qt5base-lts/examples/widgets/doc/flowlayout.qdoc

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/*!
    \example layouts/flowlayout
    \title Flow Layout Example

    The Flow Layout example demonstrates a custom layout that arranges child 
    widgets from left to right and top to bottom in a top-level widget.

    \image flowlayout-example.png Screenshot of the Flow Layout example

    The items are first laid out horizontally and then vertically when each line
    in the layout runs out of space. 

	The Flowlayout class mainly uses QLayout and QWidgetItem, while the
	Window uses QWidget and QLabel. We will only document the definition 
	and implementation of \c FlowLayout below.

    \section1 FlowLayout Class Definition
    
    The \c FlowLayout class inherits QLayout. It is a custom layout class 
    that arranges its child widgets horizontally and vertically. 

    \snippet layouts/flowlayout/flowlayout.h 0

    We reimplement functions inherited from QLayout. These functions add items to
    the layout and handle their orientation and geometry. 
    
    We also declare two private methods, \c doLayout() and \c smartSpacing().
    \c doLayout() lays out the layout items, while the \c
    smartSpacing() function calculates the spacing between them.

    \section1 FlowLayout Class Implementation 

    We start off by looking at the constructor:

    \snippet layouts/flowlayout/flowlayout.cpp 1

    In the constructor we call \c setContentsMargins() to set the left, top,
    right and bottom margin. By default, QLayout uses values provided by
    the current style (see QStyle::PixelMetric). 

    \snippet layouts/flowlayout/flowlayout.cpp 2

    In this example we reimplement \c addItem(), which is a pure virtual 
    function. When using \c addItem() the ownership of the layout items is 
    transferred to the layout, and it is therefore the layout's 
    responsibility to delete them. 
    
    \snippet layouts/flowlayout/flowlayout.cpp 3
    
    \c addItem() is implemented to add items to the layout. 

    \snippet layouts/flowlayout/flowlayout.cpp 4

    We implement \c horizontalSpacing() and \c verticalSpacing() to get
    hold of the spacing between the widgets inside the layout. If the value 
    is less than or equal to 0, this value will be used. If not, 
    \c smartSpacing() will be called to calculate the spacing.

    \snippet layouts/flowlayout/flowlayout.cpp 5
    
    We then implement \c count() to return the number of items in the 
    layout. To navigate the list of items we use \c itemAt() and 
    takeAt() to remove and return items from the list. If an item is 
    removed, the remaining items will be renumbered. All three 
    functions are pure virtual functions from QLayout.

    \snippet layouts/flowlayout/flowlayout.cpp 6

    \c expandingDirections() returns the \l{Qt::Orientation}s in which the
    layout can make use of more space than its \c sizeHint().

    \snippet layouts/flowlayout/flowlayout.cpp 7

    To adjust to widgets of which height is dependent on width, we implement \c
    heightForWidth(). The function \c hasHeightForWidth() is used to test for this
    dependency, and \c heightForWidth() passes the width on to \c doLayout() which
    in turn uses the width as an argument for the layout rect, i.e., the bounds in
    which the items are laid out. This rect does not include the layout margin().
    
    \snippet layouts/flowlayout/flowlayout.cpp 8

    \c setGeometry() is normally used to do the actual layout, i.e., calculate
    the geometry of the layout's items. In this example, it calls \c doLayout()
    and passes the layout rect.

    \c sizeHint() returns the preferred size of the layout and \c minimumSize()
    returns the minimum size of the layout.

    \snippet layouts/flowlayout/flowlayout.cpp 9

    \c doLayout() handles the layout if \c horizontalSpacing() or \c
    verticalSpacing() don't return the default value. It uses 
    \c getContentsMargins() to calculate the area available to the 
    layout items. 
 
    \snippet layouts/flowlayout/flowlayout.cpp 10

	It then sets the proper amount of spacing for each widget in the 
	layout, based on the current style. 
	
    \snippet layouts/flowlayout/flowlayout.cpp 11

	The position of each item in the layout is then calculated by 
	adding the items width and the line height to the initial x and y 
	coordinates. This in turn lets us find out whether the next item 
	will fit on the current line or if it must be moved down to the next. 
	We also find the height of the current line based on the widgets height. 
	
    \snippet layouts/flowlayout/flowlayout.cpp 12

	\c smartSpacing() is designed to get the default spacing for either 
	the top-level layouts or the sublayouts. The default spacing for 
	top-level layouts, when the parent is a QWidget, will be determined 
	by querying the style. The default spacing for sublayouts, when 
	the parent is a QLayout, will be determined by querying the spacing 
	of the parent layout.

*/