React 源码解析

2019-08-27  本文已影响0人  优秀的javaScript

前言

基础概念

function instantiateReactComponent(node, shouldHaveDebugID) {
var instance;
if (node === null || node === false) {
instance = ReactEmptyComponent.create(instantiateReactComponent);
} else if (typeof node === 'object') {
var element = node;
// Special case string values
if (typeof element.type === 'string') {
  instance = ReactHostComponent.createInternalComponent(element);
} else if (isInternalComponentType(element.type)) {
  // This is temporarily available for custom components that are not string
  // representation, we can drop this code path.
} else {
  instance = new ReactCompositeComponentWrapper(element);
}
} else if (typeof node === 'string' || typeof node === 'number') {
instance = ReactHostComponent.createInstanceForText(node);
} else {
}
// These two fields are used by the DOM and ART diffing algorithms
// respectively. Instead of using expandos on components, we should be
// storing the state needed by the diffing algorithms elsewhere.
instance._mountIndex = 0;
instance._mountImage = null;
return instance;
}

var ReactClass = {
  createClass: function (spec) {
    // ensure that Constructor.name !== 'Constructor'
    var Constructor = identity(function (props, context, updater) {
      // Wire up auto-binding
      if (this.__reactAutoBindPairs.length) {
        bindAutoBindMethods(this);
      }
      this.props = props;
      this.context = context;
      this.refs = emptyObject;
      this.updater = updater || ReactNoopUpdateQueue;
      this.state = null;
      // ReactClasses doesn't have constructors. Instead, they use the
      // getInitialState and componentWillMount methods for initialization.
      var initialState = this.getInitialState ? this.getInitialState() : null;
      this.state = initialState;
    });
    Constructor.prototype = new ReactClassComponent();
    Constructor.prototype.constructor = Constructor;
    Constructor.prototype.__reactAutoBindPairs = [];
    injectedMixins.forEach(mixSpecIntoComponent.bind(null, Constructor));
    mixSpecIntoComponent(Constructor, spec);
    // Initialize the defaultProps property after all mixins have been merged.
    if (Constructor.getDefaultProps) {
      Constructor.defaultProps = Constructor.getDefaultProps();
    }
    // Reduce time spent doing lookups by setting these on the prototype.
    for (var methodName in ReactClassInterface) {
      if (!Constructor.prototype[methodName]) {
        Constructor.prototype[methodName] = null;
      }
    }
    return Constructor;
  }
}
var ReactClassComponent = function () {};
_assign(ReactClassComponent.prototype, ReactComponent.prototype, ReactClassMixin);

ReactComponent.js:

function ReactComponent(props, context, updater) {
  this.props = props;
  this.context = context;
  this.refs = emptyObject;
  this.updater = updater || ReactNoopUpdateQueue;
}
ReactComponent.prototype.isReactComponent = {};
ReactComponent.prototype.setState = function (partialState, callback) {
  this.updater.enqueueSetState(this, partialState);
  if (callback) {
    this.updater.enqueueCallback(this, callback, 'setState');
  }
};
ReactComponent.prototype.forceUpdate = function (callback) {
  this.updater.enqueueForceUpdate(this);
  if (callback) {
    this.updater.enqueueCallback(this, callback, 'forceUpdate');
  }
};

对象池

// 只展示部分
var oneArgumentPooler = function (copyFieldsFrom) {
  var Klass = this;
  if (Klass.instancePool.length) {
    var instance = Klass.instancePool.pop();
    Klass.call(instance, copyFieldsFrom);
    return instance;
  } else {
    return new Klass(copyFieldsFrom);
  }
};
var standardReleaser = function (instance) {
  var Klass = this;
  if (Klass.instancePool.length < Klass.poolSize) {
    Klass.instancePool.push(instance);
  }
};
var DEFAULT_POOL_SIZE = 10;
var DEFAULT_POOLER = oneArgumentPooler;
var addPoolingTo = function (CopyConstructor, pooler) {
  // Casting as any so that flow ignores the actual implementation and trusts
  // it to match the type we declared
  var NewKlass = CopyConstructor;
  NewKlass.instancePool = [];
  NewKlass.getPooled = pooler || DEFAULT_POOLER;
  if (!NewKlass.poolSize) {
    NewKlass.poolSize = DEFAULT_POOL_SIZE;
  }
  NewKlass.release = standardReleaser;
  return NewKlass;
};
var PooledClass = {
  addPoolingTo: addPoolingTo,
  oneArgumentPooler: oneArgumentPooler,
  twoArgumentPooler: twoArgumentPooler,
  threeArgumentPooler: threeArgumentPooler,
  fourArgumentPooler: fourArgumentPooler,
  fiveArgumentPooler: fiveArgumentPooler
};
module.exports = PooledClass;

