Flutter学习之构建、布局及绘制三部曲

void runApp(Widget app) {
 WidgetsFlutterBinding.ensureInitialized()//此方法是对flutter的框架做一些必要的初始化
 ..attachRootWidget(app)
 ..scheduleWarmUpFrame();
}

class WidgetsFlutterBinding extends BindingBase with GestureBinding, ServicesBinding, SchedulerBinding, PaintingBinding, SemanticsBinding, RendererBinding, WidgetsBinding {
	 static WidgetsBinding ensureInitialized() {
	 if (WidgetsBinding.instance == null)
	 WidgetsFlutterBinding();
	 return WidgetsBinding.instance;
	 }
}

void attachRootWidget(Widget rootWidget) {
 _renderViewElement = RenderObjectToWidgetAdapter<RenderBox>(
 container: renderView,
 debugShortDescription: '[root]',
 child: rootWidget
 ).attachToRenderTree(buildOwner, renderViewElement);
}

RenderObjectToWidgetElement<T> attachToRenderTree(BuildOwner owner, [RenderObjectToWidgetElement<T> element]) {
 if (element == null) {
 owner.lockState(() {
 element = createElement();
 assert(element != null);
 element.assignOwner(owner);
 });
 owner.buildScope(element, () {
 element.mount(null, null);
 });
 } else {
 element._newWidget = this;
 element.markNeedsBuild();
 }
 return element;
}

void buildScope(Element context, [VoidCallback callback]) {
 if (callback == null && _dirtyElements.isEmpty)
 return;
 Timeline.startSync('Build', arguments: timelineWhitelistArguments);
 try {
 _scheduledFlushDirtyElements = true;
 if (callback != null) {
 _dirtyElementsNeedsResorting = false;
 try {
 callback();
 } finally {}
 }
 ...
 }

@override
void mount(Element parent, dynamic newSlot) {
 assert(parent == null);
 super.mount(parent, newSlot);
 _rebuild();
}

@override
void mount(Element parent, dynamic newSlot) {
 super.mount(parent, newSlot);
 _renderObject = widget.createRenderObject(this);
 attachRenderObject(newSlot);
 _dirty = false;
}

@mustCallSuper
void mount(Element parent, dynamic newSlot) {
 _parent = parent;
 _slot = newSlot;
 _depth = _parent != null ? _parent.depth + 1 : 1;
 _active = true;
 if (parent != null) // Only assign ownership if the parent is non-null
 _owner = parent.owner;
 if (widget.key is GlobalKey) {
 final GlobalKey key = widget.key;
 key._register(this);
 }
 _updateInheritance();
}

@override
void attachRenderObject(dynamic newSlot) {
 assert(_ancestorRenderObjectElement == null);
 _slot = newSlot;
 _ancestorRenderObjectElement = _findAncestorRenderObjectElement();//获取此RenderObjectElement最近的RenderObjectElement对象
 _ancestorRenderObjectElement?.insertChildRenderObject(renderObject, newSlot);//将renderObject插入RenderObjectElement中
 final ParentDataElement<RenderObjectWidget> parentDataElement = _findAncestorParentDataElement();
 if (parentDataElement != null)
 _updateParentData(parentDataElement.widget);
}

///RenderObjectToWidgetElement中的insertChildRenderObject方法，简单将子RenderObject赋值给父RenderObject的child字段
@override
void insertChildRenderObject(RenderObject child, dynamic slot) {
 assert(slot == _rootChildSlot);
 assert(renderObject.debugValidateChild(child));
 renderObject.child = child;
}

@protected
Element updateChild(Element child, Widget newWidget, dynamic newSlot) {
 if (newWidget == null) {//当子Widget没有的时候，直接将child deactivate掉
 if (child != null)
 deactivateChild(child);
 return null;
 }
 if (child != null) {//有子Element的时候
 if (child.widget == newWidget) {//Widget没有改变
 if (child.slot != newSlot)//再判断slot有没有改变，没有则不更新slot
 updateSlotForChild(child, newSlot);//更新child的slot
 return child;//返回child
 }
 if (Widget.canUpdate(child.widget, newWidget)) {//Widget没有改变，再判断Widget能否update，如果能还是重复上面的步骤
 if (child.slot != newSlot)
 updateSlotForChild(child, newSlot);
 child.update(newWidget);
 return child;
 }
 deactivateChild(child);//如果不能更新的话，直接将child deactivate掉，然后在inflateWidget(newWidget, newSlot)创建新的Element
 }
 return inflateWidget(newWidget, newSlot);//根据Widget对象以及slot创建新的Element
}

