The difference is that it’s now capable of detecting changes, and when it does, it will update the view automatically. You don’t have to change the view at all - the same data-bind syntax will keep working. How can KO know when parts of your view model change? Answer: you need to declare your model properties as observables, because these are special JavaScript objects that can notify subscribers about changes, and can automatically detect dependencies.įor example, rewrite the preceding view model object as follows: But one of the key benefits of KO is that it updates your UI automatically when the view model changes. OK, you’ve seen how to create a basic view model and how to display one of its properties using a binding. This restricts the activation to the element with ID someElementId and its descendants, which is useful if you want to have multiple view models and associate each with a different region of the page. For example, ko.applyBindings(myViewModel, document.getElementById('someElementId')). Optionally, you can pass a second parameter to define which part of the document you want to search for data-bind attributes. The first parameter says what view model object you want to use with the declarative bindings it activates In case you’re wondering what the parameters to ko.applyBindings do, To create a view model with KO, just declare any JavaScript object. Alternatively, you can use templates that generate HTML using data from your view model. When using KO, your view is simply your HTML document with declarative bindings to link it to the view model. It displays information from the view model, sends commands to the view model (e.g., when the user clicks buttons), and updates whenever the state of the view model changes. Keeping the view model abstract in this way lets it stay simple, so you can manage more sophisticated behaviors without getting lost.Ī view: a visible, interactive UI representing the state of the view model. When using KO, your view models are pure JavaScript objects that hold no knowledge of HTML. It’s not the persisted data model either - it holds the unsaved data the user is working with. Note that this is not the UI itself: it doesn’t have any concept of buttons or display styles. For example, if you’re implementing a list editor, your view model would be an object holding a list of items, and exposing methods to add and remove items. When using KO, you will usually make Ajax calls to some server-side code to read and write this stored model data.Ī view model: a pure-code representation of the data and operations on a UI. This data represents objects and operations in your business domain (e.g., bank accounts that can perform money transfers) and is independent of any UI. It describes how you can keep a potentially sophisticated UI simple by splitting it into three parts:Ī model: your application’s stored data. Model-View-View Model (MVVM) is a design pattern for building user interfaces. But before that, let’s examine the MVVM pattern and the concept of a view model. On this page, you’ll learn about the first of these three. Knockout is built around three core features: Usage with AMD using RequireJs (Asynchronous Module Definition).Overview: What components and custom elements offer.Preprocessing: Extending the binding syntax. How KO works and what benefits it brings.
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