使用例子(ReactUpdate.js)：

var transaction = ReactUpdatesFlushTransaction.getPooled();
destructor: function () {
    this.dirtyComponentsLength = null;
    CallbackQueue.release(this.callbackQueue);
    this.callbackQueue = null;
    ReactUpdates.ReactReconcileTransaction.release(this.reconcileTransaction);
    this.reconcileTransaction = null;
  }
ReactUpdatesFlushTransaction.release(transaction);

事务机制

image

代码(Transaction.js)：

var TransactionImpl = {
  perform: function (method, scope, a, b, c, d, e, f) {
    var errorThrown;
    var ret;
    try {
      this._isInTransaction = true;
      // Catching errors makes debugging more difficult, so we start with
      // errorThrown set to true before setting it to false after calling
      // close -- if it's still set to true in the finally block, it means
      // one of these calls threw.
      errorThrown = true;
      this.initializeAll(0);
      ret = method.call(scope, a, b, c, d, e, f);
      errorThrown = false;
    } finally {
      try {
        if (errorThrown) {
          // If `method` throws, prefer to show that stack trace over any thrown
          // by invoking `closeAll`.
          try {
            this.closeAll(0);
          } catch (err) {}
        } else {
          // Since `method` didn't throw, we don't want to silence the exception
          // here.
          this.closeAll(0);
        }
      } finally {
        this._isInTransaction = false;
      }
    }
    return ret;
  },
  // 执行所有 wrapper 中的 initialize 方法
  initializeAll: function (startIndex) {
  },
  // 执行所有 wrapper 中的 close 方法
  closeAll: function (startIndex) {
  }
};
module.exports = TransactionImpl;

事件分发

image

生命周期

image 
mountComponent: function (transaction, hostParent, hostContainerInfo, context) {
    var _this = this;
    this._context = context;
    this._mountOrder = nextMountID++;
    this._hostParent = hostParent;
    this._hostContainerInfo = hostContainerInfo;
    var publicProps = this._currentElement.props;
    var publicContext = this._processContext(context);
    var Component = this._currentElement.type;
    var updateQueue = transaction.getUpdateQueue();
    // Initialize the public class
    var doConstruct = shouldConstruct(Component);
    // 最终会调用 new Component()
    var inst = this._constructComponent(doConstruct, publicProps, publicContext, updateQueue);
    var renderedElement;
    // Support functional components
    if (!doConstruct && (inst == null || inst.render == null)) {
      renderedElement = inst;
      inst = new StatelessComponent(Component);
      this._compositeType = CompositeTypes.StatelessFunctional;
    } else {
      // 大家经常在用户端用到的 PureComponent，会对 state 进行浅比较然后决定是否执行 render
      if (isPureComponent(Component)) {
        this._compositeType = CompositeTypes.PureClass;
      } else {
        this._compositeType = CompositeTypes.ImpureClass;
      }
    }
    // 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
    // 以 element 为 key，存在了 Map 中，之后会用到
    ReactInstanceMap.set(inst, this);
    var initialState = inst.state;
    if (initialState === undefined) {
      inst.state = initialState = null;
    }
    this._pendingStateQueue = null;
    this._pendingReplaceState = false;
    this._pendingForceUpdate = false;
    var markup;
    if (inst.unstable_handleError) {
      markup = this.performInitialMountWithErrorHandling(renderedElement, hostParent, hostContainerInfo, transaction, context);
    } else {
      markup = this.performInitialMount(renderedElement, hostParent, hostContainerInfo, transaction, context);
    }
    if (inst.componentDidMount) {
       transaction.getReactMountReady().enqueue(inst.componentDidMount, inst);
    }
    return markup;
  }
function shouldConstruct(Component) {
  return !!(Component.prototype && Component.prototype.isReactComponent);
}

performInitialMount: function (renderedElement, hostParent, hostContainerInfo, transaction, context) {
    var inst = this._instance;
    var debugID = 0;
    if (inst.componentWillMount) {
      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
    // 返回 ReactElement，这也就是上文说的 render 返回 ReactElement
    if (renderedElement === undefined) {
      renderedElement = this._renderValidatedComponent();
    }
    var nodeType = ReactNodeTypes.getType(renderedElement);
    this._renderedNodeType = nodeType;
    var child = this._instantiateReactComponent(renderedElement, nodeType !== ReactNodeTypes.EMPTY);
    this._renderedComponent = child;
    var markup = ReactReconciler.mountComponent(child, transaction, hostParent, hostContainerInfo, this._processChildContext(context), debugID);
    return markup;
  }