@protected
Element inflatinflateWidgeteWidget(Widget newWidget, dynamic newSlot) {
 final Key key = newWidget.key;
 if (key is GlobalKey) {//newWidget的key是GlobalKey
 final Element newChild = _retakeInactiveElement(key, newWidget);//复用Inactive状态的Element
 if (newChild != null) {
 newChild._activateWithParent(this, newSlot);//activate 此Element（将newChild出入到Element树）
 final Element updatedChild = updateChild(newChild, newWidget, newSlot);//直接将newChild更新
 return updatedChild;//返回更新后的Element
 }
 }
 final Element newChild = newWidget.createElement();//调用createElement()进行创建
 newChild.mount(this, newSlot);//继续调用newChild Element的mount方法（如此就行一直递归下去，当递归完成，整个构建过程也就结束了）
 return newChild;//返回子Element
}

@override
void mount(Element parent, dynamic newSlot) {
 super.mount(parent, newSlot);
 _firstBuild();
}

void _firstBuild() {
 rebuild();//调用了Element的rebuild()方法
}

//Element的rebuild方法，通常被三处地方调用
//1.当BuildOwner.scheduleBuildFor被调用标记此Element为dirty时
//2.当Element第一次构建由mount方法去调用
//3.当Widget改变时，被update方法调用
void rebuild() {
 if (!_active || !_dirty)
 return;
 performRebuild();//调用performRebuild方法（抽象方法）
}

//ComponentElement的performRebuild实现
@override
void performRebuild() {
 Widget built;
 try {
 built = build();//构建Widget（StatelessElement直接调用build方法，StatefulElement直接调用state.build方法）
 } catch (e, stack) {
 built = ErrorWidget.builder(_debugReportException('building $this', e, stack));//有错误的化就创建一个ErrorWidget
 } finally {
 _dirty = false;
 }
 try {
 _child = updateChild(_child, built, slot);//让后还是根据Wdiget来更新子Element
 } catch (e, stack) {
 built = ErrorWidget.builder(_debugReportException('building $this', e, stack));
 _child = updateChild(null, built, slot);
 }
}

@override
void mount(Element parent, dynamic newSlot) {
 super.mount(parent, newSlot);
 _children = List<Element>(widget.children.length);
 Element previousChild;
 for (int i = 0; i < _children.length; i += 1) {
 final Element newChild = inflateWidget(widget.children[i], previousChild);//遍历children直接inflate根据Widget创建新的Element
 _children[i] = newChild;
 previousChild = newChild;
 }
}

void buildScope(Element context, [VoidCallback callback]) {
 if (callback == null && _dirtyElements.isEmpty)
 return;
 Timeline.startSync('Build', arguments: timelineWhitelistArguments);
 try {
 _scheduledFlushDirtyElements = true;
 if (callback != null) {
 Element debugPreviousBuildTarget;
 _dirtyElementsNeedsResorting = false;
 try {
 callback();//调用callback
 } finally {}
 }
 _dirtyElements.sort(Element._sort);
 _dirtyElementsNeedsResorting = false;
 int dirtyCount = _dirtyElements.length;
 int index = 0;
 while (index < dirtyCount) {
 try {
 _dirtyElements[index].rebuild();//遍历dirtyElements并执行他们的rebuild方法来使这些Element进行rebuild
 } catch (e, stack) {}
 index += 1;
 if (dirtyCount < _dirtyElements.length || _dirtyElementsNeedsResorting) {
 _dirtyElements.sort(Element._sort);
 _dirtyElementsNeedsResorting = false;
 dirtyCount = _dirtyElements.length;
 while (index > 0 && _dirtyElements[index - 1].dirty) {
 index -= 1;
 }
 }
 }
 } finally {
 for (Element element in _dirtyElements) {//最后解除Element的dirty标记，以及清空dirtyElements
 assert(element._inDirtyList);
 element._inDirtyList = false;
 }
 _dirtyElements.clear();
 _scheduledFlushDirtyElements = false;
 _dirtyElementsNeedsResorting = null;
 Timeline.finishSync();
 }
}

