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<td >Qt 5.12</td><td ><a href="qtqml-index.html">Qt QML</a></td><td ><a href="qtqml-qmlmodule.html">QML Types</a></td><td >Binding QML Type</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right"><a href="qtqml-index.html">Qt 5.12.6 Reference Documentation</a></td>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#properties">Properties</a></li>
<li class="level1"><a href="#details">Detailed Description</a></li>
<li class="level2"><a href="#binding-to-an-inaccessible-property">Binding to an Inaccessible Property</a></li>
<li class="level2"><a href="#conditional-bindings">Conditional Bindings</a></li>
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<h1 class="title">Binding QML Type</h1>
<span class="subtitle"></span>
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<p>Enables the arbitrary creation of property bindings. <a href="#details">More...</a></p>
<!-- @@@Binding -->
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> Import Statement:</td><td class="memItemRight bottomAlign"> import QtQml 2.12</td></tr></table></div><ul>
<li><a href="qml-qtqml-binding-members.html">List of all members, including inherited members</a></li>
</ul>
<a name="properties"></a>
<h2 id="properties">Properties</h2>
<ul>
<li class="fn"><b><b><a href="qml-qtqml-binding.html#delayed-prop">delayed</a></b></b> : bool</li>
<li class="fn"><b><b><a href="qml-qtqml-binding.html#property-prop">property</a></b></b> : string</li>
<li class="fn"><b><b><a href="qml-qtqml-binding.html#target-prop">target</a></b></b> : Object</li>
<li class="fn"><b><b><a href="qml-qtqml-binding.html#value-prop">value</a></b></b> : any</li>
<li class="fn"><b><b><a href="qml-qtqml-binding.html#when-prop">when</a></b></b> : bool</li>
</ul>
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<a name="details"></a>
<h2 id="details">Detailed Description</h2>
<p>In QML, property bindings result in a dependency between the properties of different objects.</p>
<a name="binding-to-an-inaccessible-property"></a>
<h2 id="binding-to-an-inaccessible-property">Binding to an Inaccessible Property</h2>
<p>Sometimes it is necessary to bind an object's property to that of another object that isn't directly instantiated by QML, such as a property of a class exported to QML by C++. You can use the Binding type to establish this dependency; binding any value to any object's property.</p>
<p>For example, in a C++ application that maps an "app.enteredText" property into QML, you can use Binding to update the enteredText property.</p>
<pre class="cpp">
TextEdit { id: myTextField; text: <span class="string">"Please type here..."</span> }
Binding { target: app; property: <span class="string">"enteredText"</span>; value: myTextField<span class="operator">.</span>text }
</pre>
<p>When <code>text</code> changes, the C++ property <code>enteredText</code> will update automatically.</p>
<a name="conditional-bindings"></a>
<h2 id="conditional-bindings">Conditional Bindings</h2>
<p>In some cases you may want to modify the value of a property when a certain condition is met but leave it unmodified otherwise. Often, it's not possible to do this with direct bindings, as you have to supply values for all possible branches.</p>
<p>For example, the code snippet below results in a warning whenever you release the mouse. This is because the value of the binding is undefined when the mouse isn't pressed.</p>
<pre class="qml">
</pre>
<p>The Binding type can prevent this warning.</p>
<pre class="qml">
</pre>
<p>The Binding type restores any previously set direct bindings on the property.</p>
<p><b>See also </b><a href="qtqml-index.html">Qt QML</a>.</p>
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<h2>Property Documentation</h2>
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<div class="qmlitem"><div class="qmlproto">
<div class="table"><table class="qmlname">
<tr valign="top" class="odd" id="delayed-prop">
<td class="tblQmlPropNode"><p>
<a name="delayed-prop"></a><span class="name">delayed</span> : <span class="type"><a href="qml-bool.html">bool</a></span></p></td></tr>
</table></div>
</div><div class="qmldoc"><p>This property holds whether the binding should be delayed.</p>
<p>A delayed binding will not immediately update the target, but rather wait until the event queue has been cleared. This can be used as an optimization, or to prevent intermediary values from being assigned.</p>
<pre class="cpp">
Binding {
target: contactName; property: <span class="char">'text'</span>
value: givenName <span class="operator">+</span> <span class="string">" "</span> <span class="operator">+</span> familyName; when: list<span class="operator">.</span>ListView<span class="operator">.</span>isCurrentItem
delayed: <span class="keyword">true</span>
}
</pre>
<p>This property was introduced in Qt 5.8.</p>
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<br/>
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<div class="qmlitem"><div class="qmlproto">
<div class="table"><table class="qmlname">
<tr valign="top" class="odd" id="property-prop">
<td class="tblQmlPropNode"><p>
<a name="property-prop"></a><span class="name">property</span> : <span class="type"><a href="qml-string.html">string</a></span></p></td></tr>
</table></div>
</div><div class="qmldoc"><p>The property to be updated.</p>
<p>This can be a group property if the expression results in accessing a property of a <a href="qtqml-typesystem-basictypes.html">value type</a>. For example:</p>
<pre class="qml">
</pre>
</div></div><!-- @@@property -->
<br/>
<!-- $$$target -->
<div class="qmlitem"><div class="qmlproto">
<div class="table"><table class="qmlname">
<tr valign="top" class="odd" id="target-prop">
<td class="tblQmlPropNode"><p>
<a name="target-prop"></a><span class="name">target</span> : <span class="type">Object</span></p></td></tr>
</table></div>
</div><div class="qmldoc"><p>The object to be updated.</p>
</div></div><!-- @@@target -->
<br/>
<!-- $$$value -->
<div class="qmlitem"><div class="qmlproto">
<div class="table"><table class="qmlname">
<tr valign="top" class="odd" id="value-prop">
<td class="tblQmlPropNode"><p>
<a name="value-prop"></a><span class="name">value</span> : <span class="type">any</span></p></td></tr>
</table></div>
</div><div class="qmldoc"><p>The value to be set on the target object and property. This can be a constant (which isn't very useful), or a bound expression.</p>
</div></div><!-- @@@value -->
<br/>
<!-- $$$when -->
<div class="qmlitem"><div class="qmlproto">
<div class="table"><table class="qmlname">
<tr valign="top" class="odd" id="when-prop">
<td class="tblQmlPropNode"><p>
<a name="when-prop"></a><span class="name">when</span> : <span class="type"><a href="qml-bool.html">bool</a></span></p></td></tr>
</table></div>
</div><div class="qmldoc"><p>This property holds when the binding is active. This should be set to an expression that evaluates to true when you want the binding to be active.</p>
<pre class="cpp">
Binding {
target: contactName; property: <span class="char">'text'</span>
value: name; when: list<span class="operator">.</span>ListView<span class="operator">.</span>isCurrentItem
}
</pre>
<p>When the binding becomes inactive again, any direct bindings that were previously set on the property will be restored.</p>
</div></div><!-- @@@when -->
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