Data binding concepts, Bindings in WinJS George Georgiev Telerik Software Academy academy.telerik.com Technical Trainer itgeorge.net

2  Data Binding  Concepts  WinJS API for bindings  WinJS Binding Basics  Attributes, observables, binding data and styles  Binding converters  Using templates

Concepts, Advantages, WinJS APIs

4  Data binding connects UI and business logic  The UI layer describes how it visualizes data  Properties from sources mapped to UI elements  The Business layer describes the data itself  Defines objects and processes  The Data Binding API handles data updates  Visualizes data from business logic in the UI  Takes UI data & updates object properties  In good implementations, business logic has no idea there is UI

 Advantages to Data Binding  Mechanism to detach UI from business logic  Developer writes less (none) UI interaction code  UI design is independent of development  Data sources can be easily mocked and simulated  Disadvantages  A bit slower than direct UI interaction  Overhead in detecting data changes 5

 WinJS supports Data binding  Through WinJS.Binding namespace  One-way  Changes to JS objects trigger changes to UI  UI changes DON'T update JS objects  Provides attributes to bind UI controls  HTML elements  WinJS controls 6

Element attributes, bindable objects, data & style binding

 WinJS bindings use data attributes plus JS code to bind HTML elements  data-win-bind – object to element properties  Format:  Example:  WinJS.Binding.processAll()  Takes a root element and binding context object  Traverses elements and binds them with context 8 "elementProperty1: objectProperty1; elementProperty2: objectProperty2.subProperty" elementProperty2: objectProperty2.subProperty" WinJS.Binding.processAll(document.body, {text: 'hi'}) Same as calling with null

 More detail on WinJS.Binding.processAll():  Parses & applies data-win-bind attributes  No data binding occurs if not called  Second parameter defines object context  Determines which object is searched for the properties referenced in data-win-bind  When called with no parameters:  Traverses the entire DOM and applies bindings  Only bindings to global objects will work 9

 Notes on binding  Always have WinJS.Binding.optimizeBindingReferences  Prevents memory leaks from binding  Never bind to element IDs  Have a collection of your objects and bind them to the document  E.g. a namespace with all objects to display  Access appropriate object for element through "." operator 10

Live Demo

Enabling automatic updates to UI on object changes

 Binding simple objects doesn't enable updates  WinJS needs notification of property changes  Property changes typically handled in setters  Setter gives value then calls an API's function  Mixins in WinJS.Binding enable this on objects  Several methods to convert JS objects to WinJS observables in WinJS.Binding:  as() – object to observable  define() – object to constructor for observables  Note: work properly ONLY with DTOs DTO 13

 Using WinJS.Binding.as()  Takes an object to make observable  Returns the observable object  Keeps all members and their values  All members become observable properties  Methods and properties don't bind properly  Changes to the object are reflected in its binding 14 var personObservable = WinJS.Binding.as({ name: 'Joe', age: 21}); name: 'Joe', age: 21}); WinJS.Binding.processAll(null, personObservable); setInterval(function(){personObservable.age++;}, 1000)

 Using WinJS.Binding.define()  Takes a normal object template and makes a constructor for such observable objects  Returns the object constructor  All instances become observable properties  Methods & properties don't bind properly 15 var ObservablePerson = WinJS.Binding.define({ name: "", age: 0}); name: "", age: 0}); var person = new ObservablePerson(); WinJS.Binding.processAll(null, person); person.name = "Joe", person.age = 21; setInterval(function(){personObservable.age++;}, 1000)

Live Demo

Translating object info into UI appropriate info

 Bound property values often need conversion  To UI friendly data such as styles, HTML, etc.  To user-friendly texts (3.99 -> $3,99)  Not correct to have UI data in data objects  Such as styles, formatting, etc.  Remember: data should not be concerned with how it's displayed  Binding APIs usually support some type of Value Converters  Compute the representation of a value 18

 WinJS.Binding.converter() function  Receives the function to convert a value  Converter passes the function the value  Function should return converted value  Returns the converter function  Called inside a data-win-bind attribute value 19 <div data-win-bind='style.backgroundColor: frequency MyConverters.lightWavelengthToColor' frequency MyConverters.lightWavelengthToColor'</div> MyConverters.wavelengthToColor = WinJS.Binding.converters(function(wavelength){ /*return computed CSS color value*/...}); /*return computed CSS color value*/...});

Live Demo

Defining and reusing formatting for bindings

 Reusable declarative binding elements  WinJS.Binding.Template  Can contain any piece of HTML  Invisible; "instances" are visible when rendered  Can be rendered to the DOM several times with different bound objects  Two ways to initialize templates  Declaratively (needs WinJS.UI.processAll())  JS code, given element or HTML fragment URI  Both ways require some initial HTML code 22

 Creating Templates Declaratively  Write the HTML for the template  data-win-control='WinJS.Binding.Template' attribute in the container  Call WinJS.UI.processAll() to parse controls  Template container needs to be in the HTML code of the processed page to be parsed  WinJS will hide the control from the DOM  Access the element through container.winControl 23

Live Demo

 Creating Templates in JavaScript  Write the HTML for the template  Can be in an external file  Call constructor for WinJS.Binding.Template  Pass an HTML element containing the HTML for the template  Or pass an options object with a href member pointing to a HTML fragment for the template 25 var template = new WinJS.Binding.Template(element) var template = new WinJS.Binding.Template(null, { href: '/my-templates/sample-template.html}) href: '/my-templates/sample-template.html})

Live Demo

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1. Read this article (and its previous parts) on efficient conversion of objects to WinJS observables this article (and its previous parts)this article (and its previous parts) 2. Implement a Repeater control. A repeater renders a collection of objects, according to an item template  The Repeater is initialized with a WinJS.Binding.Template  Each item the repeater renders uses the template  Items can be wrapped additionally in DIVs or SPANs  The Repeater has a render method, which takes a collection of objects and a DOM element to which to render them  If the DOM element is not provided, the repeater creates a new DIV and uses it, appending to document  The Repeater should place custom CSS styles to each item it renders, as well as to the container of the items

3. Write an application which enables users to create computer descriptions.  A computer description has all the properties of the computers from demos in this lecture + rating.  Rating is not defined on creation  The application should enable users to save descriptions and should reload all descriptions and visualize them when it starts  Use the repeater from the last  Enable rating computers in the visualized list 29