knockout源码分析之computed(依赖属性)

一、序列图

knockout源码分析之computed(依赖属性)

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。

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