// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
\page qtquick-positioning-righttoleft.html
\title Right-to-left User Interfaces
\brief switching text flow and layout
\section1 Overview

This chapter discusses different approaches and options available for implementing right-to-left
language support for Qt Quick applications. Some common right-to-left languages include Arabic, Hebrew,
Persian and Urdu. Most changes include making sure that text translated to right-to-left languages
is properly aligned to the right, and horizontally ordered content in views, lists and grids flows
correctly from the right to left.

In right-to-left language speaking cultures, people naturally scan and read graphic elements and text
from the right to left. The general rule of thumb is that content (like photos, videos and maps) is not
mirrored, but positioning of the content (like application layouts and the flow of visual elements) is
mirrored. For example, photos shown in chronological order should flow from right to left, the
low end range of the horizontal sliders should be located at the right side of the slider, and
text lines should be aligned to the right side of the available text area. The location of visual
elements should not be mirrored when the position is related to a content; for example, when a
position marker is shown to indicate a location on a map. Also, there are some special cases you may
need to take into account where right-to-left language speakers are used to left-to-right
positioning, for example when using number dialers in phones and media play, pause, rewind and
forward buttons in music players.

\section1 Text Alignment

(This applies to the \l Text, \l TextInput and \l TextEdit types.)

When the horizontal alignment of a text item is not explicitly set, the text element is
automatically aligned to the natural reading direction of the text. By default left-to-right text
like English is aligned to the left side of the text area, and right-to-left text like Arabic is
aligned to the right side of the text area. The alignment of a text element with empty text takes
its alignment cue from \l QInputMethod::inputDirection(), which is based on the active
system locale.

This default locale-based alignment can be overridden by setting the \c horizontalAlignment
property for the text element, or by enabling layout mirroring using the \l LayoutMirroring attached
property, which causes any explicit left and right horizontal alignments to be mirrored.
Note that when \l LayoutMirroring is set, the \c horizontalAlignment property value remains unchanged;
the effective alignment of the text element that takes the mirroring into account can be read from the
\c effectiveHorizontalAlignment property.

\snippet qml/righttoleft.qml 0

\section1 Layout Direction of Positioners and Views

(This applies to the \l Row, \l Grid, \l Flow, \l ListView and \l GridView types.)

Types used for horizontal positioning and model views have the \c layoutDirection
property for controlling the horizontal direction of the layouts. Setting \c layoutDirection to
\c Qt.RightToLeft causes items to be laid out from the right to left. By default Qt Quick follows
the left-to-right layout direction.

The horizontal layout direction can also be reversed through the \l LayoutMirroring attached property.
This causes the effective \c layoutDirection of positioners and views to be mirrored. Note the actual value
of the \c layoutDirection property will remain unchanged; the effective layout direction of positioners and
views that takes the mirroring into account can be read from the \c effectiveLayoutDirection property.

\snippet qml/righttoleft.qml 1

\section1 Layout Mirroring

The attached property \l LayoutMirroring is provided as a convenience for easily implementing right-to-left
support for existing left-to-right Qt Quick applications. It mirrors the behavior of \l {anchor-layout}
{Item anchors}, the layout direction of \l{Item Positioners}{positioners} and
\l{qtquick-modelviewsdata-modelview.html}{model views}, and the explicit text alignment of QML text types.

You can enable layout mirroring for a particular \l Item:

\snippet qml/righttoleft.qml 2

Or set all child types to also inherit the layout direction:

\snippet qml/righttoleft.qml 3

Applying mirroring in this manner does not change the actual value of the relevant anchor,
\c layoutDirection or \c horizontalAlignment properties. The separate read-only property
\c effectiveLayoutDirection can be used to query the effective layout
direction of positioners and model views that takes the mirroring into account. Similarly the \l Text,
\l TextInput and \l TextEdit types have gained the read-only property \c effectiveHorizontalAlignment
for querying the effective visual alignment of text. For anchors, the read only
\l {Item::anchors.top}{anchors.mirrored} property reflects whether anchors have been mirrored.

Note that application layouts and animations that are defined using \l {Item::}{x} property values (as
opposed to anchors or positioner types) are not affected by the \l LayoutMirroring attached property.
Therefore, adding right-to-left support to these types of layouts may require some code changes to your application,
especially in views that rely on both the anchors and x coordinate-based positioning. Here is one way to use
the \l LayoutMirroring attached property to apply mirroring to an item that is positioned using \l {Item::}{x}
coordinates:

\snippet qml/righttoleft.qml 4

Not all layouts should necessarily be mirrored. There are cases where a visual type is positioned to
the right side of the screen for improved one-handed use, because most people are right-handed, and not
because of the reading direction. In the case that a child type should not be affected by mirroring,
set the \l {LayoutMirroring::enabled}{LayoutMirroring.enabled} property for that type to false.

Qt Quick is designed for developing animated, fluid user interfaces. When mirroring your application, remember to test that
the animations and transitions continue to work as expected. If you do not have the resources to add
right-to-left support for your application, it may be better to just keep the application layouts left
aligned and just make sure that text is translated and aligned properly.

\section1 Mirroring Icons

(This applies to \l Image, \l BorderImage and \l AnimatedImage types.)

Most images do not need to be mirrored, but some directional icons, such as arrows, may need to be mirrored.
The painting of these icons can be mirrored with a dedicated \c mirror property:

\snippet qml/righttoleft.qml 5

\section1 Default Layout Direction

Use the \l {QtQml::Qt::application}{Qt.application.layoutDirection} property
to query the active layout direction of the application. It inherits
QGuiApplication::layoutDirection(), which determines the layout direction from
the active language translation file.

To define the layout direction for a particular locale, declare the dedicated
string literal \c QT_LAYOUT_DIRECTION in the context \c QGuiApplication as
either \c LTR or \c RTL.

First, introduce this line somewhere in your QML source code:

\code
qsTr("QT_LAYOUT_DIRECTION","QGuiApplication");
\endcode

Then use \l {Using lupdate} to generate the translation source file.

This appends the following declaration to the translation file, where you can
enter either \c LTR or \c RTL as the translation for the locale.

\code
<context>
    <name>QGuiApplication</name>
    <message>
        <location filename="myapp.qml" line="33"/>
        <source>QT_LAYOUT_DIRECTION</source>
        <translation type="unfinished">RTL</translation>
    </message>
</context>
\endcode

Next, add the following bindings to the root QML component of your application:
\code
LayoutMirroring.enabled: Qt.application.layoutDirection === Qt.RightToLeft
LayoutMirroring.childrenInherit: true
\endcode

The first binding ensures that the UI will be mirrored appropriately when a
right-to-left locale is set. The second binding ensures that child items of the
root component will also respect mirroring.

You can test that the layout direction works as expected by running your Qt Quick application with
the compiled translation file:

\code
qml myapp.qml -translation myapp.qm
\endcode

You can test your application in right-to-left layout direction by calling the
static function \l QGuiApplication::setLayoutDirection():

\code
QGuiApplication app(argc, argv);
app.setLayoutDirection(Qt::RightToLeft);
\endcode

*/
