knockout.js - Possible to fire custom binding update except initially?

Question

Let's say I have this:

ko.bindingHandlers.test= {
    update: function (element, valueAccessor) {
        alert("Test");
    }
};

The alert fires every time an observable is changed, but also initally when the binding is first evaluated. How can I make the alert fire on every change except initially?

Answer 1

Here's one way: keep an array of elements that the update populates with its element if it's not there (which is the first time it runs) and otherwise does whatever action. Since you've got a custom binding handler, you can just hang the array off of that.

ko.bindingHandlers.test = {
  update: function(element, valueAccessor) {
    var seenElements = ko.bindingHandlers.test.seen,
      val = valueAccessor()();
    if (seenElements.indexOf(element) >= 0) {
      alert("Test");
    } else {
      seenElements.push(element);
    }
  }
};
ko.bindingHandlers.test.seen = [];

var vm = {
  alertOn: ko.observable(0),
  raiseAlert: function() {
    vm.alertOn.notifySubscribers();
  }
};
ko.applyBindings(vm);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<div data-bind="test:alertOn"></div>
<button data-bind="click:raiseAlert">Update</button>