主要是通过底层源码来分享一下Flutter是怎么渲染的。

1、Widget树

树是一种数据结构，Widget树就是存储Widget的树结构。Widget的功能是“描述一个UI元素的配置数据”，它就是说，Widget其实并不是表示最终绘制在设备屏幕上的显示元素，而它只是描述显示元素的一个配置数据。下面就是Flutter官方Demo的Widget树展示：

image.png

image.png

2、Rander树

上面说到了Flutter引擎是直接渲染Rander树的，Rander树里面存的是RanderObject。但不是所有的Widget都会变成独立RanderObject，只有继承RenderObjectWidget的Widget才会生成独立RanderObject并且加到Rander树里面，进行独立渲染。RenderObjectWidget的定义如下：

/// RenderObjectWidgets provide the configuration for [RenderObjectElement]s,
/// which wrap [RenderObject]s, which provide the actual rendering of the
/// application.
abstract class RenderObjectWidget extends Widget {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const RenderObjectWidget({ Key? key }) : super(key: key);

  /// RenderObjectWidgets always inflate to a [RenderObjectElement] subclass.
  @override
  @factory
  RenderObjectElement createElement();

  /// Creates an instance of the [RenderObject] class that this
  /// [RenderObjectWidget] represents, using the configuration described by this
  /// [RenderObjectWidget].
  ///
  /// This method should not do anything with the children of the render object.
  /// That should instead be handled by the method that overrides
  /// [RenderObjectElement.mount] in the object rendered by this object's
  /// [createElement] method. See, for example,
  /// [SingleChildRenderObjectElement.mount].
  @protected
  @factory
  RenderObject createRenderObject(BuildContext context);

  /// Copies the configuration described by this [RenderObjectWidget] to the
  /// given [RenderObject], which will be of the same type as returned by this
  /// object's [createRenderObject].
  ///
  /// This method should not do anything to update the children of the render
  /// object. That should instead be handled by the method that overrides
  /// [RenderObjectElement.update] in the object rendered by this object's
  /// [createElement] method. See, for example,
  /// [SingleChildRenderObjectElement.update].
  @protected
  void updateRenderObject(BuildContext context, covariant RenderObject renderObject) { }

  /// A render object previously associated with this widget has been removed
  /// from the tree. The given [RenderObject] will be of the same type as
  /// returned by this object's [createRenderObject].
  @protected
  void didUnmountRenderObject(covariant RenderObject renderObject) { }
}

/// A superclass for RenderObjectWidgets that configure RenderObject subclasses
/// that have no children.
abstract class LeafRenderObjectWidget extends RenderObjectWidget {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const LeafRenderObjectWidget({ Key? key }) : super(key: key);

  @override
  LeafRenderObjectElement createElement() => LeafRenderObjectElement(this);
}

RenderObjectWidget是Widget的子类，这里面我们需要关注两个方法：RenderObjectElement createElement()和RenderObject createRenderObject(BuildContext context)，这两个都是抽象方法，需要子类去实现的。

要看createRenderObject，首先找到哪些Widget是继承RenderObjectWidget，比较常见的有Flow、Flex、Column、Row等。Row来说看一下其继承关系：

class Row extends Flex{...}
class Flex extends MultiChildRenderObjectWidget {...}
abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {...}

Row是继承Flex的，在Row定义里面没有看到createRenderObject的实现，那去Flex里面去找：

image.png

可以找到，在Flex里面把这个方法实现了，所以继承Flex的Widget都实现了这个方法。返回一个RenderFlex对象，看一下它的继承关系：

class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, FlexParentData>,
                                        RenderBoxContainerDefaultsMixin<RenderBox, FlexParentData>,
                                        DebugOverflowIndicatorMixin{...}
abstract class RenderBox extends RenderObject {...}

3、Elment树

Element就是Widget在UI树具体位置的一个实例化对象，每生成一个Widget都会对应生成一个Element，看看是不是这样的。
先看Widget的定义：

abstract class Widget extends DiagnosticableTree {
...

/// Inflates this configuration to a concrete instance.
  ///
  /// A given widget can be included in the tree zero or more times. In particular
  /// a given widget can be placed in the tree multiple times. Each time a widget
  /// is placed in the tree, it is inflated into an [Element], which means a
  /// widget that is incorporated into the tree multiple times will be inflated
  /// multiple times.
  @protected
  @factory
  Element createElement();
  
  ...

