CODEWITHSUNDEEP
knockout.jsknockout-2.0observable
KodeKreachor
KodeKreachor

Reputation: 8882

Get previous value of an observable in subscribe of same observable

Is it possible in knockout to get the current value of an observable within a subscription to that observable, before it receives the new value?

Example:

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
});

Upvotes: 87

Views: 54762

Answers (5)

James Johnson
James Johnson

Reputation: 512

The pull request to add this feature has some different code that winds up being better than relying on using the beforeChange event.

All credit for the solution to Michael Best 

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

To quote Michael:

I originally suggested using beforeChange to solve this problem but have since realized that it's not always reliable (for example, if you call valueHasMutated() on the observable).

Upvotes: 18

Andries
Andries

Reputation: 1527

Little change to Beagle90 answer. Always return the subscription itself to be able to access the dispose() for instance.

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;
};

Upvotes: 22

rjmunro
rjmunro

Reputation: 28076

I have found that I can call peek() from a writable computed observable to get the before value.

Something like this (see http://jsfiddle.net/4MUWp):

var enclosedObservable = ko.observable();
this.myObservable = ko.computed({
    read: enclosedObservable,
    write: function (newValue) {
        var oldValue = enclosedObservable.peek();
        alert(oldValue);
        enclosedObservable(newValue);
    }
});

Upvotes: 4

JBeagle
JBeagle

Reputation: 2660

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

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

Use the above like this:

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

});

Upvotes: 154

RP Niemeyer
RP Niemeyer

Reputation: 114792

There is a way to do a subscription to the before value like this:

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");

Upvotes: 92

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