RepaintBoundary、RelayoutBoundary

RepaintBoundary

实现方式：直接使用RepaintBoundary小部件

• RepaintBoundary子树repaint的时候，只有RepaintBoundary子树会被repaint，不影响父级和同级的其他子树
• 如果父widget repaint，则RepaintBoundary子树也会repaint
• 如果非RepaintBoundary子树repaint，则也会引起父widget repaint，但是同级的RepaintBoundary不会repaint

如下图所示，

• one或者two repaint的时候只有其自身repaint，不影响bottom和其他同级
• three repaint的时候，bottom也会repaint，但是one和two不会repaint
• bottom repaint的时候，one、two、three都会repaint

IMG_1037.PNG

RelayoutBoundary

实现方式：使用Container或者SizedBox，设定固定宽高，因为Container或者SizedBox底下都是用了RenderConstrainedBox(RenderObject类)，如果设置了固定宽高，则constraints.isTight为true，从而relayoutBoundary = this

renderObject在layout阶段做了Relayout boundary的优化，当子树进行relayout时，满足下面三种中的一种

• parentUsesSize == false
• sizedByParent == true
• constraints.isTight
  那么该renderObject设置为Relayout boundary，也就是该renderObject的重新layout不触发parent的layout，一般情况下开发人员不需要关心Relayout boundary，除非是使用CustomMultiChildLayout

if (!parentUsesSize || sizedByParent || constraints.isTight || parent is! RenderObject) {
      relayoutBoundary = this;
    } else {
      final RenderObject parent = this.parent;
      relayoutBoundary = parent._relayoutBoundary;
    }
.........
if (_relayoutBoundary != null && relayoutBoundary != _relayoutBoundary) {
      // The local relayout boundary has changed, must notify children in case
      // they also need updating. Otherwise, they will be confused about what
      // their actual relayout boundary is later.
      visitChildren((RenderObject child) {
        child._cleanRelayoutBoundary();
      });
    }
    _relayoutBoundary = relayoutBoundary;

void markNeedsLayout() {
....
    if (_relayoutBoundary != this) {
      markParentNeedsLayout();
    }
....
  }

由以上源码可知，RenderObject中，relayoutBoundary不是自己就是parent。如果是自己，当layout的时候就不会要求parent layout；如果不是自己，当layout的时候就会要求parent layout。