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<td ><a href="index.html">Qt 5.9</a></td><td >QML Coding Conventions</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h3><a name="toc">Contents</a></h3>
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<li class="level1"><a href="#qml-object-declarations">QML Object Declarations</a></li>
<li class="level1"><a href="#grouped-properties">Grouped Properties</a></li>
<li class="level1"><a href="#lists">Lists</a></li>
<li class="level1"><a href="#javascript-code">JavaScript Code</a></li>
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<h1 class="title">QML Coding Conventions</h1>
<span class="subtitle"></span>
<!-- $$$qml-codingconventions.html-description -->
<div class="descr"> <a name="details"></a>
<p>This document contains the QML coding conventions that we follow in our documentation and examples and recommend that others follow.</p>
<a name="qml-object-declarations"></a>
<h2 id="qml-object-declarations">QML Object Declarations</h2>
<p>Throughout our documentation and examples, QML object attributes are always structured in the following order:</p>
<ul>
<li>id</li>
<li>property declarations</li>
<li>signal declarations</li>
<li>JavaScript functions</li>
<li>object properties</li>
<li>child objects</li>
<li>states</li>
<li>transitions</li>
</ul>
<p>For better readability, we separate these different parts with an empty line.</p>
<p>For example, a hypothetical <i>photo</i> QML object would look like this:</p>
<pre class="qml">
<span class="type">Rectangle</span> {
<span class="name">id</span>: <span class="name">photo</span> <span class="comment">// id on the first line makes it easy to find an object</span>
property <span class="type">bool</span> <span class="name">thumbnail</span>: <span class="number">false</span> <span class="comment">// property declarations</span>
property <span class="type">alias</span> <span class="name">image</span>: <span class="name">photoImage</span>.<span class="name">source</span>
signal <span class="type">clicked</span> <span class="comment">// signal declarations</span>
<span class="keyword">function</span> <span class="name">doSomething</span>(<span class="name">x</span>) <span class="comment">// javascript functions</span>
{
<span class="keyword">return</span> <span class="name">x</span> <span class="operator">+</span> <span class="name">photoImage</span>.<span class="name">width</span>
}
<span class="name">color</span>: <span class="string">"gray"</span> <span class="comment">// object properties</span>
<span class="name">x</span>: <span class="number">20</span>; <span class="name">y</span>: <span class="number">20</span>; <span class="name">height</span>: <span class="number">150</span> <span class="comment">// try to group related properties together</span>
<span class="name">width</span>: { <span class="comment">// large bindings</span>
<span class="keyword">if</span> (<span class="name">photoImage</span>.<span class="name">width</span> <span class="operator">></span> <span class="number">200</span>) {
<span class="name">photoImage</span>.<span class="name">width</span>;
} <span class="keyword">else</span> {
<span class="number">200</span>;
}
}
<span class="type">Rectangle</span> { <span class="comment">// child objects</span>
<span class="name">id</span>: <span class="name">border</span>
<span class="name">anchors</span>.centerIn: <span class="name">parent</span>; <span class="name">color</span>: <span class="string">"white"</span>
<span class="type">Image</span> { <span class="name">id</span>: <span class="name">photoImage</span>; <span class="name">anchors</span>.centerIn: <span class="name">parent</span> }
}
<span class="name">states</span>: <span class="name">State</span> { <span class="comment">// states</span>
<span class="name">name</span>: <span class="string">"selected"</span>
<span class="type">PropertyChanges</span> { <span class="name">target</span>: <span class="name">border</span>; <span class="name">color</span>: <span class="string">"red"</span> }
}
<span class="name">transitions</span>: <span class="name">Transition</span> { <span class="comment">// transitions</span>
<span class="name">from</span>: <span class="string">""</span>; <span class="name">to</span>: <span class="string">"selected"</span>
<span class="type">ColorAnimation</span> { <span class="name">target</span>: <span class="name">border</span>; <span class="name">duration</span>: <span class="number">200</span> }
}
}
</pre>
<a name="grouped-properties"></a>
<h2 id="grouped-properties">Grouped Properties</h2>
<p>If using multiple properties from a group of properties, consider using <i>group notation</i> instead of <i>dot notation</i> if it improves readability.</p>
<p>For example, this:</p>
<pre class="qml">
<span class="type">Rectangle</span> {
<span class="name">anchors</span>.left: <span class="name">parent</span>.<span class="name">left</span>; <span class="name">anchors</span>.top: <span class="name">parent</span>.<span class="name">top</span>; <span class="name">anchors</span>.right: <span class="name">parent</span>.<span class="name">right</span>; <span class="name">anchors</span>.leftMargin: <span class="number">20</span>
}
<span class="type">Text</span> {
<span class="name">text</span>: <span class="string">"hello"</span>
