awesome/docs/04-new-widgets.md

Creating new widget

All widgets have to be generated by the wibox.widget.base.make_widget function so that the needed signals are added and mouse input handling is set up.

The returned widget will have a :buttons member function that can be used to register a set of mouse button events with the widget.

To implement your own widget, you can implement some member functions on a freshly-created widget. Note that all of these functions should be deterministic in the sense that they will show the same behavior if they are repeatedly called with the same arguments (same width and height). If your widget is updated and needs to change, suitable signals have to be emitted. This will be explained later.

The first callback is :fit. This function is called to select the size of your widget. The arguments to this function is the available space and it should return its desired size. Note that this function only provides a hint which is not necessarily followed. The widget must also be able to draw itself at different sizes than the one requested.

function widget:fit(context, width, height)
    -- Find the maximum square available
    local m = math.min(width, height)
    return m, m
end

The next callback is :draw. As the name suggests, this function is called to draw the widget. The arguments to this widget are the context that the widget is drawn in, the Cairo context on which it should be drawn and the widget's size. The Cairo context is set up in such a way that the widget as its top-left corner at (0, 0) and its bottom-right corner at (width, height). In other words, no special transformation needs to be done. Note that during this callback a suitable clip will already be applied to the Cairo context so that this callback will not be able to draw outside of the area that was registered for the widget by the layout that placed this widget. You should not call cr:reset_clip(), as redraws will not be handled correctly in this case.

function widget:draw(context, cr, width, height)
    cr:move_to(0, 0)
    cr:line_to(width, height)
    cr:move_to(0, height)
    cr:line_to(width, 0)
    cr:stroke()
end

There are two signals configured for a widget. When the result that :fit would return changes, the widget::layout_changed signal has to be emitted. If this actually causes layout changes, the affected areas will be redrawn. The other signal is widget::redraw_needed. This signal signals that :draw has to be called to redraw the widget, but it is safe to assume that :fit does still return the same values as before. If in doubt, you can emit both signals to be safe.

If your widget only needs to draw something to the screen, the above is all that is needed. The following callbacks can be used when implementing layouts which place other widgets on the screen.

The :layout callback is used to figure out which other widgets should be drawn relative to this widget. Note that it is allowed to place widgets outside of the extents of your own widget, for example at a negative position or at twice the size of this widget. Use this mechanism if your widget needs to draw outside of its own extents. If the result of this callback changes, widget::layout_changed has to be emitted. You can use :fit_widget to call the :fit callback of other widgets. Never call :fit directly! For example, if you want to place another widget child inside of your widget, you can do it like this:

-- For readability
local base = wibox.widget.base
function widget:layout(width, height)
    local result = {}
    table.insert(result, base.place_widget_at(child, width/2, 0, width/2, height)
    return result
end

Finally, if you want to influence how children are drawn, there are four callbacks available that all get similar arguments:

function widget:before_draw_children(context, cr, width, height)
function widget:after_draw_children(context, cr, width, height)
function widget:before_draw_child(context, index, child, cr, width, height)
function widget:after_draw_child(context, index, child, cr, width, height)

All of these are called with the same arguments as the :draw() method. Please note that a larger clip will be active during these callbacks that also contains the area of all children. These callbacks can be used to influence the way in which children are drawn, but they should not cause the drawing to cover a different area. As an example, these functions can be used to draw children translucently:

function widget:before_draw_children(context, cr, width, height)
    cr:push_group()
end
function widget:after_draw_children(context, cr, width, height)
    cr:pop_group_to_source()
    cr:paint_with_alpha(0.5)
end

In pseudo-code, the call sequence for the drawing callbacks during a redraw looks like this:

widget:draw(context, cr, width, height)
widget:before_draw_children(context, cr, width, height)
for child do
    widget:before_draw_child(context, cr, child_index, child, width, height)
    cr:save()
    -- Draw child and all of its children recursively, taking into account the
    -- position and size given to base.place_widget_at() in :layout().
    cr:restore()
    widget:after_draw_child(context, cr, child_index, child, width, height)
end
widget:after_draw_children(context, cr, width, height)

The :set_children() method gets called recursively when setting a widget with the declarative layout system, therefore the method should be well defined. It should probably hook into the :add or :add_widget methods or be overridden to do nothing.