在订阅相同的observable中获取observable的先前值

Question

在获得新值之前,是否可以在knockout中获取可观察的可观察量的当前值？

例:

this.myObservable = ko.observable();
this.myObservable.subscribe(function(newValue){
    //I'd like to get the previous value of 'myObservable' here before it's set to newValue
});

Answer 1

ko.subscribable.fn.subscribeChanged = function (callback) {
    var oldValue;
    this.subscribe(function (_oldValue) {
        oldValue = _oldValue;
    }, this, 'beforeChange');

    this.subscribe(function (newValue) {
        callback(newValue, oldValue);
    });
};

使用上面这样:

MyViewModel.MyObservableProperty.subscribeChanged(function (newValue, oldValue) {

});

Answer 2

有一种方法可以像这样订阅之前的值:

this.myObservable = ko.observable();
this.myObservable.subscribe(function(previousValue){
    //I'd like to get the previous value of 'myObservable' here before it's set to newValue
}, this, "beforeChange");

Answer 3

Beagle90的答案变化不大.始终返回订阅本身以便能够访问dispose().

ko.subscribable.fn.subscribeChanged = function (callback) {
    var oldValue;
    this.subscribe(function (_oldValue) {
        oldValue = _oldValue;
    }, this, 'beforeChange');

    var subscription = this.subscribe(function (newValue) {
        callback(newValue, oldValue);
    });

    // always return subscription
    return subscription;
};

Answer 4

添加此功能的pull请求有一些不同的代码,最终比依赖使用beforeChange事件更好.

迈克尔·贝斯特解决方案的所有功劳

ko.subscribable.fn.subscribeChanged = function (callback) {
    var savedValue = this.peek();
    return this.subscribe(function (latestValue) {
        var oldValue = savedValue;
        savedValue = latestValue;
        callback(latestValue, oldValue);
    });
};

引用迈克尔:

我最初建议beforeChange用来解决这个问题,但后来意识到它并不总是可靠的(例如,如果你调用valueHasMutated()了observable).