path: root/src/qml/doc/src/qmllanguageref/documents/structure.qdoc

// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtqml-documents-structure.html
\title Structure of a QML Document
\brief Description of the structure of QML documents


A QML document is a self contained piece of QML source code that consists of three parts:

    \list
    \li An optional list of pragmas
    \li Its \e import statements
    \li A single root object declaration
    \endlist

By convention, a single empty line separates the imports from the object hierarchy definition.

QML documents are always encoded in UTF-8 format.



\section1 Pragmas

Pragmas are instructions to the QML engine itself that can be used to specify
certain characteristics of objects in the current file or to modify how the
engine interprets code. The following pragmas are exaplained in details below.

\list
\li \c Singleton
\li \c ListPropertyAssignBehavior
\li \c ComponentBehavior
\li \c FunctionSignatureBehavior
\li \c NativeMethodBehavior
\li \c ValueTypeBehavior
\li \c Translator
\endlist

\section2 Singleton

\c{pragma Singleton} declares the component defined in the QML document as
singleton. Singletons are created only once per QML engine. In order to use
a QML-declared singleton you also have to register it with its module. See
\l{qt_target_qml_sources} for how to do this with CMake.

\section2 ListPropertyAssignBehavior

With this pragma you can define how assignments to list properties shall be
handled in components defined in the QML document. By default, assigning to a
list property appends to the list. You can explicitly request this behavior
using the value \c{Append}. Alternatively, you can request the contents of list
properties to always be replaced using \c{Replace}, or replaced if the property
is not the default property using \c{ReplaceIfNotDefault}. For example:

\qml
pragma ListPropertyAssignBehavior: ReplaceIfNotDefault
\endqml

\note The same declaration can also be given for C++-defined types, by adding
the \l{QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND},
\l{QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE}, and
\l{QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT} macros to the
class declaration.

\section2 ComponentBehavior

You may have multiple components defined in the same QML file. The root
scope of the QML file is a component, and you may additionally have
elements of type \l QQmlComponent, explicitly or implicitly created
as properties, or inline components. Those components are nested. Each
of the inner components is within one specific outer component. Most of
the time, IDs defined in an outer component are accessible within all
its nested inner components. You can, however, create elements from a
component in any a different context, with different IDs available.
Doing so breaks the assumption that outer IDs are available. Therefore,
the engine and the QML tooling cannot generally know in advance what
type, if any, such IDs will resolve to at run time.

With the ComponentBehavior pragma you can restrict all inner components
defined in a file to only create objects within their original context.
If a component is bound to its context, you can safely use IDs from
outer components in the same file within the component. QML tooling will
then assume the outer IDs with their specific types to be available.

In order to bind the components to their context specify the \c{Bound}
argument:

\qml
pragma ComponentBehavior: Bound
\endqml

This implies that, in case of name clashes, IDs defined outside a bound
component override local properties of objects created from the
component. Otherwise it wouldn't actually be safe to use the IDs since
later versions of a module might add more properties to the component.
If the component is not bound, local properties override IDs defined
outside the component, but not IDs defined inside the component.

The example below prints the \e r property of the ListView object with
the id \e color, not the \e r property of the rectangle's color.

\qml
pragma ComponentBehavior: Bound
import QtQuick

ListView {
 id: color
 property int r: 12
 model: 1

 delegate: Rectangle {
  Component.onCompleted: console.log(color.r)
 }
}
\endqml

The default value of \c ComponentBehavior is \c{Unbound}. You can also
specify it explicitly. In a future version of Qt the default will change
to \c{Bound}.

Delegate components bound to their context don't receive their own
private contexts on instantiation. This means that model data can only
be passed via \l{Required Properties}{required properties} in this case.
Passing model data via context properties will not work. This concerns
delegates to e.g. \l{Instantiator}, \l{Repeater}, \l{ListView},
\l{TableView}, \l{GridView}, \l{TreeView} and in general anything that
uses \l{DelegateModel} internally.

For example, the following will \e{not} work:

\qml
pragma ComponentBehavior: Bound
import QtQuick

ListView {
 delegate: Rectangle {
     color: model.myColor
 }
}
\endqml

The \c{delegate} property of \l{ListView} is a component. Therefore, a
\l{Component} is implicitly created around the \l{Rectangle} here. That
component is bound to its context. It doesn't receive the context property
\c{model} provided by \l{ListView}. To make it work, you'd have to write
it this way:

\qml
pragma ComponentBehavior: Bound
import QtQuick

ListView {
 delegate: Rectangle {
     required property color myColor
     color: myColor
 }
}
\endqml

You can nest components in a QML file. The pragma holds for all components in
the file, no matter how deeply nested.

\section2 FunctionSignatureBehavior

With this pragma you can change the way type annotations on functions
are handled. Since Qt 6.7 type annotations are enforced when calling
functions. Before, only the \l{QML script compiler} enforced the type
annotations. The interpreter and JIT compiler ignored them. Always
enforcing the type annotations is a behavior change in comparison to
earlier versions since you could call functions with mismatched
arguments before.

Specifying \c{Ignored} as value makes the QML engine and the
\l{QML script compiler} ignore any type annotations and therefore
restores the pre-6.7 behavior of the interpreter and JIT. As a result
less code is compiled to C++ ahead of time, and more code has to be
interpreted or JIT-compiled.

Specifying \c{Enforced} as value explicitly states the default: Type
annotations are always enforced.

