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Timestamp:
Feb 11, 2010, 11:19:06 PM (15 years ago)
Author:
Dmitry A. Kuminov
Message:

trunk: Merged in qt 4.6.1 sources.

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trunk
Files:
2 edited

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  • trunk

  • trunk/doc/src/examples/flowlayout.qdoc

    r2 r561  
    22**
    33** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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    … …  
    4444    \title Flow Layout Example
    4545
    46     The Flow Layout example demonstrates a custom layout that arranges child widgets from
    47     left to right and top to bottom in a top-level widget.
     46    The Flow Layout example demonstrates a custom layout that arranges child
     47    widgets from left to right and top to bottom in a top-level widget.
    4848
    49     \image flowlayout-example.png
     49    \image flowlayout-example.png Screenshot of the Flow Layout example
     50
     51    The items are first laid out horizontally and then vertically when each line
     52    in the layout runs out of space.
     53
     54        The Flowlayout class mainly uses QLayout and QWidgetItem, while the
     55        Window uses QWidget and QLabel. We will only document the definition
     56        and implementation of \c FlowLayout below.
     57
     58    \section1 FlowLayout Class Definition
     59   
     60    The \c FlowLayout class inherits QLayout. It is a custom layout class
     61    that arranges its child widgets horizontally and vertically.
     62
     63    \snippet examples/layouts/flowlayout/flowlayout.h 0
     64
     65    We reimplement functions inherited from QLayout. These functions add items to
     66    the layout and handle their orientation and geometry.
     67   
     68    We also declare two private methods, \c doLayout() and \c smartSpacing().
     69    \c doLayout() lays out the layout items, while the \c
     70    smartSpacing() function calculates the spacing between them.
     71
     72    \section1 FlowLayout Class Implementation
     73
     74    We start off by looking at the constructor:
     75
     76    \snippet examples/layouts/flowlayout/flowlayout.cpp 1
     77
     78    In the constructor we call \c setContentsMargins() to set the left, top,
     79    right and bottom margin. By default, QLayout uses values provided by
     80    the current style (see QStyle::PixelMetric).
     81
     82    \snippet examples/layouts/flowlayout/flowlayout.cpp 2
     83
     84    In this example we reimplement \c addItem(), which is a pure virtual
     85    function. When using \c addItem() the ownership of the layout items is
     86    transferred to the layout, and it is therefore the layout's
     87    responsibility to delete them.
     88   
     89    \snippet examples/layouts/flowlayout/flowlayout.cpp 3
     90   
     91    \c addItem() is implemented to add items to the layout.
     92
     93    \snippet examples/layouts/flowlayout/flowlayout.cpp 4
     94
     95    We implement \c horizontalSpacing() and \c verticalSpacing() to get
     96    hold of the spacing between the widgets inside the layout. If the value
     97    is less than or equal to 0, this value will be used. If not,
     98    \c smartSpacing() will be called to calculate the spacing.
     99
     100    \snippet examples/layouts/flowlayout/flowlayout.cpp 5
     101   
     102    We then implement \c count() to return the number of items in the
     103    layout. To navigate the list of items we use \c itemAt() and
     104    takeAt() to remove and return items from the list. If an item is
     105    removed, the remaining items will be renumbered. All three
     106    functions are pure virtual functions from QLayout.
     107
     108    \snippet examples/layouts/flowlayout/flowlayout.cpp 6
     109
     110    \c expandingDirections() returns the \l{Qt::Orientation}s in which the
     111    layout can make use of more space than its \c sizeHint().
     112
     113    \snippet examples/layouts/flowlayout/flowlayout.cpp 7
     114
     115    To adjust to widgets of which height is dependent on width, we implement \c
     116    heightForWidth(). The function \c hasHeightForWidth() is used to test for this
     117    dependency, and \c heightForWidth() passes the width on to \c doLayout() which
     118    in turn uses the width as an argument for the layout rect, i.e., the bounds in
     119    which the items are laid out. This rect does not include the layout margin().
     120   
     121    \snippet examples/layouts/flowlayout/flowlayout.cpp 8
     122
     123    \c setGeometry() is normally used to do the actual layout, i.e., calculate
     124    the geometry of the layout's items. In this example, it calls \c doLayout()
     125    and passes the layout rect.
     126
     127    \c sizeHint() returns the preferred size of the layout and \c minimumSize()
     128    returns the minimum size of the layout.
     129
     130    \snippet examples/layouts/flowlayout/flowlayout.cpp 9
     131
     132    \c doLayout() handles the layout if \c horizontalSpacing() or \c
     133    verticalSpacing() don't return the default value. It uses
     134    \c getContentsMargins() to calculate the area available to the
     135    layout items.
     136 
     137    \snippet examples/layouts/flowlayout/flowlayout.cpp 10
     138
     139        It then sets the proper amount of spacing for each widget in the
     140        layout, based on the current style.
     141       
     142    \snippet examples/layouts/flowlayout/flowlayout.cpp 11
     143
     144        The position of each item in the layout is then calculated by
     145        adding the items width and the line height to the initial x and y
     146        coordinates. This in turn lets us find out whether the next item
     147        will fit on the current line or if it must be moved down to the next.
     148        We also find the height of the current line based on the widgets height.
     149       
     150    \snippet examples/layouts/flowlayout/flowlayout.cpp 12
     151
     152        \c smartSpacing() is designed to get the default spacing for either
     153        the top-level layouts or the sublayouts. The default spacing for
     154        top-level layouts, when the parent is a QWidget, will be determined
     155        by querying the style. The default spacing for sublayouts, when
     156        the parent is a QLayout, will be determined by querying the spacing
     157        of the parent layout.
     158
    50159*/
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