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<td ><a href="index.html">Qt 5.12</a></td><td >QML Coding Conventions</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.12.6 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>
<li class="level1"><a href="#related-information">Related Information</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">
Rectangle {
id: photo // id on the first line makes it easy to find an object
property bool thumbnail: false // property declarations
property alias image: photoImage.source
signal clicked // signal declarations
function doSomething(x) // javascript functions
{
return x + photoImage.width
}
color: "gray" // object properties
x: 20 // try to group related properties together
y: 20
height: 150
width: { // large bindings
if (photoImage.width > 200) {
photoImage.width;
} else {
200;
}
}
Rectangle { // child objects
id: border
anchors.centerIn: parent; color: "white"
Image {
id: photoImage
anchors.centerIn: parent
}
}
states: State { // states
name: "selected"
PropertyChanges { target: border; color: "red" }
}
transitions: Transition { // transitions
from: ""
to: "selected"
ColorAnimation { target: border; duration: 200 }
}
}
</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">
Rectangle {
anchors.left: parent.left; anchors.top: parent.top; anchors.right: parent.right; anchors.leftMargin: 20
}
Text {
text: "hello"
font.bold: true; font.italic: true; font.pixelSize: 20; font.capitalization: Font.AllUppercase
}
</pre>
<p>could be written like this:</p>
<pre class="qml">
Rectangle {
anchors { left: parent.left; top: parent.top; right: parent.right; leftMargin: 20 }
}
Text {
text: "hello"
font { bold: true; italic: true; pixelSize: 20; capitalization: Font.AllUppercase }
}
</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">
states: [
State {
name: "open"
PropertyChanges { target: container; width: 200 }
}
]
</pre>
<p>we will write this:</p>
<pre class="qml">
states: State {
name: "open"
PropertyChanges { target: container; width: 200 }
}
</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">
Rectangle { color: "blue"; width: parent.width / 3 }
</pre>
<p>If the script is only a couple of lines long, we generally use a block:</p>
<pre class="qml">
Rectangle {
color: "blue"
width: {
var w = parent.width / 3
console.debug(w)
return w
}
}
</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">
function calculateWidth(object)
{
var w = object.width / 3
// ...
// more javascript code
// ...
console.debug(w)
return w
}
Rectangle { color: "blue"; width: calculateWidth(parent) }
</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
Rectangle { color: "blue"; width: Script.calculateWidth(parent) }
</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">
MouseArea {
anchors.fill: parent
onClicked: {
var scenePos = mapToItem(null, mouseX, mouseY);
console.log("MouseArea was clicked at scene pos " + scenePos);
}
}
</pre>
<a name="related-information"></a>
<h2 id="related-information">Related Information</h2>
<ul>
<li>Best Practices for QML and Qt Quick</li>
</ul>
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