% File src/library/grDevices/man/getGraphicsEvent.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2014 R Core Team
% Distributed under GPL 2 or later

\name{getGraphicsEvent}
\alias{getGraphicsEvent}
\alias{setGraphicsEventHandlers}
\alias{getGraphicsEventEnv}
\alias{setGraphicsEventEnv}
\title{Wait for a mouse or keyboard event from a graphics window}
\description{
This function waits for input from a graphics window in the
form of a mouse or keyboard event.
}
\usage{
getGraphicsEvent(prompt = "Waiting for input",
                 onMouseDown = NULL, onMouseMove = NULL,
                 onMouseUp = NULL, onKeybd = NULL,
                 onIdle = NULL,
                 consolePrompt = prompt)
setGraphicsEventHandlers(which = dev.cur(), ...)
getGraphicsEventEnv(which = dev.cur())
setGraphicsEventEnv(which = dev.cur(), env)

}
\arguments{
  \item{prompt}{prompt to be displayed to the user in the graphics window}
  \item{onMouseDown}{a function to respond to mouse clicks}
  \item{onMouseMove}{a function to respond to mouse movement}
  \item{onMouseUp}{a function to respond to mouse button releases}
  \item{onKeybd}{a function to respond to key presses}
  \item{onIdle}{a function to call when no events are pending}
  \item{consolePrompt}{prompt to be displayed to the user in the console}
  \item{which}{which graphics device does the call apply to?}
  \item{...}{items including handlers to be placed in the event environment}
  \item{env}{an environment to be used as the event environment}
}
\details{
These functions allow user input from some graphics devices (currently
only the \code{windows()}, \code{X11(type = "Xlib")}
and \code{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 \code{getGraphicsEvent} are given, then it uses those
in a call to \code{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 \code{setGraphicsEventHandlers} or \code{setGraphicsEventEnv} on
one or more graphics devices, and then use \code{getGraphicsEvent()} with
no arguments to retrieve event data.
\code{getGraphicsEventEnv()} may be used to save the event environment
for use later.

The names of the arguments in \code{getGraphicsEvent} are special.  When
handling events, the graphics system will look through the event
environment for functions named \code{onMouseDown}, \code{onMouseMove},
\code{onMouseUp}, \code{onKeybd}, and \code{onIdle}, and use them as
event handlers.  It will use
\code{prompt} for a label on the graphics device.  Two other special names are
\code{which}, which will identify the graphics device, and
\code{result}, where the result of the last event
handler will be stored before being returned by \code{getGraphicsEvent()}.

The mouse event handlers should be functions with header
\code{function(buttons, x, y)}.  The coordinates \code{x}
and \code{y} will be passed to mouse event handlers in device independent
coordinates (i.e., the lower left corner of the window is \code{(0,0)},
the upper right is \code{(1,1)}).  The \code{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
\code{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 \code{shift-a} will be passed as
\code{"A"}.  The following special keys may also be passed to the handler:
\itemize{
     \item Control keys, passed as \code{"Ctrl-A"}, etc.
     \item Navigation keys, passed as one of\cr
     \code{"Left", "Up", "Right", "Down", "PgUp", "PgDn", "End", "Home"}
     \item Edit keys, passed as one of \code{"Ins", "Del"}
     \item Function keys, passed as one of \code{"F1", "F2", ...}
}

The idle event handler \code{onIdle} should be a function with no
arguments. If the function is undefined or \code{NULL}, then R will
typically call a system function which (efficiently) waits for the next
event to appear on a file handle. 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, \code{onIdle} is only implemented for \code{X11()} devices.

Note that calling \code{Sys.sleep()} is not recommended within an idle
handler - \code{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
\code{Sys.sleep()} are lost, and currently do not trigger the event
handlers registered via \code{getGraphicsEvent} or
\code{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
\itemize{
     \item one of the event handlers returns
a non-\code{NULL} value which will be returned as the value of
\code{getGraphicsEvent}, or
      \item the user interrupts the function from the
console.
}
}
\value{
When run interactively,
\code{getGraphicsEvent} returns a non-\code{NULL} value returned from one of the event handlers.
In a non-interactive session, \code{getGraphicsEvent} will return \code{NULL} immediately.  It
will also return \code{NULL} if the user closes the last window that has graphics handlers.

\code{getGraphicsEventEnv} returns the current event environment for the graphics device,
or \code{NULL} if none has been set.

\code{setGraphicsEventEnv} and \code{setGraphicsEventHandlers} return the previous
event environment for the graphics device.
}
\author{Duncan Murdoch}
\examples{
# This currently only works on the Windows, X11(type = "Xlib"), and
# X11(type = "cairo") screen devices...
\dontrun{
savepar <- par(ask = FALSE)
dragplot <- function(..., xlim = NULL, ylim = NULL, xaxs = "r", yaxs = "r") {
    plot(..., xlim = xlim, ylim = ylim, xaxs = xaxs, yaxs = yaxs)
    startx <- NULL
    starty <- NULL
    prevx <- NULL
    prevy <- NULL
    usr <- NULL

    devset <- function()
        if (dev.cur() != eventEnv$which) dev.set(eventEnv$which)

    dragmousedown <- function(buttons, x, y) {
        startx <<- x
        starty <<- y
        prevx <<- 0
        prevy <<- 0
        devset()
        usr <<- par("usr")
        eventEnv$onMouseMove <- dragmousemove
        NULL
    }

    dragmousemove <- function(buttons, x, y) {
        devset()
        deltax <- diff(grconvertX(c(startx, x), "ndc", "user"))
        deltay <- diff(grconvertY(c(starty, y), "ndc", "user"))
	if (abs(deltax-prevx) + abs(deltay-prevy) > 0) {
	    plot(..., xlim = usr[1:2]-deltax, xaxs = "i",
		      ylim = usr[3:4]-deltay, yaxs = "i")
	    prevx <<- deltax
	    prevy <<- deltay
	}
        NULL
    }

    mouseup <- function(buttons, x, y) {
    	eventEnv$onMouseMove <- NULL
    }	

    keydown <- function(key) {
        if (key == "q") return(invisible(1))
        eventEnv$onMouseMove <- NULL
        NULL
    }

    setGraphicsEventHandlers(prompt = "Click and drag, hit q to quit",
                     onMouseDown = dragmousedown,
                     onMouseUp = mouseup,
                     onKeybd = keydown)
    eventEnv <- getGraphicsEventEnv()
}

dragplot(rnorm(1000), rnorm(1000))
getGraphicsEvent()
par(savepar)
}
}
\keyword{iplot}
