Force a computed property function to run

Question

Given a computed property

vm.checkedValueCount = ko.computed(function(){   var observables = getCurrentValues();  //an array of ko.observable[]   return _.filter(observables, function(v) { return v() }).length; }); 

suppose getCurrentValues() can return different sets of observables which are modified elsewhere in the code (and comes from a more complex structure than an observableArray).

I need checkedValueCount to update whenever

• one of its dependencies change
• getCurrentValues() returns a different set of observables.

The problem is that ko.computed seems to memoize the last returned value and only update when a dependency updates. This handles the first case but not the latter.

What I'm looking for is a way to force checkedValueCount to re-run. Something which I can use like:

changeCurrentValues(); vm.checkeValueCount.recalculate(); 

Put most simply, given that I have

a = ko.computed(function() { return Math.random() }) 

how can I force invoking a() twice to return different values.

Answer 1

I realized my first answer missed your point, and won't solve your issue.

The problem is that a computed will only reevaluate if there is some observable that forces it to re-evaluate. There is no native way to force a computed to re-evaluate.

However, you can get around this with some hackery by creating a dummy observable value and then telling its subscribers that it has changed.

(function() {      var vm = function() {         var $this = this;          $this.dummy = ko.observable();          $this.curDate = ko.computed(function() {             $this.dummy();             return new Date();         });          $this.recalcCurDate = function() {             $this.dummy.notifySubscribers();         };             };      ko.applyBindings(new vm());  }());​ 

Here is a Fiddle showing this approach

Answer 2

There is a method to force recalculation of all observables depending on yours:

getCurrentValues.valueHasMutated()