Add option to zoom to map center instead of pointer position/focal point

[JaffaKetchup]

What do you want implemented?

At the moment, zooming with a cursor zooms into the cursor location, which is what most use cases would require. However, some applications may prefer to have the map zoom into the center, regardless of the cursor location.

This has been requested twice in the Discord server: https://discord.com/channels/951867686378409984/1317675189701705778/1317675189701705778, https://discord.com/channels/951867686378409984/1166739850137980958/1166739850137980958

This should work for trackpad as well, as requested in #1905.

What other alternatives are available?

The second link's workaround was to play an animation immediately after the gesture has stopped to move the map back to the desired center, and move the desired marker/widget around to stay in the correct geographic position instead of screen center.

It may also be possible to use constraints to prevent moving during zoom?

Can you provide any other information?

No response

Severity

Annoying: Currently have to use workarounds

[JaffaKetchup]

(This may be related (the solution) to #1905)

[monsieurtanuki]

https://m.youtube.com/watch?v=kR2r-A9H3Kg

(Sorry about that)

[JaffaKetchup]

(Also see #1905 (comment))

[JaffaKetchup]

Trackpad zooming is bugged. See these lines:

flutter_map/lib/src/gestures/map_interactive_viewer.dart

Lines 630 to 647 in 48832da

	LatLng _calculatePinchZoomAndMove(
	ScaleUpdateDetails details,
	double zoomAfterPinchZoom,
	) {
	final oldCenterPt =
	_camera.projectAtZoom(_camera.center, zoomAfterPinchZoom);
	final newFocalLatLong =
	_camera.offsetToCrs(_focalStartLocal, zoomAfterPinchZoom);
	final newFocalPt =
	_camera.projectAtZoom(newFocalLatLong, zoomAfterPinchZoom);
	final oldFocalPt =
	_camera.projectAtZoom(_focalStartLatLng, zoomAfterPinchZoom);
	final zoomDifference = oldFocalPt - newFocalPt;
	final moveDifference = _rotateOffset(_focalStartLocal - _lastFocalLocal);
	final newCenterPt = oldCenterPt + zoomDifference + moveDifference;
	return _camera.unprojectAtZoom(newCenterPt, zoomAfterPinchZoom);
	}

These lines are also used when a touchscreen zoom is used - on some platforms. I'm not sure whether there is an easy way to change this, but it's causing issues.

The existing lines (shown below) works correctly on touchscreen and when trying to move using two fingers on a trackpad.
However, when zooming on a trackpad, it causes the map to move excessively, especially toward the bottom-right corner, although it does depend on the position of the cursor. Removing moveDifference (as shown) fixes the issue. Unfortunately, it breaks two finger movement.

- final newCenterPt = oldCenterPt + zoomDifference + moveDifference;
+ final newCenterPt = oldCenterPt + zoomDifference;

[roylenz]

what are the workarounds? couldnt open the discord link.
Im trying to set the trackpad zoom to work as expected.
btw, on mac it works perfectly so maybe its possible to figure out what is the difference between the platforms.
should fixing the trackpad bug and adding the feature for the center zoom be on the same issue?

[roylenz]

@JaffaKetchup Ive done some more digging and found that the problem is in the flutter source code (I think)
I dont know enough, but applying this commit: Dusk-afk/flutter@59c8ea6 seemed to solve the trackpad problems completely

[JaffaKetchup]

Indeed it looks like the Flutter engine has an issue here. I intend to investigate and hopefully resolve it (flutter/flutter#136029 (comment)) at the source, but in the meantime, the best workaround is to introduce a flag for flutter_map to manually toggle between zooming to the focal point or to the viewport center.

[JaffaKetchup]

Unmerged from #1905, which has also been split into #2152 (linking flutter/flutter#136029). These detail the more precise problem/desired workaround of correcting the behaviour of zooming into the pointer position/focal point.

This is a separate feature request.