防止计算收集依赖项

Question

有没有好办法大致实现如下概念：

var computed = ko.computed(function() {
    readSomeObservables(); //<-- if these change, notify computed
    ko.stopCollectingDependencies();
    readSomeMoreObservables(); //<-- if these change, do not notify computed
    ko.resumeCollectingDependencies();
});

我知道peek()，但在这种情况下，计算是调用从外部模块提供的方法，如果这些方法碰巧涉及可观察对象，设计要求它纯粹是偶然的。

我有一个解决办法，大致是这样的：

window.setTimeout(function() {
    readSomeMoreObservables();
}, 0);

但由于显而易见的原因，这并不理想，并且在某些情况下会导致不良行为。

Answer 1

为以后的访问者...

3.3 版公开了 ko.ignoreDependencies(callback, callbackTarget, callbackArgs)。这是绑定处理程序处理在内部使用的方法，以避免从 init 函数调用创建依赖关系。

见http://www.knockmeout.net/2015/02/knockout-3-3-released.html

Answer 2

我有点晚了，但这是我在这种情况下使用的解决方案：

var computed = ko.computed(function() {
    var a = readSomeObservables(); //<-- if these change, notify computed
    var b;

    // capture any dependencies from readSomeMoreObservables
    // in a temporary computed, then immediately dispose
    // of it to release those captured dependencies
    ko.computed(function() { b = readSomeMoreObservables(); }).dispose();

    return a + b; // or whatever
});

这会创建一个临时计算，我们称之为readSomeMoreObservables。临时计算设置了一个新的依赖捕获帧，因此所有读取的可观察数据都被捕获到我们的临时计算中。然后我们立即处理临时计算以释放它捕获的任何依赖项。

Answer 3

组合怎么样。为您需要阅读但不想订阅的可订阅内容创建一个临时计算。更改它们会更新计算的温度，但这可能是一个便宜的操作。您的实际计算读取 tempComputed 并 peek 访问当前缓存的值。

// this one is updated 
// if any of the subscribeables used in readSomeMoreObservables changes
// but that is hopefully cheap
var tempComputed = ko.computed(function() {
    readSomeMoreObservables();
});


var computed = ko.computed(function() {
    readSomeObservables(); //<-- if these change, notify computed

    // do not update on readSomeMoreObservables
    tempComputed.peek(); 
});

Answer 4

Knockout 的依赖检测有一个ko.dependencyDetection.ignore 函数。如果我理解正确，您可以使用它来读取可订阅的值，而无需创建对它们的依赖。

至少运行以下测试：

it('Should not subscribe to subscribeables called by ignore', function() {


    var observableInner = ko.observable('initial'),
        observableOuter = ko.observable(),
        called = 0,
        computedInner = ko.computed(function() { return observableInner(); }),
        computedOuter = ko.computed(function() { 
            called += 1;
            // read dependend
            observableOuter();

            // read ignored
            var result = ko.dependencyDetection.ignore(computedInner, null)
            expect(result).toEqual('initial');

            return true;
        });

    expect(called).toEqual(1);

    // update the one we are depending on
    observableOuter(1);
    expect(called).toEqual(2);        

    // update the inner one which should trigger an update to computedInner but not to computedOuter
    observableInner('ignore');
    expect(called).toEqual(2);
});

Answer 5

如果您对这些可观察对象的新值不感兴趣，为什么不将可观察对象的读数移到计算范围之外？

http://jsfiddle.net/uUXWv/2

var observableTwoInitialState = this.observableTwo(); 

this.computed = ko.computed(function() {
    return this.observableOne() + observableTwoInitialState;
}, this);