Android: Allow custom Typeface to be added ReactInstanceManager

[mfenniak]

Attached PR adds support for custom, non-system fonts in Android applications. It also adds a demonstration of this capability in the Android UIExplorer application.

Android doesn't have an equivalent to iOS's approach, where you can add fonts to your application's resources and they are automatically available by their font-family name. In fact, Android doesn't have any "global" mechanism to provide fonts in an application that are available by their name. So, the approach taken here is a reasonable platform-specific solution given the disparity in this area between iOS and Android.

To add a custom font, you register a Typeface object on your ReactInstanceManager.Builder, like this:

    mReactInstanceManager = ReactInstanceManager.builder()
        .setApplication(getApplication())
        ...
        .addTypeFace("DancingScript",
            Typeface.createFromAsset(getAssets(), "DancingScript-Regular.ttf"))
        .build();

And this allows you to use the font, like this:

<Text style={{fontFamily: 'DancingScript'}}>
    Custom font loaded from an application asset TTF file
</Text>

Internally, I've created an interface called a TypefaceProvider that is provided by the ReactContext, and then can be passed through to any text-related view manager to retrieve fonts. The existing caching behavior that was in CustomStyleSpan has been preserved and moved to TypefaceProviderImpl, augmented with the ability to reference the typefaces that have been added to the react instance manager. I don't love the fact that the TypefaceProvider needs to be passed around so much, but it seems like something would need to be passed into the text view managers to accomplish this without using global state, so I can't think of a better approach.

Would resolve #2919.

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @mkonicek, @kmagiera and @arbesfeld to be potential reviewers.

[facebook-github-bot]

@mfenniak updated the pull request.

[vovacodes]

Looks neat! Hope this will be merged. The "custom typeface" problem on Android is very annoying. Thanks for your effort

[facebook-github-bot]

@mfenniak updated the pull request.

[satya164]

+1

[mfenniak]

Closing this PR; equivalent functionality has been added in bfeaa6a.