></li>
<li><code class="code-voice"><span class="c">// Rolled a 5</span></code></li>
<li><code class="code-voice"><span class="c">// The game lasted for 4 turns</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_355"></a>
<h3 class="section-name" tabindex="0">Adding Protocol Conformance with an Extension</h3>
<p class="para">
You can extend an existing type to adopt and conform to a new protocol, even if you do not have access to the source code for the existing type. Extensions can add new properties, methods, and subscripts to an existing type, and are therefore able to add any requirements that a protocol may demand. For more about extensions, see <span class="x-name"><a href="Extensions.html#TP40014097-CH24-XID_191" data-id="TP40014097-CH24-XID_191">Extensions</a></span>.
</p><div class="note">
<a name="TP40014097-CH25-XID_356"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">Existing instances of a type automatically adopt and conform to a protocol when that conformance is added to the instance’s type in an extension.
</p>
</aside>
</div><p class="para">
For example, this protocol, called <code class="code-voice">TextRepresentable</code>, can be implemented by any type that has a way to be represented as text. This might be a description of itself, or a text version of its current state:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">TextRepresentable</span> {</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">asText</span>() -> <span class="n"></span></code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
The <code class="code-voice">Dice</code> class from earlier can be extended to adopt and conform to <code class="code-voice">TextRepresentable</code>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">extension</span> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">asText</span>() -> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">return</span> <span class="s">"A </span>\(<span class="vc">sides</span>)<span class="s">-sided dice"</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
This extension adopts the new protocol in exactly the same way as if <code class="code-voice">Dice</code> had provided it in its original implementation. The protocol name is provided after the type name, separated by a colon, and an implementation of all requirements of the protocol is provided within the extension’s curly braces.
</p><p class="para">
Any <code class="code-voice">Dice</code> instance can now be treated as <code class="code-voice">TextRepresentable</code>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">d12</span> = <span class="vc">Dice</span>(<span class="vc">sides</span>: <span class="m">12</span>, <span class="vc">generator</span>: <span class="vc">LinearCongruentialGenerator</span>())</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="vc">d12</span>.<span class="vc">asText</span>())</code></li>
<li><code class="code-voice"><span class="c">// prints "A 12-sided dice"</span></code></li>
</ul>
</div>
</section><p class="para">
Similarly, the <code class="code-voice">SnakesAndLadders</code> game class can be extended to adopt and conform to the <code class="code-voice">TextRepresentable</code> protocol:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">extension</span> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">asText</span>() -> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">return</span> <span class="s">"A game of Snakes and Ladders with </span>\(<span class="vc">finalSquare</span>)<span class="s"> squares"</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="vc">game</span>.<span class="vc">asText</span>())</code></li>
<li><code class="code-voice"><span class="c">// prints "A game of Snakes and Ladders with 25 squares"</span></code></li>
</ul>
</div>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_357"></a>
<h3 class="section-name" tabindex="0">Declaring Protocol Adoption with an Extension</h3>
<p class="para">
If a type already conforms to all of the requirements of a protocol, but has not yet stated that it adopts that protocol, you can make it adopt the protocol with an empty extension:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">struct</span> <span class="vc">Hamster</span> {</code></li>
<li><code class="code-voice"> <span class="kt">var</span> <span class="vc">name</span>: <span class="n"></span></code></li>
<li><code class="code-voice"> <span class="kt">func</span> <span class="vc">asText</span>() -> <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="kt">return</span> <span class="s">"A hamster named </span>\(<span class="vc">name</span>)<span class="s">"</span></code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="kt">extension</span> <span class="n"></span> {}</code></li>
</ul>
</div>
</section><p class="para">
Instances of <code class="code-voice">Hamster</code> can now be used wherever <code class="code-voice">TextRepresentable</code> is the required type:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">simonTheHamster</span> = <span class="vc">Hamster</span>(<span class="vc">name</span>: <span class="s">"Simon"</span>)</code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">somethingTextRepresentable</span>: <span class="n"></span> = <span class="vc">simonTheHamster</span></code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="vc">somethingTextRepresentable</span>.<span class="vc">asText</span>())</code></li>
<li><code class="code-voice"><span class="c">// prints "A hamster named Simon"</span></code></li>
</ul>
</div>
</section><div class="note">
<a name="TP40014097-CH25-XID_358"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">Types do not automatically adopt a protocol just by satisfying its requirements. They must always explicitly declare their adoption of the protocol.
</p>
</aside>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_359"></a>
<h3 class="section-name" tabindex="0">Collections of Protocol Types</h3>
<p class="para">
A protocol can be used as the type to be stored in a collection such as an array or a dictionary, as mentioned in <span class="x-name"><a href="#TP40014097-CH25-XID_352" data-id="TP40014097-CH25-XID_352">Protocols as Types</a></span>. This example creates an array of <code class="code-voice">TextRepresentable</code> things:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">things</span>: <span class="n"></span>[] = [<span class="vc">game</span>, <span class="vc">d12</span>, <span class="vc">simonTheHamster</span>]</code></li>
</ul>
</div>
</section><p class="para">
It is now possible to iterate over the items in the array, and print each item’s textual representation:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">for</span> <span class="vc">thing</span> <span class="kt">in</span> <span class="vc">things</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="vc">thing</span>.<span class="vc">asText</span>())</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// A game of Snakes and Ladders with 25 squares</span></code></li>
<li><code class="code-voice"><span class="c">// A 12-sided dice</span></code></li>
<li><code class="code-voice"><span class="c">// A hamster named Simon</span></code></li>
</ul>
</div>
</section><p class="para">
Note that the <code class="code-voice">thing</code> constant is of type <code class="code-voice">TextRepresentable</code>. It is not of type <code class="code-voice">Dice</code>, or <code class="code-voice">DiceGame</code>, or <code class="code-voice">Hamster</code>, even if the actual instance behind the scenes is of one of those types. Nonetheless, because it is of type <code class="code-voice">TextRepresentable</code>, and anything that is <code class="code-voice">TextRepresentable</code> is known to have an <code class="code-voice">asText</code> method, it is safe to call <code class="code-voice">thing.asText</code> each time through the loop.
</p>
</section>
<section class="section">
<a name="TP40014097-CH25-XID_360"></a>
<h3 class="section-name" tabindex="0">Protocol Inheritance</h3>
<p class="para">
A protocol can <em>inherit</em> one or more other protocols and can add further requirements on top of the requirements it inherits. The syntax for protocol inheritance is similar to the syntax for class inheritance, but with the option to list multiple inherited protocols, separated by commas:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">protocol</span> <span class="vc">InheritingProtocol</span>: <span class="n"></span> {</code></li>
<li><code class="code-voice"> <span class="c">// protocol definition goes here</span></code></li>
<li><code class="code-voice">}</code></li>
</ul>