qtquick-bestpractices.qdoc

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// Copyright (C) 2018 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qtquick-bestpractices.html
\meta {keywords} {qmltopic}
\title Best Practices for QML and Qt Quick
\brief Lists best practices for working with QML and Qt Quick.
\ingroup best-practices
\ingroup explanations-programminglanguages
 
Despite all of the benefits that QML and Qt Quick offer, they can be
challenging in certain situations. The following sections elaborate on some of
the best practices that will help you get better results when developing
applications.
 
\section1 Use Built-in Controls over Custom UI Controls
 
A fluid and modern UI is key for any application's success in today's world, and
that's where QML makes so much sense for a designer or developer. Qt offers the
most basic UI controls that are necessary to create a fluid and modern-looking
UI. It is recommended to browse this list of UI controls before creating your
own custom UI control.
 
Besides these basic UI controls offered by Qt Quick itself, a rich set of UI
controls are also available with Qt Quick Controls. They cater to the most
common use cases without any change, and offer a lot more possibilities with their
customization options. In particular, Qt Quick Controls provides styling
options that align with the latest UI design trends. If these UI controls do not
satisfy your application's needs, only then it is recommended to create a
custom control.
 
You can use the controls when you design UIs in Qt Design Studio. In addition,
it provides timeline-based animations, visual effects, layouts, and a
live-preview for prototyping applications.
 
\section2 Related Information
\list
\li \l{Qt Quick Controls}
\li \l{Customizing Qt Quick Controls}
\li \l{Qt Quick}
\li \l{Qt Design Studio Manual}
\endlist
 
\omit
\section1 Keep it Short and Simple or "KiSS"
 
QML being a declarative language, a lot of the details are worked out by the underlying
engine. So it is important for any QML application, especially one with a
larger codebase, to have its code organized in smaller and simpler \c .qml files.
 
TODO: need a few snippet or example applications that showcase this.
\endomit
 
\section1 Coding Conventions
 
See \l{QML Coding Conventions}.
 
\section1 Bundle Application Resources
 
Most applications depend on resources, such as images and icons, to provide a
rich user experience. It can often be a challenge to make these resources
available to the application regardless of the target OS. Most popular OS-es
employ security policies that restrict access to the file system,
making it harder to load these resources. As an alternative, Qt offers its own
\l {The Qt Resource System}{resource system} that is built into the
application binary, enabling access to the application's resources regardless
of the target OS.
 
For example, for a C++ project, consider the following directory structure:
 
\badcode
MyModule
├── images
│   ├── image1.png
│   └── image2.png
├── CMakeLists.txt
└── main.qml
\endcode
 
You may represent this structure as a \l{qt_add_qml_module}{CMake QML Module} in the following way:
 
\code
qt_add_qml_module(my_module
   URI MyModule
   VERSION 1.0
   QML_FILES
       main.qml
   RESOURCES
       images/image1.png
       images/image2.png
   # ...
)
\endcode
 
All QML files listed under \c {QML_FILES} will automatically get compiled \l {Ahead-of-Time Compilation}{ahead of time}.
 
You should keep the QML files in the \e same directory as the CMakeLists.txt with
the \l {qt_add_qml_module}. Otherwise, their \l{The Implicit Import}{implicit imports}
will be different from the \l{QML Modules} they belong to. This is a frequent
source of mistakes.
 
\section2 Related Information
\list
 \li \l{The Qt Resource System}
\endlist
 
\section1 Separate UI from Business Logic
 
One of the key goals that most application developers want to achieve is to
create a maintainable application. One of the ways to achieve this goal is
to separate the user interface, the frontend, from the business logic, the backend.
The following are a few reasons why an application's UI should be written in QML:
 
\list
 \li Declarative languages are strongly suited for defining UIs.
 \li QML code is simpler to write, as it is less verbose than C++, and is not
     strongly typed. This also results in it being an excellent language to
     prototype in, a quality that is vital when collaborating with designers,
     for example.
 \li JavaScript can easily be used in QML to respond to events.
\endlist
 
Strongly typed languages, such as C++, are best suited for an application's
business logic, the backend. Typically, such code performs tasks such as complex calculations
or data processing, which are faster in strongly typed languages than in QML.
 
Qt offers various approaches to integrate QML with a strongly typed language in an application.
A typical use case is displaying a list of data in a user interface.
If the data set is static, simple, and small, a model written in QML can be
sufficient.
 
The following snippet demonstrates examples of models written in QML:
 
\qml
    model: [ "Item 1", "Item 2", "Item 3" ]
 
    model: 10
\endqml
 
For larger and more dynamic data sets, use a strongly typed language, such as
\l {QAbstractItemModel Subclass}{C++}, to handle the business logic.
 
