Runtime changes to text-field are not immediate

[friedbunny]

On the current iOS release branch (dbfa8dc), changing the value of MGLStyleValue.textField (in this case, the language labels via #6819) is not reflected immediately on the map.

Instead, the labels go blank.

• Zooming several levels (and triggering a [re]load?), then zooming back to the original zoom level will cause the labels to appear correctly at the original zoom level.
• Panning outside of the viewport at the original zoom level (to an area that was not shown) will show the labels in those new areas.

/cc @boundsj @1ec5 @ivovandongen

[1ec5]

This might affect all layout properties, not just text-field.

/cc @bsudekum @jfirebaugh

[1ec5]

Given #6903, this is definitely affecting all layout properties on both iOS and Android. It’s a regression of #5701.

[jfirebaugh]

The bug in this case is specific to text-field, not all properties.

GeometryTileWorker expects to be notified when the main thread has loaded additional glyphs which may be needed in order to proceed with symbol layout. However the notification mechanism, GeometryTileWorker::setPlacementConfig, is overloaded. Its primary purpose is to notify GeometryTileWorker when the camera changes in a way that affects label placement, such as rotating. In order to avoid needless updates for camera changes that don't affect placement, the provided value for PlacementConfig is compared against the last used value, and short circuited if unchanged. This short circuiting conflicts with the use of GeometryTileWorker::setPlacementConfig as a notification that new glyphs have been loaded. That needs to be a separate notification pathway.

[jfirebaugh]

Related issues: #2851, #2511. This has perpetually been a problematic area of the code.