Flutter开发进阶之瞧瞧RenderObject

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Flutter开发进阶之瞧瞧RenderObject

通过上回我们了解到Flutter执行buildTree的逻辑线，当Tree构建完成后会交给Flutter底层的渲染事件循环去执行将内容渲染到屏幕的操作。
但是渲染的操作到底是如何串起来的呢？这篇文章将会从Element联系到RenderObject去瞧瞧逻辑线形成闭环。

在Element的源码中有一个方法，如下。

/*
### RebuildingDirty elements are rebuilt during the next frame. Precisely how this isdone depends on the kind of element. A [StatelessElement] rebuilds byusing its widget's [StatelessWidget.build] method. A [StatefulElement]rebuilds by using its widget's state's [State.build] method. A[RenderObjectElement] rebuilds by updating its [RenderObject].In many cases, the end result of rebuilding is a single child widgetor (for [MultiChildRenderObjectElement]s) a list of children widgets.These child widgets are used to update the [widget] property of theelement's child (or children) elements. The new [Widget] is considered tocorrespond to an existing [Element] if it has the same [Type] and [Key].(In the case of [MultiChildRenderObjectElement]s, some effort is put intotracking widgets even when they change order; see[RenderObjectElement.updateChildren].)
*/
@pragma('vm:prefer-inline')
void rebuild({bool force = false})

我们通过注释可知，Element在构建好Tree后，Dirty elements在下一帧重建，无论是多子Element还是单子Element都是通过RenderObjectElement更新它的RenderObject完成。
让我们来看看RenderObjectElement的mount方法，以下。

@overridevoid mount(Element? parent, Object? newSlot) {super.mount(parent, newSlot);assert(() {_debugDoingBuild = true;return true;}());_renderObject = (widget as RenderObjectWidget).createRenderObject(this);assert(!_renderObject!.debugDisposed!);assert(() {_debugDoingBuild = false;return true;}());assert(() {_debugUpdateRenderObjectOwner();return true;}());assert(_slot == newSlot);attachRenderObject(newSlot);super.performRebuild(); // clears the "dirty" flag}
@overridevoid attachRenderObject(Object? newSlot) {assert(_ancestorRenderObjectElement == null);_slot = newSlot;_ancestorRenderObjectElement = _findAncestorRenderObjectElement();_ancestorRenderObjectElement?.insertRenderObjectChild(renderObject, newSlot);final ParentDataElement<ParentData>? parentDataElement = _findAncestorParentDataElement();if (parentDataElement != null) {_updateParentData(parentDataElement.widget as ParentDataWidget<ParentData>);}}

在RenderObjectElement装载RenderObject的过程中，会创建一个RenderObject与其对应，然后将RenderObject插入对应的卡槽（Slot），并且还需保证子RenderObject遵循上级Widget的相关配置。
根据事件线的逻辑，接下来将看到RenderObject的逻辑，源码代码太长就不一一贴了。

abstract class RenderObjectwith DiagnosticableTreeMixinimplements HitTestTarget

RenderObject不仅负责Layout、Paint还需要负责hit，这里我们只考虑Layout和Paint。
让我们看看markNeedsLayout，以下。

void markNeedsLayout() {assert(_debugCanPerformMutations);if (_needsLayout) {assert(_debugSubtreeRelayoutRootAlreadyMarkedNeedsLayout());return;}if (_relayoutBoundary == null) {_needsLayout = true;if (parent != null) {// _relayoutBoundary is cleaned by an ancestor in RenderObject.layout.// Conservatively mark everything dirty until it reaches the closest// known relayout boundary.markParentNeedsLayout();}return;}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();}}}

可以看到渲染的逻辑线是推迟到parent的，当合成的Layout需要执行渲染时，会交给owner并添加进需要Layout的集合里，然后通过owner!.requestVisualUpdate();去通知渲染管线进行渲染。
让我们看看源码中怎么说，以下。

/// The owner for this render object (null if unattached).////// The entire render tree that this render object belongs to/// will have the same owner.PipelineOwner? get owner => _owner;PipelineOwner? _owner;

可知在同一个Tree以下都对应同一个PipelineOwner，这个owner负责管理具体的渲染工作，相当于前文说到的Element与BuildOwner的关系。
接下来来我们看看markNeedsPaint方法，以下。

void markNeedsPaint() {assert(!_debugDisposed);assert(owner == null || !owner!.debugDoingPaint);if (_needsPaint) {return;}_needsPaint = true;// If this was not previously a repaint boundary it will not have// a layer we can paint from.if (isRepaintBoundary && _wasRepaintBoundary) {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(_layerHandle.layer is OffsetLayer);if (owner != null) {owner!._nodesNeedingPaint.add(this);owner!.requestVisualUpdate();}} else if (parent is RenderObject) {parent!.markNeedsPaint();} else {assert(() {if (debugPrintMarkNeedsPaintStacks) {debugPrintStack(label: 'markNeedsPaint() called for $this (root of render tree)');}return true;}());// If we are the root of the render tree and not a repaint boundary// 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.//// Trees rooted at a RenderView do not go through this// code path because RenderViews are repaint boundaries.if (owner != null) {owner!.requestVisualUpdate();}}}

