flutter RenderObject中relayoutBou

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。