In Flutter, how can a positioned Widget feel taps outside of its parent Stack area?

Question

A Stack contains MyWidget inside of a Positioned.

Stack(
  overflow: Overflow.visible,
  children: [
    Positioned(
          


        

Answer 1

This behavior occurs because the stack checks whether the pointer is inside its bounds before checking whether a child got hit:

Class: RenderBox (which RenderStack extends)

bool hitTest(BoxHitTestResult result, { @required Offset position }) {

    ...

    if (_size.contains(position)) {
      if (hitTestChildren(result, position: position) || hitTestSelf(position)) {
        result.add(BoxHitTestEntry(this, position));
        return true;
      }
    }
    return false;
}

My workaround is deleting the

if (_size.contains(position))

check. Unfortunately, this is not possible without copying code from the framework.

Here is what I did:

• Copied the Stack class and named it Stack2
• Copied RenderStack and named it RenderStack2
• Made Stack2 reference RenderStack2
• Added the hitTest method from above without the _size.contains check
• Copied Positioned and named it Positioned2 and made it reference Stack2 as its generic parameter
• Used Stack2 and Positioned2 in my code

This solution is by no means optimal, but it achieves the desired behavior.

Answer 2

Ok, I did a workaround about this, basically I added a GestureDetector on the parent and implemented the onTapDown. Also you have to keep track your Widget using GlobalKey to get the current position.

When the Tap at the parent level is detected check if the tap position is inside your widget.

The code below:

final GlobalKey key = new GlobalKey();

      void onTapDown(BuildContext context, TapDownDetails details) {
        final RenderBox box = context.findRenderObject();
        final Offset localOffset = box.globalToLocal(details.globalPosition);
        final RenderBox containerBox = key.currentContext.findRenderObject();
        final Offset containerOffset = containerBox.localToGlobal(localOffset);
        final onTap = containerBox.paintBounds.contains(containerOffset);
        if (onTap){
          print("DO YOUR STUFF...");
        }
      }

      @override
      Widget build(BuildContext context) {
        return GestureDetector(
          onTapDown: (TapDownDetails details) => onTapDown(context, details),
          child: Container(
            color: Colors.red,
            width: MediaQuery.of(context).size.width,
            height: MediaQuery.of(context).size.height,
            child: Align(
              alignment: Alignment.topLeft,
                      child: SizedBox(
                width: 200.0,
                height: 400.0,
                child: Container(
                  color: Colors.black,
                    child: Stack(
                      overflow: Overflow.visible,
                      children: [
                        Positioned(
                          top: 0.0, left: 0.0,
                                          child: Container(
                            key: key,
                            width: 500.0,
                            height: 200.0,
                            color: Colors.blue,
                          ),
                        ),
                      ],
                    ),

                ),
              ),
            ),
          ),
        );
      } 

Answer 3

I had a similar issue. Basically since the stack's children don't use the fully overflown box size for their hit testing, i used a nested stack and an arbitrary big height so that i can capture the clicks of the nested stack's overflown boxes. Not sure if it can work for you but here goes nothing :)

So in your example maybe you could try something like that

Stack(
  overflow: Overflow.visible,
  children: [
    Positioned(
    top: 0.0,
    left: 0.0,
    height : 500.0 // biggest possible child size or just very big 
    child: Stack(
      children: [MyWidget()]
    ),
  )],
);