Graphics Contexts
A graphics
context is a low-level component of the drawing system representing a
destination for drawing commands that will be executed and rendered.
Instances of the
NSGraphicsContext
class represent these contexts. The
graphics context also provides an interface to manage graphics states
and configure a number of rendering options, such as anti-aliasing,
image interpolation, and various settings for drawing paths such as
line width and join styles.
At any time in the execution of an application, a current context
defines the graphics context for all graphics operations. You can
obtain this object using the class method
currentContext
. Multiple graphics
contexts are stored on a stack, so that different parts of an
application may configure a context specific to their needs. To push
a new context onto the stack, use the method
saveGraphicsState
. Contexts lower in the stack are
restored by invoking restoreGraphicsState
, which
pulls the current context off the stack, making the next lower one
current.
You can use a graphics context to determine whether or not the screen
is the current drawing destination. This is useful if you have to
handle onscreen and off-screen drawing differently (e.g., to a
printer). The isDrawingToScreen
method returns
YES
if drawing is done onscreen, and
NO
otherwise. The convenience class method
currentContextDrawingToScreen
does the same thing,
saving you the step of first invoking
currentContext
.
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