MimicableHandle startMimic()

Start the mimicking process.

The child of this object will no longer be built at this location in the tree. Instead, this widget will build a transparent placeholder with the same dimensions as the widget had when the mimicking process started.

If you use startMimic, it is your responsibility to do something with the returned MimicableHandle; typically, passing it to a Mimic widget. To mimic the child in the Overlay, consider using liftToOverlay() instead.

To end the mimic process, call MimicableHandle.stopMimic on the returned object.

Source 

MimicableHandle startMimic() {
  assert(() {
    if (_placeholderSize != null) {
      throw new FlutterError(
        'Mimicable started while already active.\n'
        'When startMimic() or liftToOverlay() is called on a MimicableState, the mimic becomes active. '
        'While active, it cannot be reactivated until it is stopped. '
        'To stop a Mimicable started with startMimic(), call the MimicableHandle object\'s stopMimic() method. '
        'To stop a Mimicable started with liftToOverlay(), call dispose() on the MimicOverlayEntry.'
      );
    }
    return true;
  });
  setState(() {
    _placeholderSize = context.size;
  });
  return new MimicableHandle._(this);
}