Cocoa: NSRectFill on button click does not work

Question

Hello I am new to Cocoa programming and I met a problem about NSRectFill.

There is one button in the window, and the following is my AppDelegate.m file:

Answer 1

Some background, applicationDidFinishLaunching: is called at the launch of a program once the program is 'done' loading resources.

But at this point, there could be many windows and many views displayed by the application. Simply calling NSRectFill() is not enough. The application wouldn't know where to draw said rectangle (is it in this window or that one? This NSView or that one). Even if there's only one window and it might seem obvious... there might be multiple NSViews displayed that you're unaware of.... and with computers you have to be very explicit with your commands.

The bottom line is: there is no 'context' established for where the drawing actions to occur. As d00dle points out, you should read up on Drawing Guide.

When an NSView's drawRect: is called, a context (itself) has already been set/defined. You could draw directly from your delegate's applicationDidFinishLaunching: but a 'context' needs to be defined.