qtquicklayouts-overview.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page qtquicklayouts-overview.html
    \title Qt Quick Layouts Overview
    \brief A set of APIs for arranging QML items in a user interface.
 
    Use Qt Quick Layouts to arrange items in a user interface. Qt Quick Layouts
    resize their items, which makes them well suited for resizable user
    interfaces.
 
    \section1 Key features
 
    Some of the key features of Qt Quick Layouts are:
 
    \list
        \li \l{Layout.alignment}{Align} items with the
            \l{Layout.alignment} property.
        \li Specify \l{Layout.fillWidth}{resizable items} with the
            \l{Layout.fillWidth} and
            \l{Layout.fillHeight} properties.
        \li Set \l{Size constraints}{size constraints} with the
            \l{Layout.minimumWidth},
            \l{Layout.preferredWidth}, and
            \l{Layout.maximumWidth} properties -- "Width"
            can be replaced with "Height" for specifying similar constraints to
            the height.
        \li You can specify \l {RowLayout::}{spacing} with
            \l{RowLayout::}{spacing},
            \l{GridLayout::}{rowSpacing},
            or \l{GridLayout::}{columnSpacing}.
        \li Stretch items both horizontally and vertically with
            \l{Layout.horizontalStretchFactor}{stretch factors}.
    \endlist
 
    In addition, GridLayout adds these features:
    \list
        \li \l{Layout.row}{Grid coordinates}, controlled by the
            \l{Layout.row} and \l{Layout.column}
            properties.
        \li \l{GridLayout.flow}{Automatic grid coordinates} used together with
            the \l{GridLayout::}{flow}, \l{GridLayout::}{rows}, and
            \l{GridLayout::}{columns} properties.
        \li \l{Layout.columnSpan}{Spans} across rows or columns, that you can
            specify with the \l{Layout.rowSpan} and
            \l{Layout.columnSpan} properties.
    \endlist
 
    \section1 Getting started
 
    To get started using Qt Quick Layouts, import the QML types into your
    application with the following import statement in your \c {.qml} file:
 
    \qml
    import QtQuick.Layouts
    \endqml
 
    The next step is to create a \l {A simple layout}{simple layout}. You can
    also study the \l {Qt Quick Layouts - Basic Example}.
 
    \section2 A simple layout
 
    The intention of using a layout is to rearrange its children whenever the
    layout changes size. This means the application must ensure that the layout
    gets resized. In the following snippet, the \e RowLayout ensures that by
    specifying \c{anchors.fill: parent}. However, you can also achieve this by
    other means, such as specifying the \l{Item::}{width} and
    \l{Item::}{height} properties. In the same snippet, the \e {orange
    Rectangle} has a fixed size of \e 100 by \e 150 pixels, and the \e {plum
    Rectangle} will expand to occupy all the space it gets allocated.
 
    \snippet qml/layout-simple.qml 1
    \target simple-layout-snippet
 
    Layouts are responsible for their children's geometry.
    This includes properties such as \l{Item::}{width},
    \l{Item::}{height}, \l{Item::}{x}, \l{Item::}{y},
    \l{Item::}{anchors}), etc.
 
    \important Don't specify properties that influence the geometry of child
    items in your application. Setting these properties on a child item causes
    a conflict of interest, and the result is undefined. This also applies when
    the child item is a layout. Therefore, only layouts with no parent layout
    can have \c{anchors.fill: parent}.
 
    \section3 Spacing
    As seen in the \l {simple-layout-snippet}{previous snippet}, the \e spacing
    for the \e RowLayout is set to \e 6. This ensures that all items in the
    layout have 6 pixels of spacing between them:
 
    \snippet qml/layout-simple.qml spacing
 
    If you omit specifying a spacing value, the layout will use a default of
    \e 5 pixels. The spacing, as well as the \e implicitWidth of any children,
    contributes to the \e implicitWidth of the layout. This is important to
    keep in mind if you rely on default behavior, as it may impact your layout
    design. For example, the two \e {ColumnLayout}s both set
    \e {Layout.fillWidth: true} in the following snippet. It's natural to think
    that they would both get the same width. However, because of the default 5
    pixel spacing between the items in the inner \e RowLayout, the
    \e implicitWidth of the first \e ColumnLayout becomes larger, leaving less
    room for the second one. For example:
 
    \snippet qml/layout_with_default_spacing.qml layout-with-default-spacing
 
    This snippet will produce a layout that looks like this:
 
    \image layout-with-default-spacing.png "A QML layout with default spacing"
 
    To ensure equal size of these two columns, you can either
    \list a
        \li set the spacing of the \e RowLayout to \c 0, or
        \li set \e preferredWidth to equal values on both \e {ColumnLayout}s.
    \endlist
 
    \section2 Specifying preferred size
    For each item, the effective preferred size may come from one of several
    candidate properties. For determining the effective preferred size, an item
    queries these candidate properties in the following order, and will use the
    first candidate with a valid width or height.
 
