 | Contents | Index |
| CDOM.Level2.Events |
|
| setEventHandler :: (CElement zz, CEvent a) => String -> (a -> Bool) -> zz -> CPS c zz |
| waitFor :: (CEvent e, CElement c) => c -> String -> (e -> Bool) -> Bool |
This module provides support for handling events that a program executed in a Web browser may receive. The Javascript backend uses mixed model of event handling. Although methods like addEventListener are available via automated W3C IDL conversion, it is known that not all browsers consistently support this model of event handling. On the other hand, older model of setting event handlers as on- attributes of DOM HTML element objects has been widely supported by many generations of Web browsers.
In the same time, attributes of Event object (see DOM.Level2.Events, DOM.Level2.Event, DOM.Level2.KeyEvent, DOM.Level2.MouseEvent, etc.) are available even with this model of setting event handlers. So, this is possible to write:
get'keyCode e $ \kci ->
if kci == cDOM_VK_ENTER
then
...
else
...
Other methods of Event objects family are also available, but their support may vary across different web browsers.
There are two paradigms of handling DOM events. One, similar to the "traditional" method of setting a permanent event handler on a DOM object is represented by the setEventHandler function. Another, oriented to using pseudo-threads (see the Control.Concurrent.JSThreads module description), is represented by the waitFor function.
| :: (CElement zz, CEvent a) | |
| => String | type of an event (e. g. "click") |
| -> (a -> Bool) | event handler, should return a Boolean value |
| -> zz | the DOM element upon which the event handler is set |
| -> CPS c zz | the return value: the sane element |
| Set an event handler of the specified type. Event handlers may be set on Element's and their subclasses. This function sets the most general event handler. The handler is expected to receive an instance of the CEvent class as its argument, and return a Bool value. The rules covering the interpretation of handler's return value are same as in traditional Javascript programming: True means proceed with default action; False means prevent default action. | |
The waitFor function sets the event handler on a given DOM object so that it invokes the current continuation. Right after the event fires, the same event handler on the DOM object is removed (set to null). So, between calls to waitFor the DOM object remains irresponsive to users actions. Threads using waitFor usually pass events received to other threads via message boxes. Below is an example of such thread used in an activator for a Button element.
actb v mb par = (waitFor par "click" :: CPS Bool TMouseEvent) $ \e -> sendMsg_ mb v |>>| actb v mb par
The thread contains an infinite loop which waits for an event, sends a message using sendMsg_, then repeats again.
Return valus of waitFor depends on whether any other thread has already been waiting for the same event from this DOM element. If not analyzed, this may alter the handler's default action. Use waitForB to explicitly set the handler return value.
| :: (CEvent e, CElement c) | |
| => c | the DOM element upon which the event handler is set |
| -> String | type of an event (e. g. "click") |
| -> (e -> Bool) | event handler, should return a Boolean value |
| -> Bool | return value |
| Wait for an event from a DOM element. Execution of the continuation given is resumed as the event has been received. True is returned if no other thread was waiting for the same event on the same element, False otherwise. | |