[Spec] Use Android Fast Renderers by Default

[samhouts]

Use Android Fast Renderers by Default

Android Fast Renderers were introduced in #845 in version 2.3.5. These removed an extra ViewGroup wrapper around the basic controls, which made view rendering faster by reducing the number of views inflated.

These controls have a Fast Renderer that you must currently opt in to use via experimental flag:

• Button
• Image
• Label
• Frame

Since these new renderers were not identical to the legacy renderers, they were put under an experimental flag to prevent unexpected behavior changes in existing applications.

These renderers have now been tested heavily for almost two years, and it is time to make them the default renderers on Android!

From this point forward, all applications targeting FormsAppCompatActivity will use the Fast Renderers by default. We will also no longer plan to add new features to the legacy renderers.

As part of this change, we will also be un-sealing the Fast Renderers and ensuring that all extension points are properly open to allow developers to create custom renderers.

Backward Compatibility

You can override the new default by:

1. Using custom renderers targeting the legacy renderers. Any existing custom renderers will continue to function with legacy renderers.
2. Specifying a different View.Visual, such as Material. Note that these are already different renderers that are also (mostly) following the Fast Renderer pattern.
3. Enabling the Forms.SetFlags("UseLegacyRenderers") flag in your MainActivity.

[adrianknight89]

Should there be a warning in the console for those who have added the following line?

Forms.SetFlags("FastRenderers_Experimental");

If fast renderers are used by default, there is no need for the above flag.

[dansiegel]

One additional thing that should be done here... when we do this, make the fast renderers public and unseal them so that people can inherit from the fast renderers.

[samhouts]

@dansiegel ;)

As part of this change, we will also be un-sealing the Fast Renderers and ensuring that all extension points are properly open to allow developers to create custom renderers.

[samhouts]

@adrianknight89

Should there be a warning in the console for those who have added the following line?

We designed the flags to just no-op when they're no longer necessary. It should have no adverse affects on your application to continue to have it there. If we add a console warning, you'll definitely have a "larger" (read: still very small) performance hit. But if this is something that is strongly desired by the community, we'll consider it for sure. Thanks!

[adrianknight89]

@samhouts Frame doesn't seem to be fully tested yet. Can #5766 and #3150 be resolved before fast renderers get turned on by default? There is also #2989.