<span class="name">font</span>.bold: <span class="number">true</span>; <span class="name">font</span>.italic: <span class="number">true</span>; <span class="name">font</span>.pixelSize: <span class="number">20</span>; <span class="name">font</span>.capitalization: <span class="name">Font</span>.<span class="name">AllUppercase</span>
}
</pre>
<p>could be written like this:</p>
<pre class="qml">
<span class="type">Rectangle</span> {
<span class="type">anchors</span> { <span class="name">left</span>: <span class="name">parent</span>.<span class="name">left</span>; <span class="name">top</span>: <span class="name">parent</span>.<span class="name">top</span>; <span class="name">right</span>: <span class="name">parent</span>.<span class="name">right</span>; <span class="name">leftMargin</span>: <span class="number">20</span> }
}
<span class="type">Text</span> {
<span class="name">text</span>: <span class="string">"hello"</span>
<span class="type">font</span> { <span class="name">bold</span>: <span class="number">true</span>; <span class="name">italic</span>: <span class="number">true</span>; <span class="name">pixelSize</span>: <span class="number">20</span>; <span class="name">capitalization</span>: <span class="name">Font</span>.<span class="name">AllUppercase</span> }
}
</pre>
<a name="lists"></a>
<h2 id="lists">Lists</h2>
<p>If a list contains only one element, we generally omit the square brackets.</p>
<p>For example, it is very common for a component to only have one state.</p>
<p>In this case, instead of:</p>
<pre class="qml">
<span class="name">states</span>: [
<span class="type">State</span> {
<span class="name">name</span>: <span class="string">"open"</span>
<span class="type">PropertyChanges</span> { <span class="name">target</span>: <span class="name">container</span>; <span class="name">width</span>: <span class="number">200</span> }
}
]
</pre>
<p>we will write this:</p>
<pre class="qml">
<span class="name">states</span>: <span class="name">State</span> {
<span class="name">name</span>: <span class="string">"open"</span>
<span class="type">PropertyChanges</span> { <span class="name">target</span>: <span class="name">container</span>; <span class="name">width</span>: <span class="number">200</span> }
}
</pre>
<a name="javascript-code"></a>
<h2 id="javascript-code">JavaScript Code</h2>
<p>If the script is a single expression, we recommend writing it inline:</p>
<pre class="qml">
<span class="type">Rectangle</span> { <span class="name">color</span>: <span class="string">"blue"</span>; <span class="name">width</span>: <span class="name">parent</span>.<span class="name">width</span> <span class="operator">/</span> <span class="number">3</span> }
</pre>
<p>If the script is only a couple of lines long, we generally use a block:</p>
<pre class="qml">
<span class="type">Rectangle</span> {
<span class="name">color</span>: <span class="string">"blue"</span>
<span class="name">width</span>: {
var <span class="name">w</span> = <span class="name">parent</span>.<span class="name">width</span> <span class="operator">/</span> <span class="number">3</span>
<span class="name">console</span>.<span class="name">debug</span>(<span class="name">w</span>)
<span class="keyword">return</span> <span class="name">w</span>
}
}
</pre>
<p>If the script is more than a couple of lines long or can be used by different objects, we recommend creating a function and calling it like this:</p>
<pre class="qml">
<span class="keyword">function</span> <span class="name">calculateWidth</span>(<span class="name">object</span>)
{
var <span class="name">w</span> = <span class="name">object</span>.<span class="name">width</span> <span class="operator">/</span> <span class="number">3</span>
<span class="comment">// ...</span>
<span class="comment">// more javascript code</span>
<span class="comment">// ...</span>
<span class="name">console</span>.<span class="name">debug</span>(<span class="name">w</span>)
<span class="keyword">return</span> <span class="name">w</span>
}
<span class="type">Rectangle</span> { <span class="name">color</span>: <span class="string">"blue"</span>; <span class="name">width</span>: <span class="name">calculateWidth</span>(<span class="name">parent</span>) }
</pre>
<p>For long scripts, we will put the functions in their own JavaScript file and import it like this:</p>
<pre class="qml">
import "myscript.js" as Script
<span class="type">Rectangle</span> { <span class="name">color</span>: <span class="string">"blue"</span>; <span class="name">width</span>: <span class="name">Script</span>.<span class="name">calculateWidth</span>(<span class="name">parent</span>) }
</pre>
<p>If the code is longer than one line and hence within a block, we use semicolons to indicate the end of each statement:</p>
<pre class="qml">
<span class="type">MouseArea</span> {
<span class="name">anchors</span>.fill: <span class="name">parent</span>
<span class="name">onClicked</span>: {
var <span class="name">scenePos</span> = <span class="name">mapToItem</span>(<span class="number">null</span>, <span class="name">mouseX</span>, <span class="name">mouseY</span>);
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"MouseArea was clicked at scene pos "</span> <span class="operator">+</span> <span class="name">scenePos</span>);
}
}
</pre>
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