react中context到底是如何传递的-源码分析

发布时间：2020-12-15 06:41:02 所属栏目：百科来源：网络整理

导读：title: react-Context date: 2017-04-25 12:36:00 categories: react tags : context comments : true updated : layout : 题外话，个人不喜欢有魔法性的代码，比如context到底如何传递的，为嘛Provider可以直接通过context上下文传递我的其他React源码分析

title: react-Context
date: 2017-04-25 12:36:00
categories: react
tags : context
comments : true
updated :
layout :

题外话，个人不喜欢有魔法性的代码，比如context到底如何传递的，为嘛Provider可以直接通过context上下文传递

我的其他React源码分析系列 https://github.com/jimwmg/JiM...

https://github.com/jimwmg/React-
react源码

react中使用context

基本要求就是

父组件中声明Parent.prototype.getChildContext
父组件中声明Parent.childContextType
子组件声明 Child.contextType

1 先看一个组件

class BaseDataSelect extends Component {
    //只在组件重新加载的时候执行一次
    constructor(props) {
        super(props);
      //..
    }
      //other methods
}
//super其实就是下面这个函数
function ReactComponent(props,context,updater) {
  this.props = props;
  this.context = context;
  this.refs = emptyObject;
  // We initialize the default updater but the real one gets injected by the
  // renderer.
  this.updater = updater || ReactNoopUpdateQueue;
}

//自执行函数
var Provider = function (_Component) {
  _inherits(Provider,_Component);
//父组件需要声明
  Provider.prototype.getChildContext = function getChildContext() {
    return { store: this.store };
  };
//这里其实就产生了闭包
  function Provider(props,context) {
    _classCallCheck(this,Provider);

    var _this = _possibleConstructorReturn(this,_Component.call(this,props,context));
    //这行代码是我加的测试代码,在控制台输出的就是一个Provider对象
    console.log(_this);
    
    _this.store = props.store;
    return _this;
  }

  Provider.prototype.render = function render() {
    return _react.Children.only(this.props.children);
  };
  //父组件需要声明
Provider.childContextTypes = {store:PropTypes.storeShape.isRequired,}
  return Provider;
}(_react.Component);

对，就是我们常用的Provider组件；

实际中的运用(App. 是经过connect过的组件)

const store = createStore(reducer)
ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,document.getElementById('root')
)；

2 那么传递context的工作是由谁来做的呢？当然是react了；

ReacrDOM.render其实就是ReactMount.render函数；

以下是react如何将ReactElement挂载到实际DOM元素上的step过程；

ReactMount.js源码地址

var ReactMount = {
  //nextElement就是ReactELement，jsx语法将组件或者div，span等转化为一个ReactElement对象
  //这里就是Provider组件生成的ReactElement对象；
  //step1 
  render: function (nextElement,container,callback) {
    //将ReactElement对象和container元素传递给_renderSubtreeIntoContainer函数；
    return ReactMount._renderSubtreeIntoContainer(null,nextElement,callback);
  },//step2 
  _renderSubtreeIntoContainer: function (parentComponent,callback){
    .....//具体源码看上面源码地址
    var nextContext;
    if (parentComponent) {
      //parentComponent为null ;
      var parentInst = ReactInstanceMap.get(parentComponent);
      nextContext = parentInst._processChildContext(parentInst._context);
    } else {
      //所以传递下去的nextContext = enmtyObject;
      nextContext = emptyObject;
    }
    //.....
    var component = ReactMount._renderNewRootComponent(nextWrappedElement,shouldReuseMarkup,nextContext)  ._renderedComponent.getPublicInstance();

    return component;
  },//step3 
  //下面这个函数实现将ReactElement元素，转化为DOM元素并且插入到对应的Container元素中去；
  _renderNewRootComponent: function (nextElement,context) {
    //instantiateReactComponent(nextElement,false)函数返回一个组件的实例，该函数源码下面会解释；
    var componentInstance = instantiateReactComponent(nextElement,false);

