This is the part that holds the data and which image is selected, and later on we’ll also deal with what happens if the user changes the selection. If we take the terms in the MVVM pattern name in order then the view is up next, but it will make more sense to cover the view model first. Image 1 Caption Image 2 Caption Image 3 Caption. In this way the basic image list will still be available to browsers that can’t apply the richer UI. We’ll replicate this data in a new HTML structure, so in the spirit of progressive enhancement we can hide the original markup using JavaScript during page load. First is the data, or model, which in this case comes from a list of links to images in an HTML document.įrom here we can use a DOM query to extract the URL of each image and its related caption and supply them to the view model using an initialisation function. Let’s work through the three main parts which make up the demo. This a great blog by Ryan Niemeyer on getting to know Knockout and using it as an essential part of your development toolkit (opens in new tab) Getting started I’m using jQuery to initialise the page and view model, but there’s no reason why you couldn’t replace this with your framework of choice – or pure JavaScript. Knockout is compatible down to IE6 and doesn’t depend on any other JavaScript library, so I’ve kept the demo framework-agnostic where possible. The view model is purely dealing with data, and this loose coupling means it’s really easy to build logical, testable components you can fit together however you like. You can bind as many elements as you like to the same part of the view model, and if a binding is not made then there are no ill-effects so you can reuse the same logic in many different situations. UI updates are handled through data bindings in the HTML if these are present the app will work regardless of how it looks. The key principle to remember when developing with a view model is that it doesn’t have, or need, knowledge of how the DOM is structured or laid out. The online tutorials that introduce Knockout are brilliant pieces of work in themselves (opens in new tab) Knockout is a JavaScript implementation of the Model-View-View Model pattern, a way to define data in a model object and then bind DOM elements or templates to that data. It’s also more robust and testable, so whether you’re working on your own or in a team it’s a great way to make life easier for everyone. In rich UI development this can greatly simplify the process of loading and updating data. You might already use event handler bindings in jQuery or other JavaScript libraries to connect user actions with parts of a page, but with Knockout we can go one step further and use JavaScript to automatically populate the interface – so whenever the data or state changes so does the UI. Perhaps a better way to approach things is to have a clean separation of presentation and logic, and that’s where frameworks such as Knockout come in. But as soon as a reasonable amount of data or the need to keep track of UI state in your application is added to the mix then it can start to cause a bit of a problem.įor interfaces where the user can page through data, change the appearance or position of components on the page or make selections or filters that need to persist, trying to rely on DOM inspection to understand where things are is likely to end in frustration. load callback i now do this: if(loadedPages = doc.pages().If you’re working with a fairly simple, content-based website then your JavaScript needn’t get too complicated perhaps a lightbox effect for an image gallery and some form validation. load because it only updates the DOM once every network request has finished, allowing me to up a counter for every iteration within the code block to indicate to the user that the load is underway (e.g. load in order to populate an array with actual image data: images.pagenumber() - 1] = $("").attr('src', path).load(function() That was the why, and heres the description of my current "how":ġ: I use jQuery. Also, i wan't to provide an indicator to the user that something is going on during the long wait. Therefore my UI (which can browse between sets of images) aims to only download these sets when necessary in order to reduce load. The point of the exercise is that the images I am loading take quite some time to download, in addition to being processed first on the server. Using knockout.js I'm trying to achieve what essentially is an image preloader and cacher.
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