getOffsetToReveal method Null safety

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

This is used by RenderViewportBase.showInViewport, which is itself used by RenderObject.showOnScreen for RenderViewportBase, which is in turn used by the semantics system to implement scrolling for accessibility tools.

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. Other viewports in between this viewport and the target will not be adjusted.

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:

• RevealedOffset, which describes the return value of this method.