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<td >Qt 5.12</td><td ><a href="qtqml-index.html">Qt QML</a></td><td ><a href="qtqml-module.html">C++ Classes</a></td><td >QQmlExpression</td></tr></table><table class="buildversion"><tr>
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<h3><a name="toc">Contents</a></h3>
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<li class="level1"><a href="#public-functions">Public Functions</a></li>
<li class="level1"><a href="#signals">Signals</a></li>
<li class="level1"><a href="#static-public-members">Static Public Members</a></li>
<li class="level1"><a href="#details">Detailed Description</a></li>
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<h1 class="title">QQmlExpression Class</h1>
<!-- $$$QQmlExpression-brief -->
<p>The <a href="qqmlexpression.html">QQmlExpression</a> class evaluates JavaScript in a QML context. <a href="#details">More...</a></p>
<!-- @@@QQmlExpression -->
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> Header:</td><td class="memItemRight bottomAlign">   <span class="preprocessor">#include &lt;QQmlExpression&gt;</span>
</td></tr><tr><td class="memItemLeft rightAlign topAlign"> qmake:</td><td class="memItemRight bottomAlign"> QT += qml</td></tr><tr><td class="memItemLeft rightAlign topAlign"> Since:</td><td class="memItemRight bottomAlign">  Qt 5.0</td></tr></table></div><ul>
<li><a href="qqmlexpression-members.html">List of all members, including inherited members</a></li>
</ul>
<a name="public-functions"></a>
<h2 id="public-functions">Public Functions</h2>
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#QQmlExpression">QQmlExpression</a></b>()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#QQmlExpression-1">QQmlExpression</a></b>(QQmlContext *<i>ctxt</i>, QObject *<i>scope</i>, const QString &amp;<i>expression</i>, QObject *<i>parent</i> = nullptr)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#QQmlExpression-2">QQmlExpression</a></b>(const QQmlScriptString &amp;<i>script</i>, QQmlContext *<i>ctxt</i> = nullptr, QObject *<i>scope</i> = nullptr, QObject *<i>parent</i> = nullptr)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> virtual </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#dtor.QQmlExpression">~QQmlExpression</a></b>() override</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#clearError">clearError</a></b>()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> int </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#columnNumber">columnNumber</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QQmlContext *</td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#context">context</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QQmlEngine *</td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#engine">engine</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QQmlError </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#error">error</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QVariant </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#evaluate">evaluate</a></b>(bool *<i>valueIsUndefined</i> = nullptr)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QString </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#expression">expression</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#hasError">hasError</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> int </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#lineNumber">lineNumber</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#notifyOnValueChanged">notifyOnValueChanged</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QObject *</td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#scopeObject">scopeObject</a></b>() const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#setExpression">setExpression</a></b>(const QString &amp;<i>expression</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#setNotifyOnValueChanged">setNotifyOnValueChanged</a></b>(bool <i>notifyOnChange</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#setSourceLocation">setSourceLocation</a></b>(const QString &amp;<i>url</i>, int <i>line</i>, int <i>column</i> = 0)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QString </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#sourceFile">sourceFile</a></b>() const</td></tr>
</table></div>
<a name="signals"></a>
<h2 id="signals">Signals</h2>
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#valueChanged">valueChanged</a></b>()</td></tr>