    \table
    \header
        \li Candidate properties
        \li Description
    \row
        \li \l{Layout.preferredWidth} or \l{Layout.preferredHeight}
        \li These properties are supposed to be modified by the application if
            the default implicit size does not give the optimal arrangement.
    \row
        \li \l{Item::}{implicitWidth} or \l{Item::}{implicitHeight}.
        \li These properties are supposed to be supplied by each item to give a
            meaningful ideal size. For example, the size needed to display all
            the contents of a \l Text type. An implicit width or height of \c 0
            is interpreted as invalid.
    \endtable
 
    An item can specify \l{Layout.preferredWidth} without having to specify
    \l{Layout.preferredHeight}. In such cases, the effective preferred height
    is determined from the \l{Item::}{implicitHeight}.
 
    \note If you don't specify neither preferredWidth nor implicitWidth, the
    Layout will query \l {Item::}{width} as an ultimate value for the effective
    preferred width. However, you shouldn't rely on \l {Item::}{width} as a
    source for the effective preferred width, as that may cause unexpected
    behavior. For instance, changing the \l{Item::}{width} or
    \l{Item::}{height} properties won't trigger a layout rearrangement,
    or the layout might use the actual width and height -- not the width and
    height specified in your QML file -- when forced to do a full rebuild.
 
    \section2 Size constraints
 
    Since an item can be resized by its layout, the layout needs to know the
    \l{Layout.minimumWidth}{minimum}, \l{Layout::preferredWidth}{preferred},
    and \l{Layout.maximumWidth}{maximum} sizes of all items where
    \l{Layout.fillWidth} or \l{Layout.fillHeight} is set to \e true.
 
    The \l{Layout.preferredWidth}{preferred} width and height is the \e actual
    width and height of an item, if the layout is not bound to a specific size
    itself. If the layout is set to a specific size, it distributes additional
    space based on the ratio of preferred sizes of its items, while taking
    minimum and maximum sizes into account. The preferred and implicit sizes
    act as ratios and weights when all items set \e fillWidth and
    \e fillHeight.
 
    For instance, the following produces a layout with two rectangles lying
    side-by-side that stretches horizontally. The \e {orange Rectangle} can be
    resized from 50x150 to 300x150, and the \e {plum Rectangle} can be resized
    from 100x100 to ∞x100. As long as the minimum and maximum width of each
    item isn't exceeded, the plum rectangle will have twice the width of the
    orange one.
 
    \snippet qml/windowconstraints.qml rowlayout
 
    \image rowlayout-minimum.png "RowLayout at its minimum"
 
    Combining each item's constraints gives these implicit constraints to the
    layout element:
 
    \table
    \header
        \li
        \li minimum
        \li preferred
        \li maximum
    \row
        \li implicit constraints (width)
        \li 156
        \li 306
        \li ∞ (\c Number.POSITIVE_INFINITY)
    \row
        \li implicit constraints (heights)
        \li 150
        \li 150
        \li 150
    \endtable
 
    Thus, the layout cannot be narrower than 156, nor can it be taller or
    shorter than 150, without breaking any of the constraints of its child items.
 
    \section1 Connecting windows and layouts
    You can use normal anchoring concepts to ensure that your layout follows
    the window resizing.
 
    \snippet qml/windowconstraints.qml anchoring
 
    You can rely on the size constraints of layouts to ensure that the window
    cannot be resized beyond the layout constraints. You can take the size
    constraints from the layout and set these constraints on the \e minimumWidth,
    \e minimumHeight, \e maximumWidth, and \e maximumHeight of the
    \l [Qml]{Window} element. The following code ensures that the window cannot
    be resized beyond the constraints of the layout:
 
    \snippet qml/windowconstraints.qml bindconstraints
 
    \note Because \e {layout.Layout.maximumWidth} is infinite in this case, we
    cannot bind that to the \e maximumWidth property of Window, as that is an
    integer number. Therefore, the maximum width is set to a fixed value of
    \c 1000.
 
    Finally, set the initial size of the window to be the layout's implicit
    size:
 
    \snippet qml/windowconstraints.qml binddefaultsize
 
    \section1 Spanning and stretching Items
 
    Use \l {Layout.columnSpan}{spans} in a GridLayout to make child items
    occupy more than one cell. For example, you may have a GridLayout with six
    cells across two rows. The top row contains the \e {Item}s item1, item2,
    and item3. The bottom row contains the \e Item item4, which specifies
    \e {columnSpan: 3} and \e {alignment: Qt.AlignHCenter}. This places item4
    in the middle of the three cells that make up the bottom row. The following
    snippet serves as an example:
 
    \snippet qml/gridlayout_with_span.qml gridlayout-with-span
 
    The size of rows and columns are given implicitly by their contents.
    For example, a Button may impact the width of the column it's in, or the
    height of the row it's in. This means GridLayout doesn't have uniform
    distribution. Because of this, you can't use a span to stretch a layout.
    To manipulate the stretch of an item or layout, use
    \l {Layout.horizontalStretchFactor}{stretchFactor}s and/or size hints
    instead.
 
    \note When setting implicit or preferred sizes, don't bind the respective
    properties to the width or height of the layout itself or items it depends
    on for its size calculations, as this can cause cyclic dependencies that
    are hard to track down.
*/