Subscribe to observable array for new or removed entry only

Question

So yes I can subscribe to an observable array:

vm.myArray = ko.observableArray(); vm.myArray.subscribe(function(newVal){...}); 

The problem is the newVal passed to the function is the entire array. Is there anyway I can get only the delta part? Say the added or removed element?

Answer 1

As of KnockoutJS 3.0, there's an arrayChange subscription option on ko.observableArray.

var myArray = ko.observableArray(["Alpha", "Beta", "Gamma"]);  myArray.subscribe(function(changes) {      // For this example, we'll just print out the change info     console.log(changes);  }, null, "arrayChange");  myArray.push("newitem!"); 

In the above callback, the changes argument will be an array of change objects like this:

[     {        index: 3,        status: 'added',        value: 'newitem!'     } ] 

For your specific problem, you want to be notified of new or removed items. To implement that using Knockout 3, it'd look like this:

myArray.subscribe(function(changes) {      changes.forEach(function(change) {         if (change.status === 'added' || change.status === 'deleted') {             console.log("Added or removed! The added/removed element is:", change.value);         }     });  }, null, "arrayChange");