    // The initial render is synchronous but any updates that happen during
    // rendering,in componentWillMount or componentDidMount,will be batched
    // according to the current batching strategy.
    //这个函数是真正的将ReactElement元素插入到DOM元素的，会进入到batchedMountComponentIntoNode函数中；
    ReactUpdates.batchedUpdates(batchedMountComponentIntoNode,componentInstance,context);

    var wrapperID = componentInstance._instance.rootID;
    instancesByReactRootID[wrapperID] = componentInstance;

    return componentInstance;    

  }
}
//step 4
//====================会进入到mountComponentIntoNode函数中
function batchedMountComponentIntoNode(componentInstance,context) {
  var transaction = ReactUpdates.ReactReconcileTransaction.getPooled(
    /* useCreateElement */
    !shouldReuseMarkup && ReactDOMFeatureFlags.useCreateElement);
  transaction.perform(mountComponentIntoNode,null,transaction,context);
  ReactUpdates.ReactReconcileTransaction.release(transaction);
}
//step 5
//====================
function mountComponentIntoNode(wrapperInstance,context) {
  var markerName;
  if (ReactFeatureFlags.logTopLevelRenders) {
    var wrappedElement = wrapperInstance._currentElement.props.child;
    var type = wrappedElement.type;
    markerName = 'React mount: ' + (typeof type === 'string' ? type : type.displayName || type.name);
    console.time(markerName);
  }
  
  //markup是经过解析成功的HTML元素，该元素通过_mountImageIntoNode加载到对应的DOM元素上；
  //注意经过上面的函数层层调用，最后到这里的context还是emptyObject
  var markup = ReactReconciler.mountComponent(wrapperInstance,ReactDOMContainerInfo(wrapperInstance,container),0 /* parentDebugID */
                                             );

  if (markerName) {
    console.timeEnd(markerName);
  }

  wrapperInstance._renderedComponent._topLevelWrapper = wrapperInstance;
  ReactMount._mountImageIntoNode(markup,wrapperInstance,transaction);
}
//step 6
//_mountImageIntoNode
_mountImageIntoNode: function (markup,instance,transaction) {
  !isValidContainer(container) ? process.env.NODE_ENV !== 'production' ? invariant(false,'mountComponentIntoNode(...): Target container is not valid.') : _prodInvariant('41') : void 0;

  if (shouldReuseMarkup) {
    var rootElement = getReactRootElementInContainer(container);
    if (ReactMarkupChecksum.canReuseMarkup(markup,rootElement)) {
      ReactDOMComponentTree.precacheNode(instance,rootElement);
      return;
    } else {
      var checksum = rootElement.getAttribute(ReactMarkupChecksum.CHECKSUM_ATTR_NAME);
      rootElement.removeAttribute(ReactMarkupChecksum.CHECKSUM_ATTR_NAME);

      var rootMarkup = rootElement.outerHTML;
      rootElement.setAttribute(ReactMarkupChecksum.CHECKSUM_ATTR_NAME,checksum);

      var normalizedMarkup = markup;
      var diffIndex = firstDifferenceIndex(normalizedMarkup,rootMarkup);
      var difference = ' (client) ' + normalizedMarkup.substring(diffIndex - 20,diffIndex + 20) + 'n (server) ' + rootMarkup.substring(diffIndex - 20,diffIndex + 20);

   
  if (transaction.useCreateElement) {
    while (container.lastChild) {
      container.removeChild(container.lastChild);
    }
    DOMLazyTree.insertTreeBefore(container,markup,null);
  } else {
   // 利用innerHTML将markup插入到container这个DOM元素上
      setInnerHTML(container,markup);
      // 将instance（Virtual DOM）保存到container这个DOM元素的firstChild这个原生节点上
    ReactDOMComponentTree.precacheNode(instance,container.firstChild);

  }

  if (process.env.NODE_ENV !== 'production') {
    var hostNode = ReactDOMComponentTree.getInstanceFromNode(container.firstChild);
    if (hostNode._debugID !== 0) {
      ReactInstrumentation.debugTool.onHostOperation({
        instanceID: hostNode._debugID,type: 'mount',payload: markup.toString()
      });
    }
  }
}

