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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 examples/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 examples/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 examples/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 examples/layouts/flowlayout/flowlayout.cpp 3

    \c addItem() is implemented to add items to the layout. 

    \snippet examples/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 examples/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 examples/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 examples/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 examples/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 examples/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 examples/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 examples/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 examples/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.

*/