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Breadcrumbs go here --> </ul> </div> <div class="toolbuttons toolblock"> <ul> <li id="smallA" class="t_button">A</li> <li id="medA" class="t_button active">A</li> <li id="bigA" class="t_button">A</li> <li id="print" class="t_button"><a href="javascript:this.print();"> <span>Print</span></a></li> </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> </div> </div> <div class="ft"> <span></span> </div> </div> <div class="footer"> <p> <acronym title="Copyright">©</acronym> 2013 Digia Plc and/or its subsidiaries. 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