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Getting the Location of an Event

You can get the location of a mouse or tablet-pointer event by sending locationInWindow to an NSEvent object. But, as the name of the method denotes, this location (an NSPoint structure) is in the base coordinate system of a window, not in the coordinate system of the view that typically handles the event. Therefore a view must convert the point to its own coordinate system using the method convertPoint:fromView:, as shown in Listing 3-4.

Listing 3-4  Converting a mouse-dragged location to be in a view’s coordinate system

- (void)mouseDragged:(NSEvent *)event {

    NSPoint eventLocation = [event locationInWindow];

    center = [self convertPoint:eventLocation fromView:nil];

    [self setNeedsDisplay:YES];

}

The second parameter of convertPoint:fromView: is nil to indicate that the conversion is from the window’s base coordinate system.

Keep in mind that the locationInWindow method is not appropriate for key events, periodic events, or any other type of event other than mouse and tablet-pointer events.

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