ListenEvent

This function sets a callback function that will be executed when an event occurs.

Syntax

ListenEvent(number id, Widget source, function callback, Object extra = nil)

Parameter	Description
id	event ID to listen for, or EVENT_NONE to listen for all events
source	event source to listen for, or nil for events emitted from any source
callback	function to call when event is triggered
extra	extra object to pass to the callback function

Remarks

Internally, this function will store a weak pointer to the event source and a shared pointer to the extra field.

If the callback function returns false no more callbacks will be executed and no event will be added to the event queue.

Example

The first example shows another way to quit the program when the window is closed:

-- Define the EventCallback function
local function EventCallback(ev, extra)
    os.exit(0)
end

-- Get the displays
local displays = GetDisplays()

-- Create window
local window = CreateWindow("Ultra Engine", 0, 0, 800, 600, displays[1])

-- Create user interface
local ui = CreateInterface(window)

-- Listen for the window close event
ListenEvent(EVENT_WINDOWCLOSE, window, EventCallback)

while true do
    WaitEvent()
end

The example below demonstrates how an event listener can be used for custom resizing behavior:

function EventCallback(ev, extra)
    local panel = Widget(extra)
    local sz = panel:GetParent():ClientSize()
    panel:SetShape(50, 50, sz.x - 100, sz.y - 100)
    panel:SetText(tostring(sz.x) .. " x " .. tostring(sz.y))
    return true
end

--Get the displays
local displays = GetDisplays()

--Create window
local window = CreateWindow("Ultra Engine", 0, 0, 800, 600, displays[1], WINDOW_TITLEBAR | WINDOW_RESIZABLE | WINDOW_CENTER)

--Create user interface
local ui = CreateInterface(window)

local sz = ui.background:ClientSize()
local panel = CreatePanel(50, 50, sz.x - 100, sz.y - 100, ui.background)
panel:SetColor(0, 0, 0)

--Comment this line out to see why it is needed. ;)
ListenEvent(EVENT_WINDOWSIZE, window, EventCallback, panel)

while true do
    local ev = WaitEvent()
    if ev.id == EVENT_WINDOWCLOSE then
        if ev.source == window then
            return 0
        end
    elseif ev.id == EVENT_WINDOWSIZE then
        sz = panel:GetParent():ClientSize()
        panel:SetShape(50, 50, sz.x - 100, sz.y - 100)
        panel:SetText(tostring(sz.x) .. " x " .. tostring(sz.y))
    end
end