在markNeedsPaint中RepaintBoundary会将渲染子Tree限制在自己的范围内，与markNeedsLayout类似，而且也会通过owner去执行渲染管线的管理。
让我们看看PipelineOwner是如何执行的？

/// Calls [onNeedVisualUpdate] if [onNeedVisualUpdate] is not null.////// Used to notify the pipeline owner that an associated render object wishes/// to update its visual appearance.void requestVisualUpdate() {if (onNeedVisualUpdate != null) {onNeedVisualUpdate!();} else {_manifold?.requestVisualUpdate();}}

在Flutter中，PipelineOwner负责渲染管线的各个方面，是具体与WidgetsBinding中RendererBinding帧事件循环的交互。
让我们看看onNeedVisualUpdate的注释，以下。

/// Called when a render object associated with this pipeline owner wishes to/// update its visual appearance.////// Typical implementations of this function will schedule a task to flush the/// various stages of the pipeline. This function might be called multiple/// times in quick succession. Implementations should take care to discard/// duplicate calls quickly.////// When the [PipelineOwner] is attached to a [PipelineManifold] and/// [onNeedVisualUpdate] is provided, the [onNeedVisualUpdate] callback is/// invoked instead of calling [PipelineManifold.requestVisualUpdate].final VoidCallback? onNeedVisualUpdate;

可知作为一个调度任务可能被多次重复调用，当PipelineOwner连接到PipelineManifold并提供onNeedVisualUpdate时，会直接执行onNeedVisualUpdate。
继续查看PipelineManifold，以下。

abstract class PipelineManifold implements Listenable {/// Whether [PipelineOwner]s connected to this [PipelineManifold] should/// collect semantics information and produce a semantics tree.////// The [PipelineManifold] notifies its listeners (managed with [addListener]/// and [removeListener]) when this property changes its value.////// See also://////  * [SemanticsBinding.semanticsEnabled], which [PipelineManifold]///    implementations typically use to back this property.bool get semanticsEnabled;/// Called by a [PipelineOwner] connected to this [PipelineManifold] when a/// [RenderObject] associated with that pipeline owner wishes to update its/// visual appearance.////// Typical implementations of this function will schedule a task to flush the/// various stages of the pipeline. This function might be called multiple/// times in quick succession. Implementations should take care to discard/// duplicate calls quickly.////// A [PipelineOwner] connected to this [PipelineManifold] will call/// [PipelineOwner.onNeedVisualUpdate] instead of this method if it has been/// configured with a non-null [PipelineOwner.onNeedVisualUpdate] callback.////// See also://////  * [SchedulerBinding.ensureVisualUpdate], which [PipelineManifold]///    implementations typically call to implement this method.void requestVisualUpdate();
}

在Flutter中，PipelineManifold通常不会暴露在外，它管理PipelineOwner Tree下所有PipelineOwner，它实现了PipelineOwner访问共享，PipelineManifold通过调用SchedulerBinding.ensureVisualUpdate实现通知渲染执行。
让我们继续进行下一步的探索，以下。

/// Schedules a new frame using [scheduleFrame] if this object is not/// currently producing a frame.////// Calling this method ensures that [handleDrawFrame] will eventually be/// called, unless it's already in progress.////// This has no effect if [schedulerPhase] is/// [SchedulerPhase.transientCallbacks] or [SchedulerPhase.midFrameMicrotasks]/// (because a frame is already being prepared in that case), or/// [SchedulerPhase.persistentCallbacks] (because a frame is actively being/// rendered in that case). It will schedule a frame if the [schedulerPhase]/// is [SchedulerPhase.idle] (in between frames) or/// [SchedulerPhase.postFrameCallbacks] (after a frame).void ensureVisualUpdate() {switch (schedulerPhase) {case SchedulerPhase.idle:case SchedulerPhase.postFrameCallbacks:scheduleFrame();return;case SchedulerPhase.transientCallbacks:case SchedulerPhase.midFrameMicrotasks:case SchedulerPhase.persistentCallbacks:return;}}

继续沿SchedulerBinding的执行路径，以下。

void scheduleFrame() {if (_hasScheduledFrame || !framesEnabled) {return;}assert(() {if (debugPrintScheduleFrameStacks) {debugPrintStack(label: 'scheduleFrame() called. Current phase is $schedulerPhase.');}return true;}());ensureFrameCallbacksRegistered();platformDispatcher.scheduleFrame();_hasScheduledFrame = true;}

最终void scheduleFrame()是具体下一帧绘制的方法，由此完成闭环。
综上所述，我们了解到，Flutter具体执行渲染时会构建RenderObject Tree（通过将RenderObject插入Slot），具体操作交给PipelineOwner管理，而同一Tree下的PipelineOwner对接唯一的PipelineManifold，最后通知SchedulerBinding去执行帧绘制的操作。

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