\section2 Exposing Data from C++ to QML
 
Refactoring QML is a lot easier than refactoring C++, so in order to make
maintenance pain-free, we should strive to keep C++ types unaware of QML as
much as possible. This can be achieved by "pushing" references to C++ types
into QML.
 
This can be done by using \l {Required properties}{required properties} and
setting them via \l QQmlApplicationEngine::setInitialProperties. It is also
possible to create one or multiple \l {QML_SINGLETON}{singletons} which will
return all the data the C++ side wants to provide to QML.
 
With this approach, the C++ remains unchanged in the event that the QML needs
to be refactored in the future.
 
For a quick guide to choosing the correct approach to expose C++ types to QML,
see \l {Choosing the Correct Integration Method Between C++ and QML}.
 
\section2 Related Information
\list
\li \l{Writing QML Extensions with C++}
      {Writing QML Extensions with C++ Tutorial}
\li \l{Qt Quick Controls - Chat Tutorial}{Chat application tutorial}
\endlist
 
\section1 Using Qt Design Studio
 
Qt Design Studio uses UI files that have the filename extension \e {.ui.qml}
to separate the visual parts of the UI from the UI logic you implement in
\e {.qml} files. You should edit UI files only in the \uicontrol {2D} view in
Qt Design Studio. If you use some other tool to add code that Qt Design Studio
does not support, it displays error messages. Fix the errors to enable visual
editing of the UI files again. Typically, you should move the unsupported code
to a \e {.qml} file.
 
\section2 Related Information
 
\list
    \li \l{Qt Design Studio: UI Files}
\endlist
 
\section1 Using Qt Quick Views
 
\section2 Store State in Models
 
See \l {Avoid Storing State in Delegates}.
 
\section1 Using Qt Quick Layouts
 
Qt offers Qt Quick Layouts to arrange Qt Quick items visually in a layout.
Unlike its alternative, the item positioners, the Qt Quick Layouts can also
resize its children on window resize. Although Qt Quick Layouts are often
the desired choice for most use cases, the following \e dos and \e{don'ts}
must be considered while using them:
 
\section2 Dos
 
\list
 \li Use \l {Item::}{anchors} or the \l {Item::}{width} and \l {Item::}{height}
     properties to specify the size of the layout against its non-layout parent
     item.
 \li Use the \l Layout attached property to set the size and alignment
     attributes of the layout's immediate children.
\endlist
 
\section2 Don'ts
 
\list
 \li Do not define preferred sizes for items that provide implicitWidth and
     implicitHeight, unless their implicit sizes are not satisfactory.
 \li Do not use anchors on an item that is an immediate child of a layout.
     Instead, use \c Layout.preferredWidth and \c Layout.preferredHeight:
 
    \snippet qml/windowconstraints.qml rowlayout
\endlist
 
\note Layouts and anchors are both types of objects that take more memory and
instantiation time. Avoid using them (especially in list and table delegates,
and styles for controls) when simple bindings to x, y, width, and height
properties are enough.
 
\section2 Related Information
 
\list
 \li \l{Item Positioners}
 \li \l{Qt Quick Layouts Overview}
\endlist
 
\section1 Type Safety
 
When declaring properties in QML, it's easy and convenient to use the "var" type:
 
\code
property var name
property var size
property var optionsMenu
\endcode
 
However, this approach has several disadvantages:
\list
 \li If a value with the wrong type is assigned, the error reported will point
     to the location of the property declaration, as opposed to the location
     where the property was assigned to. This slows down the development
     process by making it more difficult to track down errors.
 \li Static anaylsis to catch errors like the ones mentioned above is not
     possible.
 \li The actual underlying type of the property is not always immediately clear
     to the reader.
\endlist
 
Instead, always use the actual type where possible:
 
\code
property string name
property int size
property MyMenu optionsMenu
\endcode
 
\section1 Property change signals
 
Prefer using the explicit interaction signals over the value changed signals to
avoid subtle bugs.
 
Using \c {valueChanged} can lead to event cascades where the value is constantly
changed because it is somehow rounded or normalized.
 
Using explicit interaction signals alone avoids this whole class of issues.
 
For example, \l Slider has these similar signals: \l {Slider::}{moved} and
\c {valueChanged}.
 
\qml
Slider {
    value: someValueFromBackend
 
    onValueChanged: pushToBackend(value)
    // or
    onMoved: pushToBackend(value)
}
\endqml
 
Both cases look similar, and you may want to use \c {valueChanged}.
 
Developers often overlook the fact that the \l Slider can automatically change
its value, for example, because of clamping to minimum/maximum values or
rounding. In this case, the \c {valueChanged} signal is emitted. If you use the
\c {valueChanged} signal, you may notice it is emitted at unexpected moments.
 
To avoid possible issues, use an interaction signal: the signal that is emitted
when user interacts with the control. In this example, if you use the
\l {Slider::}{moved} signal, the slot will only be triggered if the user
changes the control.
 
