1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
|
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en_US" lang="en_US">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<!-- basictypes.qdoc -->
<title>Qt 4.8: QML Basic Type: variant</title>
<link rel="stylesheet" type="text/css" href="style/offline.css" />
</head>
<body>
<div class="header" id="qtdocheader">
<div class="content">
<a href="index.html" class="qtref"><span>Qt Reference Documentation</span></a>
</div>
<div class="breadcrumb toolblock">
<ul>
<li class="first"><a href="index.html">Home</a></li>
<!-- Breadcrumbs go here -->
</ul>
</div>
</div>
<div class="content mainContent">
<h1 class="title">QML Basic Type: variant</h1>
<span class="subtitle"></span>
<!-- $$$variant-description -->
<div class="descr"> <a name="details"></a>
<p>A variant is a generic property type. A variant type property can hold any of the <a href="qdeclarativebasictypes.html">basic type</a> values:</p>
<pre class="qml"> <span class="type"><a href="qml-item.html">Item</a></span> {
property <span class="type">variant</span> <span class="name">aNumber</span>: <span class="number">100</span>
property <span class="type">variant</span> <span class="name">aString</span>: <span class="string">"Hello world!"</span>
property <span class="type">variant</span> <span class="name">aBool</span>: <span class="number">false</span>
}</pre>
<p>The <tt>variant</tt> type can also hold:</p>
<ul>
<li>An array of <a href="qdeclarativebasictypes.html">basic type</a> values</li>
<li>A map of key-value pairs with <a href="qdeclarativebasictypes.html">basic-type</a> values</li>
</ul>
<p>For example, below is an <tt>items</tt> array and an <tt>attributes</tt> map. Their contents can be examined using JavaScript <tt>for</tt> loops. Individual array values are accessible by index, and individual map values are accessible by key:</p>
<pre class="qml"> <span class="type"><a href="qml-item.html">Item</a></span> {
property <span class="type">variant</span> <span class="name">items</span>: [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="string">"four"</span>, <span class="string">"five"</span>]
property <span class="type">variant</span> <span class="name">attributes</span>: { 'color': <span class="string">'red'</span>, 'width': 100 }
<span class="name">Component</span>.onCompleted: {
<span class="keyword">for</span> (<span class="keyword">var</span> <span class="name">i</span>=<span class="number">0</span>; <span class="name">i</span><span class="operator"><</span><span class="name">items</span>.<span class="name">length</span>; i++)
<span class="name">console</span>.<span class="name">log</span>(<span class="name">items</span>[<span class="name">i</span>])
<span class="keyword">for</span> (<span class="keyword">var</span> <span class="name">prop</span> in <span class="name">attributes</span>)
<span class="name">console</span>.<span class="name">log</span>(<span class="name">prop</span>, <span class="string">"="</span>, <span class="name">attributes</span>[<span class="name">prop</span>])
}
}</pre>
<p>While this is a convenient way to store array and map-type values, you must be aware that the <tt>items</tt> and <tt>attributes</tt> properties above are <i>not</i> QML objects (and certainly not JavaScript object either) and the key-value pairs in <tt>attributes</tt> are <i>not</i> QML properties. Rather, the <tt>items</tt> property holds an array of values, and <tt>attributes</tt> holds a set of key-value pairs. Since they are stored as a set of values, instead of as an object, their contents <i>cannot</i> be modified individually:</p>
<pre class="qml"> <span class="type"><a href="qml-item.html">Item</a></span> {
property <span class="type">variant</span> <span class="name">items</span>: [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="string">"four"</span>, <span class="string">"five"</span>]
property <span class="type">variant</span> <span class="name">attributes</span>: { 'color': <span class="string">'red'</span>, 'width': 100 }
<span class="name">Component</span>.onCompleted: {
<span class="name">items</span>[<span class="number">0</span>] <span class="operator">=</span> <span class="number">10</span>
<span class="name">console</span>.<span class="name">log</span>(<span class="name">items</span>[<span class="number">0</span>]) <span class="comment">// This will still be '1'!</span>
<span class="name">attributes</span>.<span class="name">color</span> <span class="operator">=</span> <span class="string">'blue'</span>
<span class="name">console</span>.<span class="name">log</span>(<span class="name">attributes</span>.<span class="name">color</span>) <span class="comment">// This will still be 'red'!</span>
}
}</pre>
<p>Additionally, since <tt>items</tt> and <tt>attributes</tt> are not QML objects, changing their individual values do not trigger property change notifications. If the above example had <tt>onNumberChanged</tt> or <tt>onAnimalChanged</tt> signal handlers, they would not have been called. If, however, the <tt>items</tt> or <tt>attributes</tt> properties themselves were reassigned to different values, then such handlers would be called.</p>
<p>One way to "update" the contents of an array or map is to copy the property to a JavaScript object, modify the copy as desired, and then reassign the property to the updated copy. Note, however, that this is not efficient. In the example below, which reassigns the <tt>attributes</tt> property, the <i>entire</i> set of key-value pairs must be serialized and deserialized every time it is copied between a JavaScript object and a QML property:</p>
<pre class="qml"> <span class="type"><a href="qml-item.html">Item</a></span> {
property <span class="type">variant</span> <span class="name">attributes</span>: { 'color': <span class="string">'red'</span>, 'width': 100 }
<span class="name">Component</span>.onCompleted: {
<span class="comment">// Change the value of attributes.color to 'blue':</span>
var <span class="name">temp</span> = <span class="name">attributes</span> <span class="comment">// copy all values to 'temp'</span>
<span class="name">temp</span>.<span class="name">color</span> <span class="operator">=</span> <span class="string">'blue'</span>
<span class="name">attributes</span> <span class="operator">=</span> <span class="name">temp</span> <span class="comment">// copy all values back to 'attributes'</span>
}
}</pre>
<p>Since this operation is inefficient, if a list or map should be modifiable, it is better to use alternative approaches. For example, you could implement a custom C++ list element, or write to a JavaScript object defined from within a JavaScript file.</p>
<p>JavaScript programmers should also note that when a JavaScript object is copied to an array or map property, the <i>contents</i> of the object (that is, its key-value properties) are copied, rather than the object itself. The property does not hold a reference to the original JavaScript object, and extra data such as the object's JavaScript prototype chain is also lost in the process.</p>
</div>
<p><b>See also </b><a href="qdeclarativebasictypes.html">QML Basic Types</a>.</p>
<!-- @@@variant -->
<div class="ft">
<span></span>
</div>
</div>
<div class="footer">
<p>
<acronym title="Copyright">©</acronym> 2012 Nokia Corporation and/or its
subsidiaries. Documentation contributions included herein are the copyrights of
their respective owners.</p>
<br />
<p>
The documentation provided herein is licensed under the terms of the
<a href="http://www.gnu.org/licenses/fdl.html">GNU Free Documentation
License version 1.3</a> as published by the Free Software Foundation.</p>
<p>
Documentation sources may be obtained from <a href="http://www.qt-project.org">
www.qt-project.org</a>.</p>
<br />
<p>
Nokia, Qt and their respective logos are trademarks of Nokia Corporation
in Finland and/or other countries worldwide. All other trademarks are property
of their respective owners. <a title="Privacy Policy"
href="http://en.gitorious.org/privacy_policy/">Privacy Policy</a></p>
</div>
</body>
</html>
|
