Qt 5.13 | QML Coding Conventions

Qt 5.13.2 Reference Documentation

# QML Coding Conventions

This document contains the QML coding conventions that we follow in our documentation and examples and recommend that others follow.

## QML Object Declarations

Throughout our documentation and examples, QML object attributes are always structured in the following order:

- id
- property declarations
- signal declarations
- JavaScript functions
- object properties
- child objects
- states
- transitions

For better readability, we separate these different parts with an empty line.

For example, a hypothetical photo QML object would look like this:

      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 }
          }
      }

## Grouped Properties

If using multiple properties from a group of properties, consider using group notation instead of dot notation if it improves readability.

For example, this:

      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
      }

could be written like this:

      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 }
      }

## Lists

If a list contains only one element, we generally omit the square brackets.

For example, it is very common for a component to only have one state.

In this case, instead of:

      states: [
          State {
              name: "open"
              PropertyChanges { target: container; width: 200 }
          }
      ]

we will write this:

      states: State {
          name: "open"
          PropertyChanges { target: container; width: 200 }
      }

## JavaScript Code

If the script is a single expression, we recommend writing it inline:

      Rectangle { color: "blue"; width: parent.width / 3 }

If the script is only a couple of lines long, we generally use a block:

      Rectangle {
          color: "blue"
          width: {
              var w = parent.width / 3
              console.debug(w)
              return w
          }
      }

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:

      function calculateWidth(object)
      {
          var w = object.width / 3
          // ...
          // more javascript code
          // ...
          console.debug(w)
          return w
      }

      Rectangle { color: "blue"; width: calculateWidth(parent) }

For long scripts, we will put the functions in their own JavaScript file and import it like this:

      import "myscript.js" as Script

      Rectangle { color: "blue"; width: Script.calculateWidth(parent) }

If the code is longer than one line and hence within a block, we use semicolons to indicate the end of each statement:

      MouseArea {
          anchors.fill: parent
          onClicked: {
              var scenePos = mapToItem(null, mouseX, mouseY);
              console.log("MouseArea was clicked at scene pos " + scenePos);
          }
      }

## Related Information

- Best Practices for QML and Qt Quick