Flutter setState为什么不会造成性能困扰？

setState为什么不会造成性能困扰？
因为只更新当前图层树。

flutter是怎么做element diff，做到只布局、绘制、合成差异部分视图的？
见以下代码

核心代码

Flutter版本

Flutter: 1.12.13+hotfix.8

核心类

abstract class RenderObject

widget层调用

setState();

核心数据结构

链表

遍历链表寻找relayout boundary，重新布局

void markNeedsLayout() {
    assert(_debugCanPerformMutations);
    if (_needsLayout) {
      assert(_debugSubtreeRelayoutRootAlreadyMarkedNeedsLayout());
      return;
    }
    assert(_relayoutBoundary != null);
    if (_relayoutBoundary != this) {
      markParentNeedsLayout();
    } else {
      _needsLayout = true;
      if (owner != null) {
        assert(() {
          if (debugPrintMarkNeedsLayoutStacks)
            debugPrintStack(label: 'markNeedsLayout() called for $this');
          return true;
        }());
        owner._nodesNeedingLayout.add(this);
        owner.requestVisualUpdate();
      }
    }
  }

遍历链表检查renderObject是否是重绘边界（isRepaintBoundary = true）时，决定合成范围

void markNeedsCompositingBitsUpdate() {
    if (_needsCompositingBitsUpdate)
      return;
    _needsCompositingBitsUpdate = true;
    if (parent is RenderObject) {
      final RenderObject parent = this.parent;
      if (parent._needsCompositingBitsUpdate)
        return;
      if (!isRepaintBoundary && !parent.isRepaintBoundary) {
        parent.markNeedsCompositingBitsUpdate();
        return;
      }
    }
    assert(() {
      final AbstractNode parent = this.parent;
      if (parent is RenderObject)
        return parent._needsCompositing;
      return true;
    }());
    // parent is fine (or there isn't one), but we are dirty
    if (owner != null)
      owner._nodesNeedingCompositingBitsUpdate.add(this);
  }

遍历链表检查renderObject是否是重绘边界（isRepaintBoundary = true），决定绘制范围，或 没有标记isRepaintBoundary的RenderObject时全部绘制

void markNeedsPaint() {
    assert(owner == null || !owner.debugDoingPaint);
    if (_needsPaint)
      return;
    _needsPaint = true;
    if (isRepaintBoundary) {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this');
        return true;
      }());
      // If we always have our own layer, then we can just repaint
      // ourselves without involving any other nodes.
      assert(_layer is OffsetLayer);
      if (owner != null) {
        owner._nodesNeedingPaint.add(this);
        owner.requestVisualUpdate();
      }
    } else if (parent is RenderObject) {
      final RenderObject parent = this.parent;
      parent.markNeedsPaint();
      assert(parent == this.parent);
    } else {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this (root of render tree)');
        return true;
      }());
      // If we're the root of the render tree (probably a RenderView),
      // then we have to paint ourselves, since nobody else can paint
      // us. We don't add ourselves to _nodesNeedingPaint in this
      // case, because the root is always told to paint regardless.
      if (owner != null)
        owner.requestVisualUpdate();
    }
  }

结语：

1.setState的节点越远离根部，布局、合成、绘制的开销越少
2.重写isRepaintBoundary可变化绘制范围，可影响开销；如使用RepaintBoundary部件
3.绝对宽高可减少开销（constraints.isTight）

RenderObject relayoutBoundary;
    if (!parentUsesSize || sizedByParent || constraints.isTight || parent is! RenderObject) {
      relayoutBoundary = this;
    } else {
      final RenderObject parent = this.parent;
      relayoutBoundary = parent._relayoutBoundary;
    }