</table></div>
<a name="static-public-members"></a>
<h2 id="static-public-members">Static Public Members</h2>
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> const QMetaObject </td><td class="memItemRight bottomAlign"><b><a href="qqmlexpression.html#staticMetaObject-var">staticMetaObject</a></b></td></tr>
</table></div>
<a name="details"></a>
<!-- $$$QQmlExpression-description -->
<div class="descr">
<h2 id="details">Detailed Description</h2>
<p>The <a href="qqmlexpression.html">QQmlExpression</a> class evaluates JavaScript in a QML context.</p>
<p>For example, given a file <code>main.qml</code> like this:</p>
<pre class="qml">



</pre>
<p>The following code evaluates a JavaScript expression in the context of the above QML:</p>
<pre class="cpp">

  <span class="type"><a href="qqmlengine.html">QQmlEngine</a></span> <span class="operator">*</span>engine <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qqmlengine.html">QQmlEngine</a></span>;
  <span class="type"><a href="qqmlcomponent.html">QQmlComponent</a></span> component(engine<span class="operator">,</span> <span class="type">QUrl</span><span class="operator">::</span>fromLocalFile(<span class="string">&quot;main.qml&quot;</span>));

  <span class="type">QObject</span> <span class="operator">*</span>myObject <span class="operator">=</span> component<span class="operator">.</span>create();
  <span class="type"><a href="qqmlexpression.html#QQmlExpression">QQmlExpression</a></span> <span class="operator">*</span>expr <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qqmlexpression.html#QQmlExpression">QQmlExpression</a></span>(engine<span class="operator">-</span><span class="operator">&gt;</span>rootContext()<span class="operator">,</span> myObject<span class="operator">,</span> <span class="string">&quot;width * 2&quot;</span>);
  <span class="type">int</span> result <span class="operator">=</span> expr<span class="operator">-</span><span class="operator">&gt;</span>evaluate()<span class="operator">.</span>toInt();  <span class="comment">// result = 400</span>

</pre>
</div>
<!-- @@@QQmlExpression -->
<div class="func">
<h2>Member Function Documentation</h2>
<!-- $$$QQmlExpression[overload1]$$$QQmlExpression -->
<h3 class="fn" id="QQmlExpression"><a name="QQmlExpression"></a>QQmlExpression::<span class="name">QQmlExpression</span>()</h3>
<p>Create an invalid <a href="qqmlexpression.html">QQmlExpression</a>.</p>
<p>As the expression will not have an associated <a href="qqmlcontext.html">QQmlContext</a>, this will be a null expression object and its value will always be an invalid QVariant.</p>
<!-- @@@QQmlExpression -->
<!-- $$$QQmlExpression$$$QQmlExpressionQQmlContext*QObject*constQString&QObject* -->
<h3 class="fn" id="QQmlExpression-1"><a name="QQmlExpression-1"></a>QQmlExpression::<span class="name">QQmlExpression</span>(<span class="type"><a href="qqmlcontext.html">QQmlContext</a></span> *<i>ctxt</i>, <span class="type">QObject</span> *<i>scope</i>, const <span class="type">QString</span> &amp;<i>expression</i>, <span class="type">QObject</span> *<i>parent</i> = nullptr)</h3>
<p>Create a <a href="qqmlexpression.html">QQmlExpression</a> object that is a child of <i>parent</i>.</p>
<p>The <i>expression</i> JavaScript will be executed in the <i>ctxt</i> <a href="qqmlcontext.html">QQmlContext</a>. If specified, the <i>scope</i> object's properties will also be in scope during the expression's execution.</p>
<!-- @@@QQmlExpression -->
<!-- $$$QQmlExpression$$$QQmlExpressionconstQQmlScriptString&QQmlContext*QObject*QObject* -->
<h3 class="fn" id="QQmlExpression-2"><a name="QQmlExpression-2"></a>QQmlExpression::<span class="name">QQmlExpression</span>(const <span class="type"><a href="qqmlscriptstring.html">QQmlScriptString</a></span> &amp;<i>script</i>, <span class="type"><a href="qqmlcontext.html">QQmlContext</a></span> *<i>ctxt</i> = nullptr, <span class="type">QObject</span> *<i>scope</i> = nullptr, <span class="type">QObject</span> *<i>parent</i> = nullptr)</h3>
<p>Create a <a href="qqmlexpression.html">QQmlExpression</a> object that is a child of <i>parent</i>.</p>
<p>The <i>script</i> provides the expression to be evaluated, the context to evaluate it in, and the scope object to evaluate it with. If provided, <i>ctxt</i> and <i>scope</i> will override the context and scope object provided by <i>script</i>.</p>
<p><b>See also </b><a href="qqmlscriptstring.html">QQmlScriptString</a>.</p>
<!-- @@@QQmlExpression -->
