Flutter: possible to detect when a drawer is open?

Question

Is it possible to detect when a Drawer is open so that we can run some routine to update its content?

A typical use case I have would be to display the number of fol

Answer 1

Unfortunately, at the moment there is no readymade solution.

You can use the dirty hack for this: to observe the visible position of the Drawer.

For example, I used this approach to synchronise the animation of the icon on the button and the location of the Drawer box.

The code that solves this problem you can see below:

    import 'package:flutter/material.dart';
    import 'package:flutter/scheduler.dart';

    class DrawerListener extends StatefulWidget {
      final Widget child;
      final ValueChanged onPositionChange;

      DrawerListener({
        @required this.child,
        this.onPositionChange,
      });

      @override
      _DrawerListenerState createState() => _DrawerListenerState();
    }

    class _DrawerListenerState extends State {
      GlobalKey _drawerKey = GlobalKey();
      int taskID;
      Offset currentOffset;

      @override
      void initState() {
        super.initState();
        _postTask();
      }

      _postTask() {
        taskID = SchedulerBinding.instance.scheduleFrameCallback((_) {
          if (widget.onPositionChange != null) {
            final RenderBox box = _drawerKey.currentContext?.findRenderObject();
            if (box != null) {
              Offset newOffset = box.globalToLocal(Offset.zero);
              if (newOffset != currentOffset) {
                currentOffset = newOffset;
                widget.onPositionChange(
                  FractionalOffset.fromOffsetAndRect(
                    currentOffset,
                    Rect.fromLTRB(0, 0, box.size.width, box.size.height),
                  ),
                );
              }
            }
          }

          _postTask();
        });
      }

      @override
      void dispose() {
        SchedulerBinding.instance.cancelFrameCallbackWithId(taskID);
        if (widget.onPositionChange != null) {
          widget.onPositionChange(FractionalOffset(1.0, 0));
        }
        super.dispose();
      }

      @override
      Widget build(BuildContext context) {
        return Container(
          key: _drawerKey,
          child: widget.child,
        );
      }
    }

If you are only interested in the final events of opening or closing the box, it is enough to call the callbacks in initState and dispose functions.