\section1 Performance
 
For information on performance in QML and Qt Quick,
see \l {QML Performance Considerations And Suggestions}.
 
\section1 Prefer Declarative Bindings Over Imperative Assignments
 
In QML, it's possible to use imperative JavaScript code to perform tasks
such as responding to input events, send data over a network, and so on.
Imperative code has an important place in QML, but it's also important
to be aware of when not to use it.
 
For example, consider the following imperative assignment:
 
\code
Rectangle {
    Component.onCompleted: color = "red"
}
\endcode
 
This has the following disadvantages:
 
\list
\li It's slow. The color property will first be evaluated with a
    default-constructed value, and then again with "red" later on.
\li It delays errors that could be found at build time to run time, slowing
    down the development process.
\li It overwrites any declarative binding that was in place. In most cases this
    is intended, but sometimes it can be unintentional.
    See \l {Debugging overwriting of bindings} for more information.
\li It interferes with tooling; Qt Quick Designer, for example, doesn't support
    JavaScript.
\endlist
 
The code can be rewritten to be a declarative binding instead:
 
\code
Rectangle {
    color: "red"
}
\endcode
 
\section1 Don't store state in delegates
 
Don't store a state in a delegate. The issue here is that the delegate is
created and destroyed multiple times, so the saved state will be lost.
 
\qml
// Wrong approach:
ListView {
    // ...
 
    delegate: Button {
        // ...
        property bool someStateProperty
        onClicked: someStateProperty = true
    }
}
\endqml
 
Instead, store the state outside of the delegate. For example, in a model.
When the delegate is destroyed, the saved state is not lost.
 
\qml
// Right approach:
ListView {
    // ...
 
    delegate: Button {
        // ...
        onClicked: model.someStateProperty = true
    }
}
\endqml
 
\section1 Make user-facing strings translatable
 
It is recommended to make user-facing strings translatable from the beginning.
See \l {Writing Source Code for Translation}.
 
\qml
ToolButton {
    id: selectionToolButton
    // ...
    icon.source: "qrc:/images/selection.png"
 
    Tooltip.Text: qsTr("Select pixels within an area and move them")
 
    onClicked: canvas.tool = ImageCanvas.SelectionTool
}
\endqml
 
\section1 Do not customize native styles
 
Native styles (Windows and macOS styles) don't support customization.
Ensure you don't customize a native style.
 
\qml *
// Wrong approach:
import QtQuick.Controls.Windows
 
// Don't customize a native style
Button {
    background: Rectangle { /\1...\1/ }
}
\endqml
 
\include customizing-native-styles.qdocinc
 
\qml *
// Right approach:
import QtQuick.Controls.Basic
 
// You can customize a commonly available style
Button {
    background: Rectangle { /\1...\1/ }
}
\endqml
 
\section1 Tools and Utilities
 
For information on useful tools and utilies that make working with QML and
Qt Quick easier, see \l {Qt Quick Tools and Utilities}.
 
\section1 Scene Graph
 
For information on Qt Quick's scene graph, see \l {Qt Quick Scene Graph}.
 
\section1 Scalable User Interfaces
 
As display resolutions improve, a scalable application UI becomes more and
more important. One of the approaches to achieve this is to maintain several
copies of the UI for different screen resolutions, and load the appropriate one
depending on the available resolution. Although this works pretty
well, it adds to the maintenance overhead.
 
Qt offers a better solution to this problem and recommends the application
developers to follow these tips:
 
\list
 \li Use anchors or the Qt Quick Layouts module to lay out the visual items.
 \li Do not specify explicit width and height for a visual item.
 \li Provide UI resources such as images and icons for each display resolution
     that your application supports. The Qt Quick Controls gallery example
     demonstrates this well by providing the \c qt-logo.png for \c @2x, \c @3x,
     and \c @4x resolutions, enabling the application to cater to high
     resolution displays. Qt automatically chooses the appropriate
     image that is suitable for the given display, provided the high DPI scaling
     feature is explicitly enabled.
 \li Use SVG images for small icons. While larger SVGs can be slow to render,
     small ones work well. Vector images avoid the need to provide several
     versions of an image, as is necessary with bitmap images.
 \li Use font-based icons, such as Font Awesome. These scale to any display
     resolution, and also allow colorization. The
     Qt Quick Controls Text Editor example demonstrates this well.
\endlist
 
With this in place, your application's UI should scale depending
on the display resolution on offer.
 
\image qtquickcontrols-gallery-welcome.png
 
\section2 Related Information
 
\list
 \li \l{Qt Quick Controls - Gallery}{Gallery example}
 \li \l{Qt Quick Controls - Text Editor}{Text Editor example}
 \li \l{Font Awesome}
 \li \l{Scalability}
 \li \l{High DPI}
\endlist
*/