<!-- $$$~QQmlExpression[overload1]$$$~QQmlExpression -->
<h3 class="fn" id="dtor.QQmlExpression"><a name="dtor.QQmlExpression"></a><code>[override virtual] </code>QQmlExpression::<span class="name">~QQmlExpression</span>()</h3>
<p>Destroy the <a href="qqmlexpression.html">QQmlExpression</a> instance.</p>
<!-- @@@~QQmlExpression -->
<!-- $$$clearError[overload1]$$$clearError -->
<h3 class="fn" id="clearError"><a name="clearError"></a><span class="type">void</span> QQmlExpression::<span class="name">clearError</span>()</h3>
<p>Clear any expression errors. Calls to <a href="qqmlexpression.html#hasError">hasError</a>() following this will return false.</p>
<p><b>See also </b><a href="qqmlexpression.html#hasError">hasError</a>() and <a href="qqmlexpression.html#error">error</a>().</p>
<!-- @@@clearError -->
<!-- $$$columnNumber[overload1]$$$columnNumber -->
<h3 class="fn" id="columnNumber"><a name="columnNumber"></a><span class="type">int</span> QQmlExpression::<span class="name">columnNumber</span>() const</h3>
<p>Returns the source file column number for this expression. The source location must have been previously set by calling <a href="qqmlexpression.html#setSourceLocation">setSourceLocation</a>().</p>
<!-- @@@columnNumber -->
<!-- $$$context[overload1]$$$context -->
<h3 class="fn" id="context"><a name="context"></a><span class="type"><a href="qqmlcontext.html">QQmlContext</a></span> *QQmlExpression::<span class="name">context</span>() const</h3>
<p>Returns the <a href="qqmlcontext.html">QQmlContext</a> this expression is associated with, or 0 if there is no association or the <a href="qqmlcontext.html">QQmlContext</a> has been destroyed.</p>
<!-- @@@context -->
<!-- $$$engine[overload1]$$$engine -->
<h3 class="fn" id="engine"><a name="engine"></a><span class="type"><a href="qqmlengine.html">QQmlEngine</a></span> *QQmlExpression::<span class="name">engine</span>() const</h3>
<p>Returns the <a href="qqmlengine.html">QQmlEngine</a> this expression is associated with, or 0 if there is no association or the <a href="qqmlengine.html">QQmlEngine</a> has been destroyed.</p>
<!-- @@@engine -->
<!-- $$$error[overload1]$$$error -->
<h3 class="fn" id="error"><a name="error"></a><span class="type"><a href="qqmlerror.html">QQmlError</a></span> QQmlExpression::<span class="name">error</span>() const</h3>
<p>Return any error from the last call to <a href="qqmlexpression.html#evaluate">evaluate</a>(). If there was no error, this returns an invalid <a href="qqmlerror.html">QQmlError</a> instance.</p>
<p><b>See also </b><a href="qqmlexpression.html#hasError">hasError</a>() and <a href="qqmlexpression.html#clearError">clearError</a>().</p>
<!-- @@@error -->
<!-- $$$evaluate[overload1]$$$evaluatebool* -->
<h3 class="fn" id="evaluate"><a name="evaluate"></a><span class="type">QVariant</span> QQmlExpression::<span class="name">evaluate</span>(<span class="type">bool</span> *<i>valueIsUndefined</i> = nullptr)</h3>
<p>Evaulates the expression, returning the result of the evaluation, or an invalid QVariant if the expression is invalid or has an error.</p>
<p><i>valueIsUndefined</i> is set to true if the expression resulted in an undefined value.</p>
<p><b>See also </b><a href="qqmlexpression.html#hasError">hasError</a>() and <a href="qqmlexpression.html#error">error</a>().</p>
<!-- @@@evaluate -->
<!-- $$$expression[overload1]$$$expression -->
<h3 class="fn" id="expression"><a name="expression"></a><span class="type">QString</span> QQmlExpression::<span class="name">expression</span>() const</h3>
<p>Returns the expression string.</p>
<p><b>See also </b><a href="qqmlexpression.html#setExpression">setExpression</a>().</p>
<!-- @@@expression -->
<!-- $$$hasError[overload1]$$$hasError -->
<h3 class="fn" id="hasError"><a name="hasError"></a><span class="type">bool</span> QQmlExpression::<span class="name">hasError</span>() const</h3>
<p>Returns true if the last call to <a href="qqmlexpression.html#evaluate">evaluate</a>() resulted in an error, otherwise false.</p>
<p><b>See also </b><a href="qqmlexpression.html#error">error</a>() and <a href="qqmlexpression.html#clearError">clearError</a>().</p>
<!-- @@@hasError -->
<!-- $$$lineNumber[overload1]$$$lineNumber -->
<h3 class="fn" id="lineNumber"><a name="lineNumber"></a><span class="type">int</span> QQmlExpression::<span class="name">lineNumber</span>() const</h3>
<p>Returns the source file line number for this expression. The source location must have been previously set by calling <a href="qqmlexpression.html#setSourceLocation">setSourceLocation</a>().</p>