@protected
void drawFrame() {
 assert(renderView != null);
 pipelineOwner.flushLayout();//布局
 pipelineOwner.flushCompositingBits();//刷新dirty的renderobject的数据
 pipelineOwner.flushPaint();//绘制
 renderView.compositeFrame(); // 将二进制数据发送给GPU
 pipelineOwner.flushSemantics(); // 将语义发送给系统
}

@override
void performLayout() {
 _resolve();//解析padding参数
 if (child == null) {//如果没有child，直接将constraints与padding综合计算得出自己的size
 size = constraints.constrain(Size(
  _resolvedPadding.left + _resolvedPadding.right,
  _resolvedPadding.top + _resolvedPadding.bottom
 ));
 return;
 }
 final BoxConstraints innerConstraints = constraints.deflate(_resolvedPadding);//将padding减去，生成新的约束innerConstraints
 child.layout(innerConstraints, parentUsesSize: true);//用新的约束去布局child
 final BoxParentData childParentData = child.parentData;
 childParentData.offset = Offset(_resolvedPadding.left, _resolvedPadding.top);//设置childParentData的offset值（这个值是相对于parent的绘制偏移值，在paint的时候传入这个偏移值）
 size = constraints.constrain(Size(//将constraints与padding以及child的sieze综合计算得出自己的size
 _resolvedPadding.left + child.size.width + _resolvedPadding.right,
 _resolvedPadding.top + child.size.height + _resolvedPadding.bottom
 ));
}

@override
void performLayout() {
 size = _getSize(constraints);//直接先确定自己的size
 int i = 0;
 _randomAccessChildren.clear();
 RenderBox child = firstChild;
 while (child != null) {//遍历孩子
 _randomAccessChildren.add(child);
 final BoxConstraints innerConstraints = _delegate.getConstraintsForChild(i, constraints);//获取child的约束，此方法为抽象
 child.layout(innerConstraints, parentUsesSize: true);//布局孩子
 final FlowParentData childParentData = child.parentData;
 childParentData.offset = Offset.zero;
 child = childParentData.nextSibling;
 i += 1;
 }
}

@override
void performLayout() {
 size = _sizeForConstraints(constraints);
}

void flushPaint() {
 try {
 final List<RenderObject> dirtyNodes = _nodesNeedingPaint;
 _nodesNeedingPaint = <RenderObject>[];
 // 对dirty nodes列表进行排序，最深的在第一位
 for (RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => b.depth - a.depth)) {
  assert(node._layer != null);
  if (node._needsPaint && node.owner == this) {
  if (node._layer.attached) {
   PaintingContext.repaintCompositedChild(node);
  } else {
   node._skippedPaintingOnLayer();
  }
  }
 }
 } finally {}
}

@override
void paint(PaintingContext context, Offset offset) {
 if (child != null)
 context.paintChild(child, offset);//直接绘制child
}

@override
void paint(PaintingContext context, Offset offset) {
 if (child != null) {
 final BoxParentData childParentData = child.parentData;
 context.paintChild(child, childParentData.offset + offset);//直接绘制child
 }
}

@override
void paint(PaintingContext context, Offset offset) {//Offset由parent去paintChild的时候传入，该值存放在child的parentdata字段中，该字段是BoxParentData或以下实例
 _painter ??= _decoration.createBoxPainter(markNeedsPaint);//获取painter画笔
 final ImageConfiguration filledConfiguration = configuration.copyWith(size: size);
 if (position == DecorationPosition.background) {//画背景
 _painter.paint(context.canvas, offset, filledConfiguration);//绘制过程，具体细节再painter中
 if (decoration.isComplex)
  context.setIsComplexHint();
 }
 super.paint(context, offset);//画child，里面直接调用了paintChild
 if (position == DecorationPosition.foreground) {//画前景
 _painter.paint(context.canvas, offset, filledConfiguration);
 if (decoration.isComplex)
  context.setIsComplexHint();
 }
}

@override
void paint(PaintingContext context, Offset offset) {
 if (_image == null)
 return;
 _resolve();
 paintImage(//直接绘制Image，具体细节再此方法中
 canvas: context.canvas,
 rect: offset & size,
 image: _image,
 scale: _scale,
 colorFilter: _colorFilter,
 fit: _fit,
 alignment: _resolvedAlignment,
 centerSlice: _centerSlice,
 repeat: _repeat,
 flipHorizontally: _flipHorizontally,
 invertColors: invertColors,
 filterQuality: _filterQuality
 );
}