flutter 渲染管道

首先从一段代码开始（flutter/rendering/binding.dart）:

void drawFrame() {
    assert(renderView != null);
    pipelineOwner.flushLayout();
    pipelineOwner.flushCompositingBits();
    pipelineOwner.flushPaint();
    if (sendFramesToEngine) {
      renderView.compositeFrame();  
      pipelineOwner.flushSemantics();
      _firstFrameSent = true;
    }
  }

这是 flutter 渲染管道的流程，也就是：

graph LR
A[flushLayout] --> B(flushCompositingBits) --> C[flushPaint]

这是我们现在最关心的几个阶段。 单击方法查看对应源码，其实都很相似。

flushLayout

flutter/rendering/Object.dart#PipeLineOwner：

 List<RenderObject> _nodesNeedingLayout = <RenderObject>[];
 	 ... (省略）
	 while (_nodesNeedingLayout.isNotEmpty) {
       final List<RenderObject> dirtyNodes = _nodesNeedingLayout;
       _nodesNeedingLayout = <RenderObject>[];
       for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => a.depth - b.depth)) {
          if (node._needsLayout && node.owner == this)
            node._layoutWithoutResize();
        }
      }
     ...

也就是说要重新布局的RenderObject ，是要先添加到这个 List 中才行，而 markNeedsLaout 正是执行此操作的方法，flutter/rendering/Object.dart#RenderObject.markNeedsLaout（简化）：

...
 if (_relayoutBoundary != this) {
      markParentNeedsLayout();
    } else {
      _needsLayout = true;
      if (owner != null) {
        owner!._nodesNeedingLayout.add(this);
        owner!.requestVisualUpdate();
      }
    }
...

如果_relayoutBoundary 不是对象本身而是parent，那就执行markParentNeedsLayout，最后调用parent.markNeedsLayout()，这是在确定由哪一方决定布局的；

_relayoutBoundary

_relayoutBoundary 是如何确定的呢，在layout方法中，是由几个条件变量确定的； flutter/rendering/Object.dart#RenderObject.layout（简化）：

...
  RenderObject? relayoutBoundary;
  if (!parentUsesSize || sizedByParent || constraints.isTight || parent is! RenderObject) {
    relayoutBoundary = this;
  } else {
    relayoutBoundary = (parent! as RenderObject)._relayoutBoundary;
  }
  if (!_needsLayout && constraints == _constraints && relayoutBoundary == _relayoutBoundary) {
    return;
  }
  _constraints = constraints;
  if (_relayoutBoundary != null && relayoutBoundary != _relayoutBoundary) {
    visitChildren(_cleanChildRelayoutBoundary);
  }
  _relayoutBoundary = relayoutBoundary; // 确定了_relayoutBoundary
  if (sizedByParent) {
    try {
      performResize();
    } catch (e, stack) {
      _debugReportException('performResize', e, stack);
    }
  }
  RenderObject? debugPreviousActiveLayout;
  try {
    performLayout();
    markNeedsSemanticsUpdate();
  } catch (e, stack) {
    _debugReportException('performLayout', e, stack);
  }
  _needsLayout = false;
  markNeedsPaint(); // 布局之后是要重新绘制的
...

布局方面的优化几乎都在这里说明了：

• parentUsesSize

• sizedByParent

• constraints.isTight

// Whether the constraints are the only input to the sizing algorithm (in
// particular, child nodes have no impact).
//
// Returning false is always correct, but returning true can be more
// efficient when computing the size of this render object because we don't
// need to recompute the size if the constraints don't change.
//
// Typically, subclasses will always return the same value. If the value can
// change, then, when it does change, the subclass should make sure to call
// [markNeedsLayoutForSizedByParentChange].
//
// Subclasses that return true must not change the dimensions of this render
// object in [performLayout]. Instead, that work should be done by
// [performResize] or - for subclasses of [RenderBox] - in
// [RenderBox.computeDryLayout].
@protected
bool get sizedByParent => false;

当要重新设计RenderObject时，可以重写sizeByParent。 当表达式返回true时，_relayoutBoundary 为它本身，并且size(RenderBox)是固定的，除非重新布局，这是因为size是要在performResize或computeDryLayout设置，并且不应在performLayout中设置size，这是因为重新布局时，只会调用performLayout。

flushCompositingBits

示例：

context.pushClipRect(needsCompositing, offset, Offset.zero & size,
	defaultPaint, oldLayer: _clipRectLayer);

调用markNeedsCompositingBitsUpdate将renderObject添加到_nodesNeedingCompositingBitsUpdate，如果needsCompositing发生改变时，将会调用markNeedsPaint; 这个步骤是合成的关键，如果我们深入探索的话，就会发现都是在layer上绘画的。

flushPaint

先看markNeedsPaint的源码，flutter/rendering/Object.dart#RenderObject.markNeedsPaint（简化）：

...
if (isRepaintBoundary) {
  if (owner != null) {
    owner!._nodesNeedingPaint.add(this);
    owner!.requestVisualUpdate();
  }
} else if (parent is RenderObject) {
  final RenderObject parent = this.parent! as RenderObject;
  parent.markNeedsPaint();
} else {
  if (owner != null)
    owner!.requestVisualUpdate();
}
...

当调用renderObject的markNeedsPaint时，会检查isRepaintBoundary的值，如果为true，parent不用重新绘制，也就是把本身标记为dirtyNodes，从而在渲染管道中被处理。

再从flutter/rendering/Object.dart#PipelineOwner.flushPaint 跟踪代码到

child._paintWithContext(childContext, Offset.zero);

_paintWithContext内部调用在RenderObject中重写的paint方法，这里就要说一下PaintingContext，在绘制child时是要调用context.paintChild的;

flutter/rendering/Object.dart#PaintingContext.paintChild：

...
void paintChild(RenderObject child, Offset offset) {
  if (child.isRepaintBoundary) {
  stopRecordingIfNeeded();
  _compositeChild(child, offset);
  } else {
  child._paintWithContext(this, offset);
  }
  ...
}
...
void _compositeChild(RenderObject child, Offset offset) {
  if (child._needsPaint) {
    repaintCompositedChild(child, debugAlsoPaintedParent: true);
  } else {
  ...
  }
}
...

现在如果我们把它们联系起来，就会发现当parent需要重新绘制时，但是child.isRepaintBoundary为true，此时只有child._needsPaint为true时，child才会重新绘制，这也就是为什么其他Widget有时要用RepaintBoundary包裹起来的原因。