Summary:When a component prop is set to null/undefined, and doesn't have a default value specified in `getDefaultProps`, the null value is sent over the bridge as a sentinel to reset to the original native value.
On iOS this is handled by creating a default view instance for each view type. The default view is then used to look up the unmodified value for any prop that is reset.
This is rather expensive however, as it means that for complex views (e.g. WebView, MapView), a minimum of two instances will be created even if only one is needed, and the default view will remain even after all actual view instances have been released.
This diff replaces the default view mechanism with a system where the default value of each prop is recorded the first time it is set. This avoids the need to keep an extra copy of the whole view.
The only exception is for props that use the `RCT_CUSTOM_VIEW_PROPERTY` macro, which includes the default view as part of the interface. To avoid a breaking change, a default view will still be created for views that use this macro, but only if they are sent a null value (so very rarely, in practice). In a future update we may deprecate or replace `RCT_CUSTOM_VIEW_PROPERTY` if there are significant benefits to doing so.
Reviewed By: javache
Differential Revision: D3012115
fb-gh-sync-id: 259348e54aa8342f444ad182b6f883d2dd684973
shipit-source-id: 259348e54aa8342f444ad182b6f883d2dd684973
62177dbb3b