fix start/finish rendering map delegate callbacks

[incanus]

Per fab65e7#commitcomment-11046360, the MapChangeWillStartRenderingMap and MapChangeDidFinishRenderingMap notification types are meant to drive delegate callbacks which have parity with Apple's MapKit. These callbacks are meant to relate to the visual appearance of render, not anything frame-based, so that common iOS post-view-load techniques can be employed.

• Capturing a snapshot of a fully-rendered complex view for use with Core Image/blur filters for view transition effects.
• Not presenting the map view until it's completely tiled in (we've had requests for this before from partners for raster use cases).
• Determining when an incomplete map view might be shown, but no more attempts to load anything will be happening (fullyRendered: NO).

Some MapKit background:

The map view calls this method when one or more tiles are revealed and require rendering.

[...]

This method lets you know when the map view finishes rendering all of the currently visible tiles to the best of its ability. This method is called regardless of whether all tiles were rendered successfully. If there were errors loading one or more tiles that prevented map view from rendering them, the fullyRendered parameter is set to NO.

Calling these sorts of methods per-frame is not useful to users, either, because any work done in there will impact the render frame rate.

If we need additional callbacks such as something like MapChangeDidRenderFrame, we should make and use them instead, optionally also exporting them to Cocoa.

/cc @kkaefer

[incanus]

Per chat we are moving this to b2 and not updating anything else around delegates — known bug that they no longer work like MapKit.

[incanus]

This is a regression currently from b1 to b2.

[friedbunny]

Seeing isChanging still firing after didChange:

2015-06-23 17:40:30.148 Mapbox GL[46461:4773694] will: z5.381543; 52.421373, 9.766954
2015-06-23 17:40:30.149 Mapbox GL[46461:4773694] is: z6.381543; 52.413178, 9.766322
2015-06-23 17:40:30.260 Mapbox GL[46461:4773694] is: z6.381543; 52.104201, 9.742608
2015-06-23 17:40:30.260 Mapbox GL[46461:4773694] is: z6.381543; 52.104201, 9.742608
2015-06-23 17:40:30.260 Mapbox GL[46461:4773694] is: z6.381543; 52.104201, 9.742608
2015-06-23 17:40:30.367 Mapbox GL[46461:4773694] is: z6.381543; 52.046701, 9.738212
2015-06-23 17:40:30.368 Mapbox GL[46461:4773694] is: z6.381543; 52.046701, 9.738212
2015-06-23 17:40:30.477 Mapbox GL[46461:4773694] is: z6.381543; 52.036550, 9.737437
2015-06-23 17:40:30.623 Mapbox GL[46461:4773694] did: z6.381543; 52.036550, 9.737437
2015-06-23 17:40:30.802 Mapbox GL[46461:4773694] is: z6.381543; 52.036550, 9.737437
2015-06-23 17:40:30.971 Mapbox GL[46461:4773694] is: z6.381543; 52.036550, 9.737437

^ zoom; lat, long

[friedbunny]

Tapping on the map also triggers an isChanging notification. (And only isChanging.)

[jfirebaugh]

@friedbunny, if you're after region change notifications, I think you wanted #1026. This ticket is about rendering notifications.

Now, as far as I can tell, b1 never sent *RenderingMap notifications, and b2 sends them but with non-MapKit semantics. @incanus, is this what you meant by regressing?

bab66c1 lays some groundwork and bd901d2 rationalizes *RenderingMap -- but not in the way we want. I'll probably rename these notifications to *RenderingFrame as suggested above, and then work on notifications with the MapKit semantics.

[jfirebaugh]

Done in 6532438.

What remains unimplemented is support for fullyRendered:NO -- we currently do not surface tile errors in a way that made this easy to implement, so I'm punting for now.