This callback is called if an associated RectTransform has its dimensions changed. The call is also made to all child rect transforms, even if the child transform itself doesn't change - as it could have, depending on its anchoring.
当自身尺寸发生变化的时候回调，可用于做排版，AspectRatioFitter，运用这个原理，在OnRectTransformDimensionsChange回调中适配recttransform大小。对于父gameobject与子gameobject回调顺序是个迷，运行中是子gameobject先调用，父gameobject后调用。