3 context如何传递的？

step2 - step5中开始出现context进行往下传递；这里传递的一直是emptyObject;

主要看下step5中

var markup = ReactReconciler.mountComponent(wrapperInstance,0 /* parentDebugID */

[ReactReconciler.js源码地址. 其实就是执行下面这个函数：

mountComponent: function (internalInstance,hostParent,hostContainerInfo,parentDebugID) // 0 in production and for roots
  {
    //这里传进去的还是emptyObject;
    var markup = internalInstance.mountComponent(transaction,parentDebugID);
    if (internalInstance._currentElement && internalInstance._currentElement.ref != null) {
      transaction.getReactMountReady().enqueue(attachRefs,internalInstance);
    }
    if (process.env.NODE_ENV !== 'production') {
      if (internalInstance._debugID !== 0) {
        ReactInstrumentation.debugTool.onMountComponent(internalInstance._debugID);
      }
    }
    return markup;
  },

对于internalInstance是React组件，而不是宿主DOM元素的情况；

ReactCompositeComponent.js源码地址

注意这里internalInstance.mountComponent其实就是ReactCompositeComponent.js中的mountComponent方法；

mountComponent: function (transaction,context) {
  var _this = this;
//这里的this指的是internalInstance,也就是经过React处理ReactElement对象之后生成的React组件实例对象；
  this._context = context;
  this._mountOrder = nextMountID++;
  this._hostParent = hostParent;
  this._hostContainerInfo = hostContainerInfo;
//internalInstance._currentElement.props
  var publicProps = this._currentElement.props;
  
  //这里这里是第一次处理context;其实是一个emptyObject;_processContext实现看上面链接，不放了，免得乱；
  var publicContext = this._processContext(context);
//这里Component就是Provider函数；
  var Component = this._currentElement.type;

  var updateQueue = transaction.getUpdateQueue();

  // Initialize the public class
  var doConstruct = shouldConstruct(Component);
  //flag1: 注意这里，这里会真的调用Provider函数，生成 new Provider实例对象
  var inst = this._constructComponent(doConstruct,publicProps,publicContext,updateQueue);
  var renderedElement;

  // These should be set up in the constructor,but as a convenience for
  // simpler class abstractions,we set them up after the fact.
  inst.props = publicProps;
  inst.context = publicContext;
  inst.refs = emptyObject;
  inst.updater = updateQueue;

  this._instance = inst;

  // Store a reference from the instance back to the internal representation
  ReactInstanceMap.set(inst,this);
 
  var markup;
  if (inst.unstable_handleError) {
    markup = this.performInitialMountWithErrorHandling(renderedElement,context);
  } else {
    //flag2 : 这里接着处理子组件
    markup = this.performInitialMount(renderedElement,context);
  }
  if (inst.componentDidMount) {
    if (process.env.NODE_ENV !== 'production') {
      transaction.getReactMountReady().enqueue(function () {
        measureLifeCyclePerf(function () {
          return inst.componentDidMount();
        },_this._debugID,'componentDidMount');
      });
    } else {
      transaction.getReactMountReady().enqueue(inst.componentDidMount,inst);
    }
  }

  return markup;
},

flag1: 注意这里，这里会真的调用Provider函数，生成 new Provider实例对象

var inst = this._constructComponent(doConstruct,updateQueue);

_constructComponent: function (doConstruct,updateQueue) {

if (process.env.NODE_ENV !== 'production' && !doConstruct) {
    ReactCurrentOwner.current = this;
    try {
      return this._constructComponentWithoutOwner(doConstruct,updateQueue);
    } finally {
      ReactCurrentOwner.current = null;
    }
  } else {
    return this._constructComponentWithoutOwner(doConstruct,updateQueue);
  }
},//然后
  
