knockout js fade transition binding handler when image src binding changes

if I have the following simple js/knockout code:

.js (viewModel):

var image = ko.observable('http://placehold.it/300x150');

view:

<img data-bind={attr: {src: image}} />

How do I create a ko binding handler that will perform a simple cross-fade when the image property changes?

I think the best solution is to create another image and fade it in.

Here is a simple binding handler that will create a new image (or a div with a background image in this case) within the bound element every time the source observable is updated and fade to it with a CSS3 animation:

ko.bindingHandlers.transitionImage = {
    init: function (element, valueAccessor) {
        // create a holder for the images
        var holder = document.createElement('div');
        holder.style['position'] = 'relative';
        holder.style['height'] = '100%';
        holder.style['width'] = '100%';
        element.appendChild(holder);

        // create initial image
        var url = ko.utils.unwrapObservable(valueAccessor());
        var fore = document.createElement('div');
        fore.style['background-image'] = 'url(' + url + ')';
        fore.style['position'] = 'absolute';
        fore.style['height'] = '100%';
        fore.style['width'] = '100%';
        fore.style['background-size'] = '100% 100%'
        holder.appendChild(fore);
    },
    update: function (element, valueAccessor) {
        var url = ko.utils.unwrapObservable(valueAccessor());

        // retrieve the holder element
        var holder = element.childNodes[0];

        // create new image to transition to
        var fore = document.createElement('div');
        fore.className = 'transition';
        fore.style['background-image'] = 'url(' + url + ')';
        fore.style['position'] = 'absolute';
        fore.style['height'] = '100%';
        fore.style['width'] = '100%';
        fore.style['background-size'] = '100% 100%'
        fore.style['opacity'] = '0'; // ensure it is hidden
        holder.appendChild(fore);

        // CSS3 animate opacity
        setTimeout(function () {
            fore.style['opacity'] = '1';
        }, 0);

        // dispose of unnecessary hidden images eventually
        setTimeout(function () {
            var toRemove = holder.childNodes[0];
            var removed = holder.removeChild(toRemove);
            delete removed;
        }, 10000);
    }
};

It requires some css in place for the opacity to get animated:

div.transition {
    -webkit-transition: opacity 0.5s linear;
    -moz-transition: opacity 0.5s linear;
    -o-transition: opacity 0.5s linear;
    transition: opacity 0.5s linear;
}

To use, bind it to a div container:

<div data-bind="transitionImage: image" style="width: 100px; height: 100px"></div>

The advantage with this approach is that it is quite easy to substitute in more complex animations like sliding by just playing with the css.

Here is a fiddle for it!