}

Widget里面定义了一个Element createElement()方法，来看看子类都是怎么实现的。

• 首先是上面提到的RenderObjectWidget的一个子类MultiChildRenderObjectWidget：

 @override
  MultiChildRenderObjectElement createElement() => MultiChildRenderObjectElement(this);

它返回的是MultiChildRenderObjectElement这个类型：

class MultiChildRenderObjectElement extends RenderObjectElement {...}
abstract class RenderObjectElement extends Element {...}

从继承关系来看，它也是继承Element的。

• 然后再看看StatelessWidget，它是继承Widget的：

@override
StatelessElement createElement() => StatelessElement(this);

它返回一个StatelessElement类型：

class StatelessElement extends ComponentElement {...}
abstract class ComponentElement extends Element {...}

从继承关系来看，它也是继承Element的。

• 最后看看StatefulWidget，它也是是继承Widget的：

  @override
  StatefulElement createElement() => StatefulElement(this);

它返回一个StatefulElement类型：

class StatefulElement extends ComponentElement {...}
abstract class ComponentElement extends Element {...}

从继承关系来看，它也是继承Element的。

从上面的分析来看，确实每一个Widget都实现了createElement()，那是什么时候调的呢？它的调用是Flutter隐试调的，肯定会调的，可以在debug模式打个断点看看。
所以说每一个Widget都会有一个Element与之对应，最后会根据Widget树创建一个Element树。

• 总结一下，我们可以认为Flutter的UI系统包含三棵树：Widget树、Element树、渲染树。他们的依赖关系是：Element树根据Widget树生成，而渲染树又依赖于Element树，如图所示：

  
  
  image.png

4 、mount函数

Widget创建的时候就会创建Element对象然后加入到Element树里面，在第一次加入Element树的时候，就会触发Flutter引擎调一个Element方法——mount（），看一下这个方法的定义：

...
  /// Add this element to the tree in the given slot of the given parent.
  ///
  /// The framework calls this function when a newly created element is added to
  /// the tree for the first time. Use this method to initialize state that
  /// depends on having a parent. State that is independent of the parent can
  /// more easily be initialized in the constructor.
  ///
  /// This method transitions the element from the "initial" lifecycle state to
  /// the "active" lifecycle state.
  ///
  /// Subclasses that override this method are likely to want to also override
  /// [update], [visitChildren], [RenderObjectElement.insertRenderObjectChild],
  /// [RenderObjectElement.moveRenderObjectChild], and
  /// [RenderObjectElement.removeRenderObjectChild].
  @mustCallSuper
  void mount(Element? parent, dynamic newSlot) {
    assert(_lifecycleState == _ElementLifecycle.initial);
    assert(widget != null);
    assert(_parent == null);
    assert(parent == null || parent._lifecycleState == _ElementLifecycle.active);
    assert(slot == null);
    _parent = parent;
    _slot = newSlot;
    _lifecycleState = _ElementLifecycle.active;
    _depth = _parent != null ? _parent!.depth + 1 : 1;
    if (parent != null) // Only assign ownership if the parent is non-null
      _owner = parent.owner;
    final Key? key = widget.key;
    if (key is GlobalKey) {
      key._register(this);
    }
    _updateInheritance();
  }
  ...

这个方法非常重要，上面是Element的mount（）的实现，来看看他的子类RenderObjectElement、StatelessElement和StatefulElement的mount（）方法都做了哪些事情。

4.1、 RenderObjectElement的mount（）

...

@override
  void mount(Element? parent, dynamic newSlot) {
    super.mount(parent, newSlot);
    assert(() {
      _debugDoingBuild = true;
      return true;
    }());
    _renderObject = widget.createRenderObject(this);
    assert(() {
      _debugDoingBuild = false;
      return true;
    }());
    assert(() {
      _debugUpdateRenderObjectOwner();
      return true;
    }());
    assert(_slot == newSlot);
    attachRenderObject(newSlot);
    _dirty = false;
  }

...

这里面最要的一个方法就是_renderObject = widget.createRenderObject(this);，它说明了RenderObjectElement创建之后Flutter引擎就会创建renderObject，创建完成之后就会调attachRenderObject(newSlot);方法添加到渲染树中。

4.2、 StatelessElement的mount()

在StatelessElement类里面并没有看到mount（）方法，在找其父类ComponentElement的mount()方法：

  @override
  void mount(Element? parent, dynamic newSlot) {
    super.mount(parent, newSlot);
    assert(_child == null);
    assert(_lifecycleState == _ElementLifecycle.active);
    _firstBuild();
    assert(_child != null);
  }

这里面调了_firstBuild();方法：

void _firstBuild() {
    rebuild();
  }

接着往下走：

 /// Called by the [BuildOwner] when [BuildOwner.scheduleBuildFor] has been
  /// called to mark this element dirty, by [mount] when the element is first
  /// built, and by [update] when the widget has changed.
  void rebuild() {
   ...
    performRebuild();
  ...
  }