updateComponent: function (transaction, prevParentElement, nextParentElement, prevUnmaskedContext, nextUnmaskedContext) {
    var inst = this._instance;
    var willReceive = false;
    var nextContext;
    // context 相关，React 建议少用 context
    // Determine if the context has changed or not
    if (this._context === nextUnmaskedContext) {
      nextContext = inst.context;
    } else {
      nextContext = this._processContext(nextUnmaskedContext);
      willReceive = true;
    }
    var prevProps = prevParentElement.props;
    var nextProps = nextParentElement.props;
    // Not a simple state update but a props update
    if (prevParentElement !== nextParentElement) {
      willReceive = true;
    }
    // An update here will schedule an update but immediately set
    // _pendingStateQueue which will ensure that any state updates gets
    // immediately reconciled instead of waiting for the next batch.
    if (willReceive && inst.componentWillReceiveProps) {
      inst.componentWillReceiveProps(nextProps, nextContext);
    }
    var nextState = this._processPendingState(nextProps, nextContext);
    var shouldUpdate = true;
    if (!this._pendingForceUpdate) {
      if (inst.shouldComponentUpdate) {
        shouldUpdate = inst.shouldComponentUpdate(nextProps, nextState, nextContext);
      } else {
        if (this._compositeType === CompositeTypes.PureClass) {
          // 这里，就是上文提到的，PureComponent 里的浅比较
          shouldUpdate = !shallowEqual(prevProps, nextProps) || !shallowEqual(inst.state, nextState);
        }
      }
    }
    this._updateBatchNumber = null;
    if (shouldUpdate) {
      this._pendingForceUpdate = false;
      // Will set `this.props`, `this.state` and `this.context`.
      this._performComponentUpdate(nextParentElement, nextProps, nextState, nextContext, transaction, nextUnmaskedContext);
    } else {
      // If it's determined that a component should not update, we still want
      // to set props and state but we shortcut the rest of the update.
      this._currentElement = nextParentElement;
      this._context = nextUnmaskedContext;
      inst.props = nextProps;
      inst.state = nextState;
      inst.context = nextContext;
    }
  }

_updateRenderedComponent: function (transaction, context) {
    var prevComponentInstance = this._renderedComponent;
    var prevRenderedElement = prevComponentInstance._currentElement;
    var nextRenderedElement = this._renderValidatedComponent();
    var debugID = 0;
    if (shouldUpdateReactComponent(prevRenderedElement, nextRenderedElement)) {
      ReactReconciler.receiveComponent(prevComponentInstance, nextRenderedElement, transaction, this._processChildContext(context));
    } else {
      var oldHostNode = ReactReconciler.getHostNode(prevComponentInstance);
      ReactReconciler.unmountComponent(prevComponentInstance, false);
      var nodeType = ReactNodeTypes.getType(nextRenderedElement);
      this._renderedNodeType = nodeType;
      var child = this._instantiateReactComponent(nextRenderedElement, nodeType !== ReactNodeTypes.EMPTY);
      this._renderedComponent = child;
      var nextMarkup = ReactReconciler.mountComponent(child, transaction, this._hostParent, this._hostContainerInfo, this._processChildContext(context), debugID);
      this._replaceNodeWithMarkup(oldHostNode, nextMarkup, prevComponentInstance);
    }
  },

可以看到，如果需要更新，则调用ReactReconciler.receiveComponent，会递归更新子组件，否则直接卸载然后挂载。所以，重点是在shouldUpdateReactComponent的判断，React 为了简化 diff，所以有一个假设：在组件层级、type、key 不变的时候，才进行比较更新，否则先 unmount 然后重新 mount。来看shouldUpdateReactComponent(shouldUpdateReactComponent.js) :

function shouldUpdateReactComponent(prevElement, nextElement) {
  var prevEmpty = prevElement === null || prevElement === false;
  var nextEmpty = nextElement === null || nextElement === false;
  if (prevEmpty || nextEmpty) {
    return prevEmpty === nextEmpty;
  }
  var prevType = typeof prevElement;
  var nextType = typeof nextElement;
  // 如果前后两次都为文本元素，则更新
  if (prevType === 'string' || prevType === 'number') {
    return nextType === 'string' || nextType === 'number';
  } else {
    // 如果为 ReactDomComponent 或 ReactCompositeComponent，则需要层级 type 和 key 相同，才进行 update（层级在递归中保证相同）
    return nextType === 'object' && prevElement.type === nextElement.type && prevElement.key === nextElement.key;
  }
}

