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MASTERING KNOCKOUTJS
MASTERING KNOCKOUTJS

MASTERING KNOCKOUTJS: Use and extend Knockout to deliver feature-rich, modern web applications

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MASTERING KNOCKOUTJS

Chapter 2. Extending Knockout with Custom Binding Handlers

Knockout's standard bindings are great. They solve most of the general problems you are likely to encounter when developing web apps. But there is always the need to provide something special, whether you are working on your own library or just trying to add a bit of style to your app. When that happens, you will want to provide that functionality through the same binding system you are using everywhere else. Luckily, Knockout makes extending this system easy. In this chapter, we will be looking at how to make our own binding handlers. We will be covering the following topics:

  • What a binding handler contains
  • Creating new binding handlers
  • Using custom binding handlers to integrate with third-party libraries
  • Managing binding contexts
  • Using the containerless control flow syntax with custom bindings

Creating custom binding handlers for new and more complex HTML interactions is a key to developing feature-rich applications....

The data binding pattern

This section is primarily philosophical. If you feel like you already have a solid understanding of the what and why behind the Model-View-ViewModel (MVVM) pattern and binding handlers, then you might want to skip to the next section, Components of a binding handler.

Okay, let's talk about patterns and practices. If you haven't worked with WPF before, then the MVVM pattern is probably the most confusing thing about Knockout. MVVM is a pattern that came out of Microsoft. It doesn't get a lot of attention outside the .NET community, and it's similar enough to the far more popular MVC pattern because of which confusion is nearly guaranteed.

In MVVM, the viewmodel is supposed to represent an abstraction of the view. Consider these two lists of message threads in iOS:

The data binding pattern

They both show a list of threads, and each thread contains a title showing the person it is with, an excerpt from the most recent message, and a timestamp. A thread can be selected or...

Components of a binding handler

Binding handlers are defined by adding objects to the ko.bindingHandlers object, just like extenders. They are composed of an init and an update function.

The init function runs when the binding is first applied to the element either when ko.applyBindings is called or when the element is created by a control flow binding, such as template or foreach. It should be used for all one-time work such as attaching event handlers or disposal callbacks to the element.

The update function runs just after init does, when the binding is first applied. It runs again anytime when any observable dependencies are changed. The update function determines its dependencies just like a computed observable does. If an observable is accessed when an update runs, it subscribes to that observable. The update function should be used to keep the UI in sync with changes from the viewmodel:

ko.bindingHandlers.yourBindingName = {
    init: function(element, valueAccessor, allBindings, viewModel...

Simple binding handlers

Binding handlers can range from very simple to whole applications by themselves. As the purpose of binding handlers is to translate between the presentation layer (HTML) and the viewmodel (JavaScript), the binding handler's complexity is directly related to the complexity of the UI interaction and the bound data. Simple tasks such as hiding or showing an element with animation will have very thin handlers, while data binding on an interactive map element will require much more logic.

Animated binding handlers

As DOM interaction in the primary use case for jQuery, and given its popularity, it is not uncommon to use jQuery inside Knockout binding handlers. The canonical custom binding handler example from the Knockout documentation is a binding to hide and show elements, with the jQuery's slideUp and slideDown methods, instead of using the standard visible binding to switch them on and off:

ko.bindingHandlers.slideVisible = {
    init: function(element, valueAccessor...

Advanced binding handlers

So far, we've been looking at binding handlers that handle one or two properties and result in a fairly simple single-purpose control. In the previous example, we started looking at binding handlers that created new child elements, and this technique allows us to create much more complex binding behaviors. Bindings can also interact with complex elements such as charts or map controls (for example, a Google Maps widget), providing a clean API that the viewmodel can interact with.

Binding complex data with charts

The first time we looked at integrating with a third-party control was with a single-property two-way binding to a datepicker. Any time we are working with third-party UI tools, the goal is to abstract them away from the view and the viewmodel through bindings; even when those tools are for complex structures such as charts.

Charts.js (http://www.chartjs.org) is a popular JavaScript library built to display data in, you guessed it right, graphical charts...

