Possible to fire custom binding update except init

问题:

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?

回答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>

回答2:

How can I make the alert fire on every change except initially?

Knockout will call the update callback initially when the binding is applied to an element and track any dependencies (observables/computeds) that you access. When any of these dependencies change, the update callback will be called once again.

I don't think it is possible to fire only by changes and not in initially.

You can create a workaround to fit with your scenario by adding change event tied up with your element inside init.

init: function(element, valueAccessor) {
   // element should be the element you want to attach change event.
   $(element).on('change', function (value)
   {
      // do your stuff.
    });
}