接下来是重头戏：setState，上文中已经提到了此 api 为:

ReactComponent.prototype.setState = function (partialState, callback) {
  this.updater.enqueueSetState(this, partialState);
  if (callback) {
    this.updater.enqueueCallback(this, callback, 'setState');
  }
};

可以看到这里只是简单的调用enqueueSetState放入队列中，而我们知道，不可能这么简单的。来看enqueueSetState(ReactUpdateQueue.js中)，this.updater会在 mount 时候赋值为updateQueue

enqueueSetState: function (publicInstance, partialState) {
    var internalInstance = getInternalInstanceReadyForUpdate(publicInstance, 'setState');
    if (!internalInstance) {
      return;
    }
    // 获取队列，如果为空则创建
    var queue = internalInstance._pendingStateQueue || (internalInstance._pendingStateQueue = []);
    // 将待 merge 的 state 放入队列
    queue.push(partialState);
    // 将待更新的组件放入队列
    enqueueUpdate(internalInstance);
  },
  function getInternalInstanceReadyForUpdate(publicInstance, callerName) {
      // 上文提到的以 element 为 key 存入 map，这里可以取到 component
    var internalInstance = ReactInstanceMap.get(publicInstance);
    if (!internalInstance) {
      return null;
    }
    return internalInstance;
  }

再来看enqueueUpdate(ReactUpdates.js)：

function enqueueUpdate(component) {
  if (!batchingStrategy.isBatchingUpdates) {
    batchingStrategy.batchedUpdates(enqueueUpdate, component);
    return;
  }
  dirtyComponents.push(component);
  if (component._updateBatchNumber == null) {
    component._updateBatchNumber = updateBatchNumber + 1;
  }
}

batchedUpdates: function (callback, a, b, c, d, e) {
    var alreadyBatchingUpdates = ReactDefaultBatchingStrategy.isBatchingUpdates;
    ReactDefaultBatchingStrategy.isBatchingUpdates = true;
    // The code is written this way to avoid extra allocations
    if (alreadyBatchingUpdates) {
      return callback(a, b, c, d, e);
    } else {
      // 注意这里，上一个代码块中可以看到，当 isBatchingUpdates 为 false 时，callback 为 enqueueUpdate 自身
      // 所以即以事务的方式处理
      return transaction.perform(callback, null, a, b, c, d, e);
    }
  }
  var transaction = new ReactDefaultBatchingStrategyTransaction();

var RESET_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: function () {
    ReactDefaultBatchingStrategy.isBatchingUpdates = false;
  }
};
var FLUSH_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};
// flushBatchedUpdates 在 ReactUpdates.js 中
var flushBatchedUpdates = function () {
  // ReactUpdatesFlushTransaction's wrappers will clear the dirtyComponents
  // asapEnqueued 为提前执行回调，暂不分析
  while (dirtyComponents.length || asapEnqueued) {
    if (dirtyComponents.length) {
      var transaction = ReactUpdatesFlushTransaction.getPooled();
      transaction.perform(, null, transaction);
      ReactUpdatesFlushTransaction.release(transaction);
    }
    if (asapEnqueued) {
    }
  }
};

function runBatchedUpdates(transaction) {
  var len = transaction.dirtyComponentsLength;
  // reconcile them before their children by sorting the array.
  dirtyComponents.sort(mountOrderComparator);
  // Any updates enqueued while reconciling must be performed after this entire
  // batch. Otherwise, if dirtyComponents is [A, B] where A has children B and
  // C, B could update twice in a single batch if C's render enqueues an update
  // to B (since B would have already updated, we should skip it, and the only
  // way we can know to do so is by checking the batch counter).
  updateBatchNumber++;
  for (var i = 0; i < len; i++) {
    // If a component is unmounted before pending changes apply, it will still
    // be here, but we assume that it has cleared its _pendingCallbacks and
    // that was is a noop.
    var component = dirtyComponents[i];
    // If performUpdateIfNecessary happens to enqueue any new updates, we
    // shouldn't execute the callbacks until the next render happens, so
    // stash the callbacks first
    var callbacks = component._pendingCallbacks;
    component._pendingCallbacks = null;
    ReactReconciler.performUpdateIfNecessary(component, transaction.reconcileTransaction, updateBatchNumber);
    if (callbacks) {
      for (var j = 0; j < callbacks.length; j++) {
        transaction.callbackQueue.enqueue(callbacks[j], component.getPublicInstance());
      }
    }
  }
}
function mountOrderComparator(c1, c2) {
  return c1._mountOrder - c2._mountOrder;
}

