在Knockout的afterAdd回调中包含fadeIn（）

Question

I am trying to learn the foreach binding and I can't understand why the $(element).fadeIn(500) line in the code below isn't working:

 ko.applyBindings({ myItems: ko.observableArray([ 'A', 'B', 'C' ]), FadeIn: function(element, index, item) { if(element.nodeType == 1){ $(element).fadeIn(500); } }, addItem: function() { this.myItems.push('New item'); } }); 

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script> <ul data-bind="foreach: { data: myItems, afterAdd: FadeIn }"> <li data-bind="text: $data"></li> </ul> <button data-bind="click: addItem">Add</button> 

The problem is that when I add a new item, it appears in the page without the fadeIn effect.

Codepen -> https://codepen.io/anon/pen/ejxeBr

Answer 1

It's because your element isn't hidden. Change it to

$(element).hide().fadeIn(500);
// --------^^^^^^^

...or set style="display: none" on it in the markup, but that could be a pain because of the pre-existing elements (which won't get afterAdd calls).

Example:

 var count = 0; ko.applyBindings({ myItems: ko.observableArray(['A', 'B', 'C']), FadeIn: function(element, index, item) { if (element.nodeType == 1) { $(element).hide().fadeIn(500); } }, addItem: function() { this.myItems.push('New item'); } }); 

 <ul data-bind="foreach: { data: myItems, afterAdd: FadeIn }"> <li data-bind="text: $data"></li> </ul> <button data-bind="click: addItem">Add</button> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>