koreader/doc/Events.md

Events

Overview

All widgets are a subclass of @{ui.widget.eventlistener}, therefore they inherit the @{ui.widget.eventlistener:handleEvent|handleEvent} method. To send an event to a widget, you can simply invoke the handleEvent method like the following:

widget_foo:handleEvent(Event:new("Timeout"))

If the widget can be destroyed during the event you should call @{ui.uimanager:sendEvent|UIManager:sendEvent} to propagate the event from the topmost widget or @{ui.uimanager:broadcastEvent|UIManager:broadcastEvent} to send the event to all widgets.

Events are passed to child Widgets (or child containers) before their own handler sees them. See the implementation of WidgetContainer:handleEvent(). So a child widget, for instance a text input widget, gets the input events before the layout manager. The child widgets can "consume" an event by returning true from the event handler. Thus a text input widget just implements an input handler and consumes left/right presses, returning true in those cases. It can even make its return code dependent on whether the cursor is on the last position (do not consume press to right) or first position (do not consume press to left) to have proper focus movement in those cases.

Builtin events

Reader events

• UpdatePos: emitted by typesetting related modules to notify other modules to recalculate the view based on the new typesetting.

• PosUpdate: emitted by readerrolling module to signal a change in pos.

Event propagation

Most UI components are a subclass of @{ui.widget.container.widgetcontainer|WidgetContainer}. A WidgetContainer is an array that stores a list of children widgets.

When @{ui.widget.container.widgetcontainer:handleEvent|WidgetContainer:handleEvent} is called with a new event, it will run roughly the following code:

-- First propagate event to its children
for _, widget in ipairs(self) do
    if widget:handleEvent(event) then
        -- stop propagating when an event handler returns true
        return true
    end
end
-- If not consumed by children, consume it ourself
return self["on"..event.name](self, unpack(event.args, 1, event.argc))

Event system

The @{ui.event|Event} system is used by widgets to communicate.

Each event is an object that has two properties: args and handler. handler is the name of function that will be called on receive. args is a table that contains all the arguments needed to be passed to the event handler. When a widget receives a event, it will first check to see if self[event.handler] exists. If yes, the self[event.handler] function will be called and the return value of the handler will be returned to UIManager.

Notice that if you don't want the event propagate after consumed in your handler, your handler must return true. Otherwise, the event will be passed to other widgets' handlers until one of the handlers returns true.

@{ui.widget.container.widgetcontainer|WidgetContainer} is a special kind of widget. When it receives an event, it will first propagate the event to all its children. If the event is still not consumed (i.e., its handler returns true), then it will try to handle the event by itself.

When you call @{ui.uimanager.show|UIManager:show} on a widget, this widget will be added to the top of the UIManager._window_stack. Events are sent to the first widget in UIManager._window_stack. If it is not consumed, then UIManager will try to send it to all active widgets (widget.is_always_active equals true) in the _window_stack.

Draw Page Code Path

• in readerview.lua: ReaderView widget flag itself dirty in ReaderView:recalculate
• in ui.lua: UI main loop calls ReaderView:paintTo
• in readerview.lua: ReaderView:paintTo calls document:drawPage
• in document.lua: document:drawPage check for cache, if found, return cache
• in document.lua: if cache not found, document:drawPage calls document:renderPage
• in document.lua: document:renderPage calls _document:openPage, page:draw and put the result into cache