diff 算法

image 
_updateChildren: function (nextNestedChildrenElements, transaction, context) {
      var prevChildren = this._renderedChildren;
      var removedNodes = {};
      var mountImages = [];
      var nextChildren = this._reconcilerUpdateChildren(prevChildren, nextNestedChildrenElements, mountImages, removedNodes, transaction, context);
      if (!nextChildren && !prevChildren) {
        return;
      }
      var updates = null;
      var name;
      // `nextIndex` will increment for each child in `nextChildren`, but
      // `lastIndex` will be the last index visited in `prevChildren`.
      var nextIndex = 0;
      var lastIndex = 0;
      // `nextMountIndex` will increment for each newly mounted child.
      var nextMountIndex = 0;
      var lastPlacedNode = null;
      for (name in nextChildren) {
        if (!nextChildren.hasOwnProperty(name)) {
          continue;
        }
        var prevChild = prevChildren && prevChildren[name];
        var nextChild = nextChildren[name];
        if (prevChild === nextChild) {
          updates = enqueue(updates, this.moveChild(prevChild, lastPlacedNode, nextIndex, lastIndex));
          lastIndex = Math.max(prevChild._mountIndex, lastIndex);
          prevChild._mountIndex = nextIndex;
        } else {
          if (prevChild) {
            // Update `lastIndex` before `_mountIndex` gets unset by unmounting.
            lastIndex = Math.max(prevChild._mountIndex, lastIndex);
            // The `removedNodes` loop below will actually remove the child.
          }
          // The child must be instantiated before it's mounted.
          updates = enqueue(updates, this._mountChildAtIndex(nextChild, mountImages[nextMountIndex], lastPlacedNode, nextIndex, transaction, context));
          nextMountIndex++;
        }
        nextIndex++;
        lastPlacedNode = ReactReconciler.getHostNode(nextChild);
      }
      // Remove children that are no longer present.
      for (name in removedNodes) {
        if (removedNodes.hasOwnProperty(name)) {
          updates = enqueue(updates, this._unmountChild(prevChildren[name], removedNodes[name]));
        }
      }
      if (updates) {
        processQueue(this, updates);
      }
      this._renderedChildren = nextChildren;
    },

综上，在开发中，保持稳定的结构有助于性能提升，当有一组节点时，除了要设置 key，也要避免将靠后的节点移动到靠前的位置

一些其他的点

interface(ReactClass.js)

var ReactClassInterface = {
  mixins: 'DEFINE_MANY',
  statics: 'DEFINE_MANY',
  propTypes: 'DEFINE_MANY',
  contextTypes: 'DEFINE_MANY',
  childContextTypes: 'DEFINE_MANY',
  // ==== Definition methods ====
  getDefaultProps: 'DEFINE_MANY_MERGED',
  getInitialState: 'DEFINE_MANY_MERGED',
  getChildContext: 'DEFINE_MANY_MERGED',
  render: 'DEFINE_ONCE',
  // ==== Delegate methods ====
  componentWillMount: 'DEFINE_MANY',
  componentDidMount: 'DEFINE_MANY',
  componentWillReceiveProps: 'DEFINE_MANY',
  shouldComponentUpdate: 'DEFINE_ONCE',
  componentWillUpdate: 'DEFINE_MANY',
  componentDidUpdate: 'DEFINE_MANY',
  componentWillUnmount: 'DEFINE_MANY',
  // ==== Advanced methods ====
  updateComponent: 'OVERRIDE_BASE'
};
function validateMethodOverride(isAlreadyDefined, name) {
  var specPolicy = ReactClassInterface.hasOwnProperty(name) ? ReactClassInterface[name] : null;
  // Disallow overriding of base class methods unless explicitly allowed.
  if (ReactClassMixin.hasOwnProperty(name)) {
    !(specPolicy === 'OVERRIDE_BASE') ? process.env.NODE_ENV !== 'production' ? invariant(false, 'ReactClassInterface: You are attempting to override `%s` from your class specification. Ensure that your method names do not overlap with React methods.', name) : _prodInvariant('73', name) : void 0;
  }
  // Disallow defining methods more than once unless explicitly allowed.
  if (isAlreadyDefined) {
    !(specPolicy === 'DEFINE_MANY' || specPolicy === 'DEFINE_MANY_MERGED') ? process.env.NODE_ENV !== 'production' ? invariant(false, 'ReactClassInterface: You are attempting to define `%s` on your component more than once. This conflict may be due to a mixin.', name) : _prodInvariant('74', name) : void 0;
  }
}

可以看到，和后端中interface(或是抽象类)还是有区别的，但是可以起到规范和检查的作用，实际项目中可以借鉴

image
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