R: Methods to add event handlers to objects

gWidgets-handlers {gWidgets} | R Documentation

## Methods to add event handlers to objects

### Description

In the gWidgets API handlers are called in reponse to certain events such as keystrokes or clicks. This set of methods makes a consistent interface to some typical events. Not all handlers are defined for each widget.

### Usage

    addHandlerChanged(obj, handler = NULL, action = NULL, ...)

    addHandlerKeystroke(obj, handler = NULL, action = NULL, ...)

    addHandlerClicked(obj, handler = NULL, action = NULL, ...)

    addHandlerDoubleclick(obj, handler = NULL, action = NULL, ...)

    addHandlerRightclick(obj, handler = NULL, action = NULL, ...)

    addHandlerFocus(obj, handler = NULL, action = NULL, ...)

    addHandlerBlur(obj, handler = NULL, action = NULL, ...)

    addHandlerMouseMotion(obj, handler = NULL, action = NULL, ...)

    addHandlerExpose(obj, handler = NULL, action = NULL, ...)

    addHandlerUnrealize(obj, handler = NULL, action = NULL, ...)

    addHandlerDestroy(obj, handler = NULL, action = NULL, ...)

    addHandlerIdle (obj, handler = NULL, action = NULL, interval = 1000,   ...)

    addPopupmenu(obj, menulist, action=NULL, ...)

    add3rdMousePopupmenu(obj, menulist, action=NULL, ...)

    removeHandler(obj, ID=NULL, ...)

    blockHandler(obj, ID=NULL, ...)

    unblockHandler(obj, ID=NULL, ...)

### Arguments

`obj` | The object to assign handler to

`handler` | A function to call if the given event occurs. The function's first argument is a list with some specific components. The component `obj` contains the object that the handler was assigned to. The `action` component contains the value given to the argument `action`. This can be used with `do.call` to make simple handlers. Or, this can be used to pass in other widgets, etc. Sometimes there are other components. For drag and drop handlers the component `dropdata` refers to the dropped data. For `ggraphics` the `addHandlerclicked` contains components `x` and `y` indicating where the click occurred.
`action` | Used to pass extra information into handlers
`interval` | For `addHandleridle` this specifies the time in milliseconds between calls to the handler.
`menulist` | For `addpopupmenu` and `add3rdmousepopupmenu` this specifies a menubar using a list which is in turn passed to `gmenu`.
`ID` | When a handler is assigned, an id is returned. This id can be used to remove or block a handler from an object.
`...` | Not documented, currently has no role.

### Details

At first these handlers were all lowercase. These functions are still availabe, although the mixed case usage is encouraged

In GTK, and other toolkits, an event causes a signal to be triggered and these handlers are called in response to that signal.

These signals have various names known to the GTK programmer. say. These functions attempt to shield the gWidgets user from needing to learn these signals. For gWidgetsRGtk, if these handlers prove insufficient then the non-exported `addHandler` function has an additional `signal` argument: `(obj,signal,handler, action,...)` for specifying a GTK signal. By avoiding this, we can make the gWidgets API non-toolkit specific.

The signals are defined to match the event described by the method name, e.g., "doubleclick."

The handlers all have signature `(h,...)` where the first argument is a list with components `obj` containing the widget the handler is added to and `action` containing the values passed along to the `action` argument. This can be used to pass in other widget's names, when they can not be found from a function closure, say.

The handlers do not have lazy evaluation. The value of `action` is the one at the time of creation of the widget. (See the example). In GTK, a means to cheat this is to pass in a gWidget instance, as the underlying GTK objects are stored as pointers, not copies, so that when queried, their current state is used.

`addHandlerChanged`:

  This handler is called when a widget is "changed." This is interpreted differently by the various widgets. For `gedit` change refers to a changed value, not a keystroke change (when ENTER is pressed). For notebooks, this is called when a page is changed.

`addHandlerKeystroke`:

  This handler is called when keys are pressed in the text widgets. The extra argument `key` is used to pass back the key code of the pressed key.

`addHandlerClicked`:

  This handler is called when a widget, such as a button or label, is clicked.

`addHandlerDoubleclick`:

  This handler is called when a widget is doubleclicked, like in the tree widget. Not all widgets receive a double click signal. Only when a single mouse click is needed for selection is this implemented.

`addHandlerRightclick`:

  This handler is called when a widget is clicked with the right mouse button

`addHandlerFocus`:

  This handler is called when a widget gains focus

`addHandlerBlur`:

  This handler is called when a widget loses focus

`addHandlerMouseMotion`:

  This handler is called when a the mouse moves over a widget. In some toolkits it is called just once per visit to the widget, for others maybe multiple times. This is like a mouseover for web pages. The drag motion handler is similar, only it is called when a drag event is dragged over a widget.

`addHandlerExpose`:

  handler is called when a widget is exposed. For instance when a page in a notebook is exposed.

`addHandlerUnrealize`:

  handler is called when a widget is being unrealized.

`addHandlerDestroy`:

  handler is called when a widget is being destroyed. For top level windows, this usually allows one to intercept the window destroy event for purposes of saving work etc.

`addHandlerIdle`:

  handler is called every so often, and can be used to update a widget's content. This method has an extra argument `interval` specifying the interval in milliseconds with a default of 1000 or 1 second.

Although not handlers, the `addPopupMenu` method adds a popup menu to a mouse click. The popup menu is specified using a list that is passed to `gmenu`.

A refinement of this is the `add3rdMousePopupmenu` method which puts the popupmenu on the right mouse click.

The method `removeHandler` is used to remove a handler from an object. If an ID is specified, just that handler is removed, otherwise all handlers will be. To temporarily disable a handler, use `blockHandler` then `unblockHandler`.

### See Also

`gWidgets-methods`

### Examples

    ## Not run:
    	## a default handler, useful for when action is enough to
    	## specify desired results

    	handler.default = function(h,...) do.call(h$action,list(svalue(h$obj)))
    	group = ggroup(horizontal=FALSE, container=gwindow("Click
    		button"))
            button = gbutton("Click me", container=group)
    	addhandlerclicked(button, handler=handler.default, action="print")

    	## use two widgets, one to update the other
    	group = ggroup(horizontal=FALSE, container=gwindow("two widgets"))
    	button = gbutton("click me", container=group)
    	label = glabel("Button has not been clicked", container=group)
    	addhandlerclicked(button, handler = function(h,...) {
    	svalue(h$obj) <-"click me again"
    	svalue(h$action) <- "Button has been clicked"
    	}, action = label)

    	## lazy evaluation is not used here
    	obj = 4
    	gbutton("click",container=TRUE, handler=function(h,...)
    	print(h$action), action=obj)
    	obj = 2
    	## now click button and value of 4 will be printed, not 2

    	## Whereas, if one uses a gWidget we get the same as lazy
           ## loading
           obj = gedit("4")
    	gbutton("click",container=TRUE, handler=function(h,...)
    		print(svalue(h$action)), action=obj)
    	svalue(obj) <- "2"
    	## Now click and "2" is printed.

          ## remove handler, block handler, unblockhandler (latter two may not be implemented)
          b <- gbutton("click", container=gwindow())
          id <- addHandlerClicked(b, handler=function(h,...) print("ouch"))
          ## click --> "ouch"
          blockHandler(b, id) ## now click -- nothing
          unblockHandler(b, id) ## now click -- "ouch"
          removeHandler(b, id)  ## all gone now

    ## End(Not run)

[Package gWidgets version 0.0-54 Index]