Switch to Plugin API, convert gesture detection to a layer

[johnpryan]

1. making all layers a "plugin" and 2. adding a gesture detection layer.

see #160 for background

[kengu]

@johnpryan: I'm currently blocked on this issue. Could we start an design discussion so that I can attempt an implementation of this issue?

[kengu]

From #153

After looking at this a little more closely, I don't think this PR should make changes to MapGestureMixin. It should be each layer's job to enter the gesture arena and immediately leave the gesture arena if the tap (or drag, for example) gesture is not on a polygon. This will be more flexible in the long run and this approach will allow for more interactive gestures by simply leaving it up to the layers themselves.

There are some challenges to doing this; the main challenge is that FlutterMapState is currently handling the gestures, but really the gesture mixing should be just a layer rendered above the tile layer.

I am going to make a separate ticket for this but for now I would like to leave this PR open so that we can come back and add this feature after 1. making all layers a "plugin" and 2. adding a gesture detection layer.

[kengu]

@johnpryan, does "entering the gesture area" imply that each layer should implement a RawGestureDetector along the same lines as Flutter Deep Dive: Gestures. Or is it just an analogy, implying something more along the lines of each layer listening for gesture events generated by the gesture layer and handling them?

If it is the latter, I guess this could easily be implemented with streams passed to each layer with the createLayer in the MapStateobject. Each layer could then listen for specific events using the filter method, and convert hits on geometries to invocation of callbacks passed to the layer in LayerOptions. This would also solve my current problem, which is implementing a geometry editor plugin. This editor needs to respond to drag events, which is hard to implement with a GestureDetector in a child widget when Stack is used.

The only issue I can see so far, is how to determine a winner when multiple geometries are located inside the same hit area. If we when for traditional callbacks instead, we could implement winning with boolean return value from each callback, or something similar to e.stopPropagation().

[johnpryan]

Yes, each layer should have the ability to handle gestures for themselves without relying on the MapGestureMixin to decide what polygon, circle, or marker was tapped. The goal is to support tap events of all layer types. #160 does not quite work for cases like that

[ruizalexandre]

@johnpryan I pushed a PR to move layers to Flutter widgets ! With that you can rebuild layer easily ! Solve performance issue also

#542 here

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