The goal of this blog entry is to explain how to perform queries using selectors when using
the WinJS library. In particular, you learn how to use
the WinJS.Utilities.query() method and
the QueryCollection class to retrieve and modify
the elements of an HTML document.
Introduction to Selectors
When you are building a Web application, you need some way of easily retrieving elements from an HTML document. For example, you might want to retrieve all of
the input elements which have a certain class. Or, you might want to retrieve
the one and only element with an id of favoriteColor.
The standard way of retrieving elements from an HTML document is by using a selector. Anyone who has ever created a Cascading Style Sheet has already used selectors. You use selectors in Cascading Style Sheets to apply formatting rules to elements in a document.
For example,
the following Cascading Style Sheet rule changes
the background color of every INPUT element with a class of .required in a document to
the color red:
input.
red { background-color:
red }
The “input.red” part is
the selector which matches all INPUT elements with a class of
red.
The W3C standard for selectors (technically, their recommendation) is entitled “Selectors Level 3” and
the standard is located here:
http://www.w3.org/TR/css3-selectors/
Selectors are not only useful for adding formatting to
the elements of a document. Selectors are also useful when you need to apply behavior to
the elements of a document. For example, you might want to select a particular BUTTON element with a selector and add a click handler to
the element so that something happens whenever you click
the button.
Selectors are not specific to Cascading Style Sheets. You can use selectors in your JavaScript code to retrieve elements from an HTML document.
jQuery is famous for its support for selectors. Using jQuery, you can use a selector to retrieve matching elements from a document and modify
the elements.
The WinJS library enables you to perform
the same types of queries as jQuery using
the W3C selector syntax.
Performing Queries with
the WinJS.Utilities.query() Method
When using
the WinJS library, you perform a query using a selector by using
the WinJS.Utilities.query() method.
The following HTML document contains a BUTTON and a DIV element:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Application1</title>
<!-- WinJS references -->
<link href="//Microsoft.WinJS.0.6/css/ui-dark.css" rel="stylesheet">
<script src="//Microsoft.WinJS.0.6/js/base.js"></script>
<script src="//Microsoft.WinJS.0.6/js/ui.js"></script>
<!-- Application1 references -->
<link href="/css/default.css" rel="stylesheet">
<script src="/js/default.js"></script>
</head>
<body>
<button>Click Me!</button>
<div style="display:none">
<h1>Secret Message</h1>
</div>
</body>
</html>
The document contains a reference to
the following JavaScript file named \js\default.js:
(function () {
"use strict";
var app = WinJS.Application;
app.onactivated = function (eventObject) {
if (eventObject.detail.kind === Windows.ApplicationModel.Activation.ActivationKind.launch) {
WinJS.Utilities.query("button").listen("click", function () {
WinJS.Utilities.query("div").clearStyle("display");
});
}
};
app.start();
})();
The default.js script uses
the WinJS.Utilities.query() method to retrieve all of
the BUTTON elements in
the page.
The listen() method is used to wire an event handler to
the BUTTON click event. When you click
the BUTTON,
the secret message contained in
the hidden DIV element is displayed.
The clearStyle() method is used to remove
the display:none style attribute from
the DIV element.
Under
the covers,
the WinJS.Utilities.query() method uses
the standard querySelectorAll() method. This means that you can use any selector which is compatible with
the querySelectorAll() method when using
the WinJS.Utilities.query() method.
The querySelectorAll() method is defined in
the W3C Selectors API Level 1 standard located here:
http://www.w3.org/TR/selectors-api/
Unlike
the querySelectorAll() method,
the WinJS.Utilities.query() method returns a QueryCollection. We talk about
the methods of
the QueryCollection class below.
Retrieving a Single Element with
the WinJS.Utilities.id() Method
If you want to retrieve a single element from a document, instead of matching a set of elements, then you can use
the WinJS.Utilities.id() method. For example,
the following line of code changes
the background color of an element to
the color red:
WinJS.Utilities.id("message").setStyle("background-color", "
red");
The statement above matches
the one and only element with an Id of message. For example,
the statement matches
the following DIV element:
<div id="message">Hello!</div>
Notice that you do not use a hash when matching a single element with
the WinJS.Utilities.id() method. You would need to use a hash when using
the WinJS.Utilities.query() method to do
the same thing like this:
WinJS.Utilities.query("#message").setStyle("background-color", "
red");
Under
the covers,
the WinJS.Utilities.id() method calls
the standard document.getElementById() method.
The WinJS.Utilities.id() method returns
the result as a QueryCollection.
If no element matches
the identifier passed to WinJS.Utilities.id() then you do not get an error. Instead, you get a QueryCollection with no elements (length=0).
Using
the WinJS.Utilities.children() method
The WinJS.Utilities.children() method enables you to retrieve a QueryCollection which contains all of
the children of a DOM element. For example, imagine that you have a DIV element which contains children DIV elements like this:
<div id="discussContainer">
<div>Message 1</div>
<div>Message 2</div>
<div>Message 3</div>
</div>
You can use
the following code to add borders around all of
the child DIV element and not
the container DIV element:
var discussContainer = WinJS.Utilities.id("discussContainer").get(0);
WinJS.Utilities.children(discussContainer).setStyle("border", "2px dashed
red");
It is important to understand that
the WinJS.Utilities.children() method only works with a DOM element and not a QueryCollection. Notice that
the get() method is used to retrieve
the DOM element which represents
the discussContainer.
Working with
the QueryCollection Class
Both
the WinJS.Utilities.query() method and
the WinJS.Utilities.id() method return an instance of
the QueryCollection class.
The QueryCollection class derives from
the base JavaScript Array class and adds several useful methods for working with HTML elements:
addClass(name) – Adds a class to every element in
the QueryCollection.
clearStyle(name) – Removes a style from every element in
the QueryCollection.
conrols(ctor, options) – Enables you to create controls.
get(index) – Retrieves
the element from
the QueryCollection at
the specified index.
getAttribute(name) – Retrieves
the value of an attribute for
the first element in
the QueryCollection.
hasClass(name) – Returns true if
the first element in
the QueryCollection has a certain class.
include(items) – Includes a collection of items in
the QueryCollection.
listen(eventType, listener, capture) – Adds an event listener to every element in
the QueryCollection.
query(query) – Performs an additional query on
the QueryCollection and returns a new QueryCollection.
removeClass(name) – Removes a class from
the every element in
the QueryCollection.
removeEventListener(eventType, listener, capture) – Removes an event listener from every element in
the QueryCollection.
setAttribute(name, value) – Adds an attribute to every element in
the QueryCollection.
setStyle(name, value) – Adds a style attribute to every element in
the QueryCollection.
template(templateElement, data, renderDonePromiseContract) – Renders a template using
the supplied data.
toggleClass(name) – Toggles
the specified class for every element in
the QueryCollection.
Because
the QueryCollection class derives from
the base Array class, it also contains all of
the standard Array methods like forEach() and slice().
Summary
In this blog post, I’ve described how you can perform queries using selectors within a Windows Metro Style application written with JavaScript. You learned how to return an instance of
the QueryCollection class by using
the WinJS.Utilities.query(), WinJS.Utilities.id(), and WinJS.Utilities.children() methods. You also learned about
the methods of
the QueryCollection class.
