knockout源码分析之computed（依赖属性）

一、序列图

二、主要代码文件

1、dependentObservable.js：主要包含ko.computed相关方法的处理
2、dependencyDetection.js：主要包含依赖的监控上下文对象。

三、主要逻辑

1、首先为某个属性定义 一个computed对象，如下源码：

var vModel = function(){    this.fName = ko.observable('fName'),    this.lName= ko.observable('lName'),    this.name= ko.computed(function () { //监控依赖对象      return this.fName() + '-' + this.lName();    },this);  };

2、当代码在执行ko.computed方法，求值方法被作为参数传入，并赋值给options的read属性
3、创建一个state字面量对象，其中包含read、write属性，如下代码：

var state = {    latestValue: undefined,    isStale: true,    isBeingEvaluated: false,    suppressDisposalUntilDisposeWhenReturnsFalse: false,    isDisposed: false,    pure: false,    isSleeping: false,    readFunction: options["read"],    evaluatorFunctionTarget: evaluatorFunctionTarget || options["owner"],    disposeWhenNodeIsRemoved: options["disposeWhenNodeIsRemoved"] || options.disposeWhenNodeIsRemoved || null,    disposeWhen: options["disposeWhen"] || options.disposeWhen,    domNodeDisposalCallback: null,    dependencyTracking: {},    dependenciesCount: 0,    evaluationTimeoutInstance: null  };

4、生成computedObservable对象(function)，然后将state附加到_state属性上，则扩展为发布/订阅对象。
5、扩展computedFn所有方法和属性到computedObservable对象上

// Inherit from 'subscribable'  if (!ko.utils.canSetPrototype) {    // 'subscribable' won't be on the prototype chain unless we put it there directly    ko.utils.extend(computedObservable, ko.subscribable['fn']);  }  ko.subscribable['fn'].init(computedObservable); //执行发布/订阅对象的init方法，用于初始化发布/订阅对象。  // Inherit from 'computed'  ko.utils.setPrototypeOfOrExtend(computedObservable, computedFn);

6、然后执行computedObservable的evaluateImmediate方法，此方法中最重的三点：
   6.1、在evaluateImmediate_CallReadWithDependencyDetection方法中，创建了依赖监控对象，并添加到依赖监控上下文中

var isInitial = state.pure ? undefined : !state.dependenciesCount,  // If we're evaluating when there are no previous dependencies, it must be the first time      dependencyDetectionContext = {        computedObservable: computedObservable,        disposalCandidates: state.dependencyTracking,        disposalCount: state.dependenciesCount      };    ko.dependencyDetection.begin({      callbackTarget: dependencyDetectionContext,      callback: computedBeginDependencyDetectionCallback,      computed: computedObservable,      isInitial: isInitial    });

    6.2、然后调用evaluateImmediate_CallReadThenEndDependencyDetection方法，参数传递的state（在ko.computed方法中定义的）、dependencyDetectionContext（依赖监控对象）
    6.3、其中用到了try catch finall方式，确保ko.dependencyDetection.end方法的执行

try {      var readFunction = state.readFunction;      return state.evaluatorFunctionTarget ? readFunction.call(state.evaluatorFunctionTarget) : readFunction();    } finally {      ko.dependencyDetection.end();      // For each subscription no longer being used, remove it from the active subscriptions list and dispose it      if (dependencyDetectionContext.disposalCount && !state.isSleeping) {        ko.utils.objectForEach(dependencyDetectionContext.disposalCandidates, computedDisposeDependencyCallback);      }      state.isStale = false;    }

7、在执行ko.computed的readFunction方法时，其中就执行了ko.observable方法(执行的是read)，这时就会去调用ko.dependencyDetection.registerDependency方法（参数为此函数对象）

function observable() {    if (arguments.length > 0) {      // Write      // Ignore writes if the value hasn't changed      if (observable.isDifferent(observable[observableLatestValue], arguments[0])) {        observable.valueWillMutate();        observable[observableLatestValue] = arguments[0];        observable.valueHasMutated();      }      return this; // Permits chained assignments    }    else {      debugger;      // Read      ko.dependencyDetection.registerDependency(observable); //执行依赖      return observable[observableLatestValue];    }  }

8、在ko.dependencyDetection中的registerDependency方法内，首先会判断ko.observable是否为订阅对象，如果是则执行begin加入的callbak函数.

registerDependency: function (subscribable) { //注入到相关依赖属性      if (currentFrame) {        if (!ko.isSubscribable(subscribable))          throw new Error("Only subscribable things can act as dependencies");        currentFrame.callback.call(currentFrame.callbackTarget, subscribable, subscribable._id || (subscribable._id = getId()));      }    }

9、执行evaluateImmediate方法后，然后注册Dom移除回调事件。

if (state.disposeWhenNodeIsRemoved && computedObservable.isActive()) {    ko.utils.domNodeDisposal.addDisposeCallback(state.disposeWhenNodeIsRemoved, state.domNodeDisposalCallback = function () {      computedObservable.dispose();    });  }

10、返回computedObservable对象

四、补充说明

1、ko.dependencyDetection中有ignore方法，他主要实现的是一个异步锁，让callbcak处于锁的状态执行

ignore: function (callback, callbackTarget, callbackArgs) { //按顺序s执行依赖，但不触发订阅。      try {        begin();        return callback.apply(callbackTarget, callbackArgs || []);      } finally {        end();      }    }

2、ko.computed 与 ko.dependentObservable是相同的。

原标题：knockout源码分析之computed（依赖属性）

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