Separate tile loading and storing

[tehmou]

This is a bit of an academic feature, but necessary for demonstrating a proper way of building more complex applications.

The loader:

• Only loads tiles and puts them into the content provider (the bitmaps to disk through bitmap cache)
• Provides an observable of error events
• Does not necessary give an observable of successes at all!

The store:

• Does not directly know about the loader, only connects to the content provider and detects loaded data through it
• Memory cache optional, though not needed

The bitmap cache:

• Remembers which bitmaps were already loaded and stores them in memory with a file path
• For performance the loader can insert the loaded bitmaps to the bitmap cache without interfering with the separation of responsibilities

The flow of loading a bitmap should be something like

• Get bitmap for tile x
• Is x in content provider?
  • No: ask loader to load it (unless loader is already loading the specific tile)
  • Yes: load bitmap from the bitmap cache
• Return the combined observable of bitmaps from the store and errors from loader. This either needs a wrapper class or we can use the RxJava Notification