\sa {Type annotations and assertions}

\section2 NativeMethodBehavior

Calling C++ methods with \c this objects different from the one they were
retrieved from is broken, due to historical reasons. The original object is
used as \c this object. You can allow the given \c this object to be used by
setting \c {pragma NativeMethodBehavior: AcceptThisObject}. Specifying
\c RejectThisObject keeps the historical behavior.

An example of this can be found under \l {C++ methods and the 'this' object}.

\section2 ValueTypeBehavior

With this pragma you can change the way value types and sequences are handled.

Usually lower case names cannot be type names in JavaScript code. This is a
problem because value type names are lower case. You can specify \c{Addressable}
as value for this pragma to change this. If \c{Addressable} is specified a
JavaScript value can be explicitly coerced to a specific, named, value type. This is
done using the \c as operator, like you would do with object types. Furthermore,
you can also check for value types using the \c instanceof operator:

\qml
pragma ValueTypeBehavior: Addressable
import QtQml

QtObject {
 property var a
 property real b: (a as rect).x
 property bool c: a instanceof rect

 property var rect // inaccessible. "rect" is a type name.
}
\endqml

If the type does not match, casting returns \c undefined. \c instanceof
only checks for inheritance, not for all possible type coercions. So, for
example, a \l{QRect} is not a \c rect value type since \c rect is \l{QRectF}
in C++, and therefore not related by inheritance. With \c as you can cast
to any type compatible via coercion.

Since \c rect in the above example is now a type name, it will shadow any
properties called \c{rect}.

Explicitly casting to the desired type helps tooling. It can allow the
\l{Qt Quick Compiler} generate efficient code where it otherwise would not be
able to. You can use \l{qmllint Reference}{qmllint} to find such occurrences.

There is also a \c{Inaddressable} value you can use to explicitly specify the
default behavior.

Value types and sequences are generally treated as references. This means, if
you retrieve a value type instance from a property into a local value, and then
change the local value, the original property is also changed. Furthermore,
if you write the original property explicitly, the local value is also updated.
This behavior is rather unintuitive in many places, and you should not rely on
it. The \c{Copy} and \c{Reference} values for the \c{ValueTypeBehavior} pragma
are experimental options to change this behavior. You should not use them.
Specifying \c{Copy} causes all value types to be treated as actual copies.
Specifying \c{Reference} explicitly states the default behavior.

Rather than using \c{Copy} you should explicitly re-load references to value
types and sequences any time they can have been affected by side effects. Side
effects can happen whenever you call a function or imperatively set a property.
\l qmllint provides guidance on this. For example, in the following code
the variable \c f is affected by side effects after writing \c width. This is
because there may be a binding in a derived type or in a \c Binding element
that updates \c font when \c width is changed.

\qml
import QtQuick
Text {
 function a() : real {
     var f = font;
     width = f.pixelSize;
     return f.pointSize;
 }
}
\endqml

In order to address this, you can avoid holding \c f across the write operation
on \c width:

\qml
import QtQuick
Text {
 function a() : real {
     var f = font;
     width = f.pixelSize;
     f = font;
     return f.pointSize;
 }
}
\endqml

This, in turn can be shortened to:

\qml
import QtQuick
Text {
 function a() : real {
     width = font.pixelSize;
     return font.pointSize;
 }
}
\endqml

You might assume that re-retrieving the \c font property is costly, but actually
the QML engine automatically refreshes value type references each time you read
from them. So this is not more expensive than the first version, but a clearer
way to express the same operations.

\sa {Type annotations and assertions}

\section2 Translator

With this pragma you can set the context for the translations in the file.

\qml
pragma Translator: myTranslationContext
\endqml

\qml
pragma Translator: "myTranslationContext"
\endqml

For more information on internationalization with QML, see \l{QML: Use qsTr()}{Use qsTr}.

\section1 Imports

A document must import the necessary modules or type namespaces to enable the
engine to load the QML object types referenced within the document. By default,
a document can access any QML object types that have been defined through
\c .qml files in the same directory; if a document needs to refer to any other
object types, it must import the type namespace into which those types have
been registered.

QML does \e not have a preprocessor that modifies the document prior to
presentation to the \l{QQmlEngine}{QML engine}, unlike C or C++.
The \c import statements do not copy and prepend the code in the document, but
instead instruct the QML engine on how to resolve type references found
in the document. Any type reference present in a QML document - such as \c
Rectangle and \c ListView - including those made within a \l {JavaScript
Expressions in QML Documents}{JavaScript block} or \l {Property Binding}{property
bindings}, are \e resolved based exclusively on the import statements. At least
one \c import statement must be present such as \c{import QtQuick 2.0}.

Please see the \l{qtqml-syntax-imports.html}{QML Syntax - Import Statements}
documentation for in-depth information about QML imports.


\section1 The Root Object Declaration

A QML document describes a hierarchy of objects which can be instantiated.
Each object definition has a certain structure; it has a type, it can have an
id and an object name, it can have properties, it can have methods, it can have
signals and it can have signal handlers.

A QML file must only contain \b {a single root object definition}. The following is invalid and will generate an error:

\code
// MyQmlFile.qml
import QtQuick 2.0

Rectangle { width: 200; height: 200; color: "red" }
Rectangle { width: 200; height: 200; color: "blue" }    // invalid!
\endcode

This is because a .qml file automatically defines a QML type, which encapsulates a \e single QML object definition. This is discussed further in \l{qtqml-documents-definetypes.html}{Documents as QML object type definitions}.

*/