Containerless syntax with custom bindings

In the first chapter, we spoke about containerless bindings; bindings applied through comments that created a virtual container around their "child" nodes. Now that we have a good understanding of how to create our own binding handlers, it's time to learn how to make them containerless bindings.

First, we are going to make a normal binding and then look at what we need to do to allow it to support the virtual elements. Let's say you want a binding that sorts its children elements. It would need to loop through them, check some property, and then rearrange the DOM so they were in order. Normally, sorting would be achieved by using a foreach binding against a sorted observableArray property, but we're going to make a sort binding that sorts on the width of the DOM node, which takes into account any CSS that may have affected it. The viewmodel would have a hard time getting this information to determine the proper sort order...

Summary

If you take away one thing from all these examples, it should be that binding handlers are solely responsible for interaction with the DOM. In our first example, we made the slideVisible binding as an animated replacement for the standard visible binding. This change from the normal "instant" hide and show to the "animated" hide and show was completely decoupled by our viewmodel. This is beneficial because it keeps these two pieces completely separated, allowing them to develop and evolve independently.

In this chapter, we covered simple and complex binding handlers, binding context management, and using the virtual elements API to support containerless bindings. In the next chapter, we will be looking at preprocessors for bindings and nodes.

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Description

If you are an experienced JavaScript developer who is looking for new tools to build web applications and get an understanding of core elements and applications, this is the book for you. A basic knowledge of DOM, JavaScript, and KnockoutJS is assumed.
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Publication date : Nov 26, 2014
Length: 270 pages
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Language : English
ISBN-13 : 9781783981007
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Table of Contents

10 Chapters
1. Knockout Essentials Chevron down icon Chevron up icon
2. Extending Knockout with Custom Binding Handlers Chevron down icon Chevron up icon
3. Extending Knockout with Preprocessors and Providers Chevron down icon Chevron up icon
4. Application Development with Components and Modules Chevron down icon Chevron up icon
5. Durandal – the Knockout Framework Chevron down icon Chevron up icon
6. Advanced Durandal Chevron down icon Chevron up icon
7. Best Practices Chevron down icon Chevron up icon
8. Plugins and Other Knockout Libraries Chevron down icon Chevron up icon
9. Under the Hood Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

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Christopher Fleming Sep 08, 2015
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I am experienced with knockout and yet this well written book yielded many new priceless undiscovered bits of information. This compliments and expands on the excellent knockout site and a must for any developer serious about mastering knockout end-to-end.
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Christian Del Bianco Oct 25, 2015
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Clear explanations, NOT boring at all. Correlated with many examples. I also appreciate the introduction to Durandal and RequireJS frameworks.
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Evan Nelson Jan 27, 2015
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In my experience, programming books always seem to have one of two problems: 1) they are targeted at first-time programmers and thus are way too dumbed down, or 2) they are targeted at people who are already experts in the subject and just need some sort of reference. Where are all the books for experienced developers who were able to sort out the basics of TechnologyX on their own, but who need a practical guide for the more advanced topics?For me, this book hits that very important sweet spot. It does not assume its audience to be beginning programmers, but neither does it throw around terms like "polyfill" without giving some sort of definition. Every chapter is littered with code samples, each one following logically from the previous. The samples are concise, including only the relevant details, but full context can be found by looking at the author's github repository (every sample has its own branch in the repo). Each technique that the book teaches you is backed by a real-world example as opposed to a generic "FooBarBaz" example, making it easy to understand *why* you would do something, instead of just telling you *how*.Overall, a great resource for anyone who already understands the basics of web development wants to learn to use Knockout to its fullest extent.
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Travis Wilson May 10, 2015
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This book actually covers expert-level Knockout. A must buy if you're serious about taking it to the next level.
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Christian Del Bianco Jul 08, 2016
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Very detailed book about knockout. Also, there is an insteresting part about Durandal - knockout framwork for SPA. I found it very usefull. Be aware that this book is not an intro. It is an advance level.
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