又调了performRebuild()：

 @protected
  void performRebuild();

到这里就点不进去了，我们可以同时按住command+alt+B，就会搜到实现：

image.png

这里选择CompentElement的performRebuild进去就可以看到实现：

  /// Calls the [StatelessWidget.build] method of the [StatelessWidget] object
  /// (for stateless widgets) or the [State.build] method of the [State] object
  /// (for stateful widgets) and then updates the widget tree.
  ///
  /// Called automatically during [mount] to generate the first build, and by
  /// [rebuild] when the element needs updating.
  @override
  void performRebuild() {
   ...
    built = build();
   ...
  }

这里调了一个build（）方法：

  /// Subclasses should override this function to actually call the appropriate
  /// `build` function (e.g., [StatelessWidget.build] or [State.build]) for
  /// their widget.
  @protected
  Widget build();

同样按住command+alt+B，搜实现：

image.png

选择StatelessElement的实现：

@override
Widget build() => widget.build(this);

这里调到了widget的bulid（）方法，并且将this做为参数传了出去，这里的this指的就是StatelessElement，那这的widget是什么呢，看一下StatelessElement的定义：

/// An [Element] that uses a [StatelessWidget] as its configuration.
class StatelessElement extends ComponentElement {
  /// Creates an element that uses the given widget as its configuration.
  StatelessElement(StatelessWidget widget) : super(widget);

  @override
  StatelessWidget get widget => super.widget as StatelessWidget;

  @override
  Widget build() => widget.build(this);

  @override
  void update(StatelessWidget newWidget) {
    super.update(newWidget);
    assert(widget == newWidget);
    _dirty = true;
    rebuild();
  }
}

StatelessElement的构造方法提供了一个参数widget，并传递给其父类的父类Element，Element通过构造方法：

    Element(Widget widget)
    : assert(widget != null),
      _widget = widget;

赋值给成员变量_widget，保存起来，并提供一个获取的方法widget来获取_widget：

/// The configuration for this element.
  @override
  Widget get widget => _widget;

在StatelessElement这个类里面重写了Element的widget方法：

@override
  StatelessWidget get widget => super.widget as StatelessWidget;

在StatelessWidget可以看到：

@override
  StatelessElement createElement() => StatelessElement(this);

在创建StatelessElement的时候是将this传给了StatelessElement的构造方法，这个this就是StatelessWidget。

根据上面的分析StatelessElement里面的：

  @override
  Widget build() => widget.build(this);

widget就是StatelessWidget，this指的就是StatelessElement。

来看Flutter官方的Demo：

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(

        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

这个build方法的参数context其实就是StatelessElement。

4.3、 StatefulElement的mount()

StatefulElement和StatelessElement一样都没有实现mount（）方法，都是在他们父类ComponentElement中实现的，流程是一样的，不一样的是build（）方法：

  @override
  Widget build() => state.build(this);

StatefulElement的build方法是调state的bulid方法，这个跟StatelessElement不一样，这个state是哪里来的呢，看一下StatefulElement的构造方法实现：

/// Creates an element that uses the given widget as its configuration.
  StatefulElement(StatefulWidget widget)
      : state = widget.createState(),
        super(widget) {
    assert(() {
      if (!state._debugTypesAreRight(widget)) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('StatefulWidget.createState must return a subtype of State<${widget.runtimeType}>'),
          ErrorDescription(
            'The createState function for ${widget.runtimeType} returned a state '
            'of type ${state.runtimeType}, which is not a subtype of '
            'State<${widget.runtimeType}>, violating the contract for createState.'
          ),
        ]);
      }
      return true;
    }());
    assert(state._element == null);
    state._element = this;
    assert(
      state._widget == null,
      'The createState function for $widget returned an old or invalid state '
      'instance: ${state._widget}, which is not null, violating the contract '
      'for createState.',
    );
    state._widget = widget;
    assert(state._debugLifecycleState == _StateLifecycle.created);
  }

这个方法里面有两个重点：

• state = widget.createState()，在创建StatefulElement的时候，调widget的createState（）方法创建一个State对象保存在成员变量state里面，这就是为什么StatefulWidget实现createState（）。
• state._widget = widget;，将widget保存在state的_widget成员变量中，在state里面提供一个获取_widget的方法——widget：

abstract class State<T extends StatefulWidget> with Diagnosticable {
   ...
   T get widget => _widget!;
   T? _widget;
   ...
  }

这就是为什么在State里面可以使用widget的原因。