_constructComponentWithoutOwner: function (doConstruct,updateQueue) {
  var Component = this._currentElement.type;

  if (doConstruct) {
    if (process.env.NODE_ENV !== 'production') {
      return measureLifeCyclePerf(function () {
        //这里其实就是new Provider(props,context) ;这个时候可以对应到Provider源码上看下；但是直到现在还是没有涉及到getChildContext()所返回的对象，是如何在子组件中可以调用的；
        //等下次循环的时候这里就是 new App(props,context) 这里的context就有Provider.prototype.getChilContext返回的对象；
        return new Component(publicProps,updateQueue);
      },this._debugID,'ctor');
    } else {
      return new Component(publicProps,updateQueue);
    }
  }

  // This can still be an instance in case of factory components
  // but we'll count this as time spent rendering as the more common case.
  if (process.env.NODE_ENV !== 'production') {
    return measureLifeCyclePerf(function () {
      return Component(publicProps,updateQueue);
    },'render');
  } else {
    return Component(publicProps,

flag2 : 这里接着处理子组件

markup = this.performInitialMount(renderedElement,context);

performInitialMount: function (renderedElement,context) {

//注意传进来的context基本上还是等于emptyObject;
var inst = this._instance;

//这个inis就是 Provider实例对象；

var debugID = 0;
if (process.env.NODE_ENV !== 'production') {
  debugID = this._debugID;
}

if (inst.componentWillMount) {
  if (process.env.NODE_ENV !== 'production') {
    measureLifeCyclePerf(function () {
      return inst.componentWillMount();
    },debugID,'componentWillMount');
  } else {
    inst.componentWillMount();
  }
  // When mounting,calls to `setState` by `componentWillMount` will set
  // `this._pendingStateQueue` without triggering a re-render.
  if (this._pendingStateQueue) {
    inst.state = this._processPendingState(inst.props,inst.context);
  }
}

// If not a stateless component,we now render
if (renderedElement === undefined) {
  //这个其实就是Provider的子组件 <App /> 也是一个ReactElement对象；
  renderedElement = this._renderValidatedComponent();
}

var nodeType = ReactNodeTypes.getType(renderedElement);
this._renderedNodeType = nodeType;
var child = this._instantiateReactComponent(renderedElement,nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */);
this._renderedComponent = child;

//这里又轮回到了ReactReconciler.js中的mountComponent

//如果child组件还是React组件，而不是宿主DOM元素，那么就会一直递归，直到child是宿主DOM元素；
//就不会轮回到ReactCompositeComponent.js中的mountComponent;
//对于还是React组件的情况下，还是会执行ReactCompositeComponent.js中mountComponent
//注意这个时候传递给该函数的context参数的值是 this._processChildContext(context)
//此时传入的child就是 App  子组件（connect后的高阶组件） 生成的React组件实例
//然后生成的高阶组件 App  就会将通过Provider传递过来的store对象上的相关接口传递给被包裹的组件，作为被包裹组件的props;
//文章开头有链接react其他源码分析，上面有Provider分析文章；
var markup = ReactReconciler.mountComponent(child,this._processChildContext(context),debugID);

//this._processChildContext(context) 此时的this指的是Provider组件经过React处理后生成的instantiateReactComponent(nextElement,false);react实例对象；上面的child也是一样的道理；

if (process.env.NODE_ENV !== 'production') {
  if (debugID !== 0) {
    var childDebugIDs = child._debugID !== 0 ? [child._debugID] : [];
    ReactInstrumentation.debugTool.onSetChildren(debugID,childDebugIDs);
  }
}

return markup;

这里重点看下

_processChildContext: function (currentContext) {
    var Component = this._currentElement.type;
  //这个inst就是Provider组件new之后的实例对象
    var inst = this._instance;
    var childContext;

    if (inst.getChildContext) {
      if ("development" !== 'production') {
        ReactInstrumentation.debugTool.onBeginProcessingChildContext();
        try {
          //这里通过Provider.prototype.getChildContext上得到context值
          childContext = inst.getChildContext();
        } finally {
          ReactInstrumentation.debugTool.onEndProcessingChildContext();
        }
      } else {
        childContext = inst.getChildContext();
      }
    }
    if (childContext) {
      return _assign({},currentContext,childContext);
    }
    return currentContext;
  },

（编辑：李大同）

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