getOffsetToReveal method

  1. @override
RevealedOffset getOffsetToReveal (RenderObject target, double alignment, { Rect rect })
override

Returns the offset that would be needed to reveal the target RenderObject.

The optional rect parameter describes which area of that target object should be revealed in the viewport. If rect is null, the entire target RenderObject (as defined by its RenderObject.paintBounds) will be revealed. If rect is provided it has to be given in the coordinate system of the target object.

The alignment argument describes where the target should be positioned after applying the returned offset. If alignment is 0.0, the child must be positioned as close to the leading edge of the viewport as possible. If alignment is 1.0, the child must be positioned as close to the trailing edge of the viewport as possible. If alignment is 0.5, the child must be positioned as close to the center of the viewport as possible.

The target might not be a direct child of this viewport but it must be a descendant of the viewport and there must not be any other RenderAbstractViewport objects between the target and this object.

This method assumes that the content of the viewport moves linearly, i.e. when the offset of the viewport is changed by x then target also moves by x within the viewport.

See also:

Implementation 

@override
RevealedOffset getOffsetToReveal(RenderObject target, double alignment, {Rect rect}) {
  // `target` is only fully revealed when in the selected/center position. Therefore,
  // this method always returns the offset that shows `target` in the center position,
  // which is the same offset for all `alignment` values.

  rect ??= target.paintBounds;

  // `child` will be the last RenderObject before the viewport when walking up from `target`.
  RenderObject child = target;
  while (child.parent != this)
    child = child.parent;

  final ListWheelParentData parentData = child.parentData;
  final double targetOffset = parentData.offset.dy; // the so-called "centerPosition"

  final Matrix4 transform = target.getTransformTo(this);
  final Rect bounds = MatrixUtils.transformRect(transform, rect);
  final Rect targetRect = bounds.translate(0.0, (size.height - itemExtent) / 2);

  return RevealedOffset(offset: targetOffset, rect: targetRect);
}