These functions allow user input from some graphics devices (currently
only the windows(), X11(type = "Xlib")
and X11(type = "cairo") screen displays
in base R). Event handlers may be installed to respond to events
involving the mouse or keyboard.
The functions are related as follows. If any of the first six
arguments to getGraphicsEvent are given, then it uses those
in a call to setGraphicsEventHandlers to replace any existing
handlers in the current device. This is for compatibility with pre-2.12.0 R
versions. The current normal way to set up event handlers is to
set them using setGraphicsEventHandlers or setGraphicsEventEnv on
one or more graphics devices, and then use getGraphicsEvent() with
no arguments to retrieve event data.
getGraphicsEventEnv() may be used to save the event environment
for use later.
The names of the arguments in getGraphicsEvent are special. When
handling events, the graphics system will look through the event
environment for functions named onMouseDown, onMouseMove,
onMouseUp, onKeybd, and onIdle, and use them as
event handlers. It will use
prompt for a label on the graphics device. Two other special names are
which, which will identify the graphics device, and
result, where the result of the last event
handler will be stored before being returned by getGraphicsEvent().
The mouse event handlers should be functions with header
function(buttons, x, y). The coordinates x
and y will be passed to mouse event handlers in device independent
coordinates (i.e., the lower left corner of the window is (0,0),
the upper right is (1,1)). The buttons argument
will be a vector listing the buttons
that are pressed at the time of the event, with 0 for left, 1 for middle, and 2
for right.
The keyboard event handler should be a function with header
function(key). A single element character vector will be passed
to this handler, corresponding to the key press. Shift and other modifier
keys will have been processed, so shift-a will be passed as
"A". The following special keys may also be passed to the handler:
Control keys, passed as "Ctrl-A", etc.
Navigation keys, passed as one of
"Left", "Up", "Right", "Down", "PgUp", "PgDn", "End", "Home"
Edit keys, passed as one of "Ins", "Del"
Function keys, passed as one of "F1", "F2", ...
The idle event handler onIdle should be a function with no
arguments. If the function is undefined or NULL, then R will
typically call a system function which (efficiently) waits for the next
event to appear on a filehandle. Otherwise, the idle event handler will
be called whenever the event queue of the graphics device was found to
be empty, i.e. in an infinite loop. This feature is intended to allow
animations to respond to user input, and could be CPU-intensive.
Currently, onIdle is only implemented for X11() devices.
Note that calling Sys.sleep() is not recommended within an idle
handler - Sys.sleep() removes pending graphics events in order to
allow users to move, close, or resize windows while it is executing.
Events such as mouse and keyboard events occurring during
Sys.sleep() are lost, and currently do not trigger the event
handlers registered via getGraphicsEvent or
setGraphicsEventHandlers.
The event handlers are standard R functions, and will be executed as though called
from the event environment.
In an interactive session, events will be processed until
one of the event handlers returns
a non-NULL value which will be returned as the value of
getGraphicsEvent, or
the user interrupts the function from the
console.