cstrings: add NSStringToString

cstrings/nsstrings_darwin.go

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2026 The Ebitengine Authors
+
+package cstrings
+
+import (
+	"fmt"
+
+	"github.com/ebitengine/purego"
+	"github.com/ebitengine/purego/internal/strings"
+	"github.com/ebitengine/purego/objc"
+)
+
+var (
+	sel_isKindOf   objc.SEL
+	sel_UTF8String objc.SEL
+
+	class_NSString objc.Class
+)
+
+func init() {
+	// Must pull in Foundation to get the NSString class.
+	_, err := purego.Dlopen("/System/Library/Frameworks/Foundation.framework/Foundation", purego.RTLD_GLOBAL|purego.RTLD_NOW)
+	if err != nil {
+		panic(fmt.Errorf("cstrings: %w", err))
+	}
+	sel_isKindOf = objc.RegisterName("isKindOfClass:")
+	sel_UTF8String = objc.RegisterName("UTF8String")
+	class_NSString = objc.GetClass("NSString")
+}
+
+// NSStringToString returns a copy of the NSString contents as a Go string.
+// If the ID is 0 then an empty string is returned.
+// If the ID is not an NSString class then the function panics.
+// This function is only available on darwin.
+func NSStringToString(str objc.ID) string {
+	if str == 0 {
+		return ""
+	}
+	if str.Send(sel_isKindOf, class_NSString) == 0 {
+		panic("cstrings: provided ID is not an NSString")
+	}
+	return strings.GoString(uintptr(str.Send(sel_UTF8String)))
+}

cstrings/nsstrings_darwin_test.go

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2026 The Ebitengine Authors
+
+package cstrings_test
+
+import (
+	"testing"
+
+	"github.com/ebitengine/purego/cstrings"
+	"github.com/ebitengine/purego/objc"
+)
+
+func TestNSStringToString(t *testing.T) {
+	t.Run("nil ID returns empty string", func(t *testing.T) {
+		result := cstrings.NSStringToString(0)
+		if result != "" {
+			t.Errorf("expected empty string for nil ID, got %q", result)
+		}
+	})
+
+	t.Run("valid NSString returns correct string", func(t *testing.T) {
+		sel := objc.RegisterName("stringWithUTF8String:")
+		nsString := objc.ID(objc.GetClass("NSString")).Send(sel, "Hello, World!\x00")
+
+		result := cstrings.NSStringToString(nsString)
+		if result != "Hello, World!" {
+			t.Errorf("expected %q, got %q", "Hello, World!", result)
+		}
+	})
+
+	t.Run("empty NSString returns empty string", func(t *testing.T) {
+		sel := objc.RegisterName("stringWithUTF8String:")
+		nsString := objc.ID(objc.GetClass("NSString")).Send(sel, "\x00")
+
+		result := cstrings.NSStringToString(nsString)
+		if result != "" {
+			t.Errorf("expected empty string, got %q", result)
+		}
+	})
+
+	t.Run("non-NSString panics", func(t *testing.T) {
+		defer func() {
+			if r := recover(); r == nil {
+				t.Error("expected panic for non-NSString ID")
+			}
+		}()
+
+		classNSNumber := objc.GetClass("NSNumber")
+		sel := objc.RegisterName("numberWithInt:")
+		nsNumber := objc.ID(classNSNumber).Send(sel, 42)
+
+		cstrings.NSStringToString(nsNumber)
+	})
+}