Get the previous observed value in the subscription of the same observed

Question

Is it possible to knock out the current value of the observable within the subscription to the observable before it receives a 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 set to newValue }); 

Answer 1

There is a way to subscribe to a value before the following:

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

Answer 2

 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:

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

Answer 3

A slight change in Beagle90's answer. Always return a subscription to be able to access the utility (), for example.

 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

pulling a request to add this function has a different code that ends better than relying on the beforeChange event.

All credits for Michael Best solution

 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 proposed using beforeChange to solve this problem, but since then I realized that it is not always reliable (for example, if you call valueHasMutated() on the observable).

Answer 5

I found that I can call peek () from a writeable record 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); } });