Sizes of Dillo Widgets

Allocation

Each widget has an allocation at a given time, this includes

the position (x, y) relative to the upper left corner of the canvas, and
the size (width, ascent, descent).

The canvas is the whole area available for the widgets, in most cases, only a part is seen in a viewport. The allocation of the toplevel widget is exactly the allocation of the canvas, i.e.

the position of the toplevel widget is always (0, 0), and
the canvas size is defined by the size of the toplevel widget.

The size of a widget is not simply defined by the width and the height, instead, widgets may have a base line, and so are vertically divided into an ascender (which height is called ascent), and a descender (which height is called descent). The total height is so the sum of ascent and descent.

Sizes of zero are allowed. The upper limit for the size of a widget is defined by the limits of the C++ type int.

Allocation of a Widget

In the example in the image, the widget has the following allocation:

x = 50
y = 50
width = 150
ascent = 150
descent = 100

The current allocation of a widget is hold in dw::core::Widget::allocation. It can be set from outside by calling dw::core::Widget::sizeAllocate. This is a concrete method, which will call dw::core::Widget::sizeAllocateImpl (see code of dw::core::Widget::sizeAllocate for details).

For trivial widgets (like dw::Bullet), dw::core::Widget::sizeAllocateImpl does not need to be implemented. For more complex widgets, the implementation should call dw::core::Widget::sizeAllocate (not dw::core::Widget::sizeAllocateImpl) on all child widgets, with appropriate child allocations. dw::core::Widget::allocation should not be changed here, this is already done in dw::core::Widget::sizeAllocate.

Requisitions

A widget may prefer a given size for the allocation. This size, the requisition, should be returned by the method dw::core::Widget::sizeRequestImpl. In the simplest case, this is independent of the context, e.g. for an image. dw::Image::sizeRequestImpl returns the following size:

If no buffer has yet been assigned (see dw::Image for more details), the size necessary for the alternative text is returned. If no alternative text has been set, zero is returned.
If a buffer has been assigned (by dw::Image::setBuffer), the root size is returned (i.e. the original size of the image to display).

This is a bit simplified, dw::Image::sizeRequestImpl should also deal with margins, borders and paddings, see dw::core::style.

From the outside, dw::Image::sizeRequest should be called, which does a bit of optimization. Notice that in dw::Image::sizeRequestImpl, no optimization like lazy evaluation is necessary, this is already done in dw::Image::sizeRequest.

A widget, which has children, will likely call dw::Image::sizeRequest on its children, to calculate the total requisition.

The caller (this is either the dw::core::Layout, or the parent widget), may, but also may not consider the requisition. Instead, a widget must deal with any allocation. (For example, dw::Image scales the image buffer when allocated at another size.)

Size Hints

Some widgets do not have an inherent size, but depend on the context, e.g. the viewport size. These widgets should adhere to size hints, i.e. implement the methods dw::core::Widget::setWidth, dw::core::Widget::setAscent and dw::core::Widget::setDescent. The values passed to the callees are

the viewport size (ascent is the heigt here, while descent is 0) for the toplevel widget, and
determined by the parent for its child widgets.

Generally, the values should define the available space for the widget.

A widget, which depends on size hints, should call dw::core::Widget::queueResize, when apropriate.

Todo:: There should be a definition of "available space".

Width Extremes

dw::Table uses width extremes for fast calculation of column widths. The structure dw::core::Extremes represents the minimal and maximal width of a widget, as defined by:

the minimal width is the smallest width, at which a widget can still display contents, and
the maximal width is the largest width, above which increasing the width does not make any sense.

Especially the latter is vaguely defined, here are some examples:

For those widgets, which do not depend on size hints, the minimal and the maximal width is the inherent width (the one returned by dw::core::Widget::sizeRequest).
For a textblock, the minimal width is the width of the widest (unbreakable) word, the maximal width is the width of the total paragraph (stretching a paragraph further would only waste space). Actually, the implementation of dw::Textblock::getExtremesImpl is a bit more complex.
dw::Table is an example, where the width extremes are calculated from the width extremes of the children.

Handling width extremes is similar to handling requisitions, a widget must implement dw::core::Widget::getExtremesImpl, but a caller will use dw::core::Widget::getExtremes.

Resizing

When the widget changes its size (requisition), it should call dw::core::Widget::queueResize. The next call of dw::core::Widget::sizeRequestImpl should then return the new size. See dw::Image::setBuffer as an example.

Interna are described in the code of dw::core::Widget::queueResize.

Incremental Resizing

A widget may calculate its size based on size calculations already done before. In this case, a widget must exactly know the reasons, why a call of dw::core::Widget::sizeRequestImpl is necessary. To make use of this, a widget must implement the following:

There is a member dw::core::Widget::parentRef, which is totally under control of the parent widget (and so sometimes not used at all). It is necessary to define how parentRef is used by a specific parent widget, and it has to be set to the correct value whenever necessary.
The widget must implement dw::core::Widget::markSizeChange and dw::core::Widget::markExtremesChange, these methods are called in two cases:
1. directly after dw::core::Widget::queueResize, with the argument ref was passed to dw::core::Widget::queueResize, and
2. if a child widget has called dw::core::Widget::queueResize, with the value of the parent_ref member of this child.

This way, a widget can exactly keep track on size changes, and so implement resizing in a faster way. A good example on how to use this is dw::Textblock.