<!-- @@@lineNumber -->
<!-- $$$notifyOnValueChanged[overload1]$$$notifyOnValueChanged -->
<h3 class="fn" id="notifyOnValueChanged"><a name="notifyOnValueChanged"></a><span class="type">bool</span> QQmlExpression::<span class="name">notifyOnValueChanged</span>() const</h3>
<p>Returns true if the <a href="qqmlexpression.html#valueChanged">valueChanged</a>() signal is emitted when the expression's evaluated value changes.</p>
<p><b>See also </b><a href="qqmlexpression.html#setNotifyOnValueChanged">setNotifyOnValueChanged</a>().</p>
<!-- @@@notifyOnValueChanged -->
<!-- $$$scopeObject[overload1]$$$scopeObject -->
<h3 class="fn" id="scopeObject"><a name="scopeObject"></a><span class="type">QObject</span> *QQmlExpression::<span class="name">scopeObject</span>() const</h3>
<p>Returns the expression's scope object, if provided, otherwise 0.</p>
<p>In addition to data provided by the expression's <a href="qqmlcontext.html">QQmlContext</a>, the scope object's properties are also in scope during the expression's evaluation.</p>
<!-- @@@scopeObject -->
<!-- $$$setExpression[overload1]$$$setExpressionconstQString& -->
<h3 class="fn" id="setExpression"><a name="setExpression"></a><span class="type">void</span> QQmlExpression::<span class="name">setExpression</span>(const <span class="type">QString</span> &amp;<i>expression</i>)</h3>
<p>Set the expression to <i>expression</i>.</p>
<p><b>See also </b><a href="qqmlexpression.html#expression">expression</a>().</p>
<!-- @@@setExpression -->
<!-- $$$setNotifyOnValueChanged[overload1]$$$setNotifyOnValueChangedbool -->
<h3 class="fn" id="setNotifyOnValueChanged"><a name="setNotifyOnValueChanged"></a><span class="type">void</span> QQmlExpression::<span class="name">setNotifyOnValueChanged</span>(<span class="type">bool</span> <i>notifyOnChange</i>)</h3>
<p>Sets whether the <a href="qqmlexpression.html#valueChanged">valueChanged</a>() signal is emitted when the expression's evaluated value changes.</p>
<p>If <i>notifyOnChange</i> is true, the <a href="qqmlexpression.html">QQmlExpression</a> will monitor properties involved in the expression's evaluation, and emit <a href="qqmlexpression.html#valueChanged">QQmlExpression::valueChanged</a>() if they have changed. This allows an application to ensure that any value associated with the result of the expression remains up to date.</p>
<p>If <i>notifyOnChange</i> is false (default), the <a href="qqmlexpression.html">QQmlExpression</a> will not montitor properties involved in the expression's evaluation, and <a href="qqmlexpression.html#valueChanged">QQmlExpression::valueChanged</a>() will never be emitted. This is more efficient if an application wants a &quot;one off&quot; evaluation of the expression.</p>
<p><b>See also </b><a href="qqmlexpression.html#notifyOnValueChanged">notifyOnValueChanged</a>().</p>
<!-- @@@setNotifyOnValueChanged -->
<!-- $$$setSourceLocation[overload1]$$$setSourceLocationconstQString&intint -->
<h3 class="fn" id="setSourceLocation"><a name="setSourceLocation"></a><span class="type">void</span> QQmlExpression::<span class="name">setSourceLocation</span>(const <span class="type">QString</span> &amp;<i>url</i>, <span class="type">int</span> <i>line</i>, <span class="type">int</span> <i>column</i> = 0)</h3>
<p>Set the location of this expression to <i>line</i> and <i>column</i> of <i>url</i>. This information is used by the script engine.</p>
<!-- @@@setSourceLocation -->
<!-- $$$sourceFile[overload1]$$$sourceFile -->
<h3 class="fn" id="sourceFile"><a name="sourceFile"></a><span class="type">QString</span> QQmlExpression::<span class="name">sourceFile</span>() const</h3>
<p>Returns the source file URL for this expression. The source location must have been previously set by calling <a href="qqmlexpression.html#setSourceLocation">setSourceLocation</a>().</p>
<!-- @@@sourceFile -->
<!-- $$$valueChanged[overload1]$$$valueChanged -->
<h3 class="fn" id="valueChanged"><a name="valueChanged"></a><code>[signal] </code><span class="type">void</span> QQmlExpression::<span class="name">valueChanged</span>()</h3>
<p>Emitted each time the expression value changes from the last time it was evaluated. The expression must have been evaluated at least once (by calling <a href="qqmlexpression.html#evaluate">QQmlExpression::evaluate</a>()) before this signal will be emitted.</p>
<!-- @@@valueChanged -->
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