Question

I have a few lines of drawing code that are duplicated in two different subclasses. When I move this drawing code to its own class and then call it from within drawRect: it is called but it is never drawn to the screen. What is the right way prevent duplicating code in two different drawRect: methods?

Details: I’m making a custom control by subclassing NSTableView and NSTableCellView. My drawing code needs to be in drawRect: in both of these subclasses.

I created a subclass of NSObject that declares one method. Here is the implementation:

@implementation TNLChartDrawingExtras

- (void)drawDividersInRect:(NSRect)rect startingAtDate:(NSDate *)startDate withZoomFactor:(NSNumber *)zoomFactor {

    float pos = 0;
    NSDate *currentDate = [startDate copy];

    while (pos < rect.size.width) {

        //draw the vertical divider
        NSBezierPath *linePath = [NSBezierPath bezierPathWithRect:NSMakeRect(pos, 0.0, 1.0, rect.size.height)];
        [[NSColor colorWithCalibratedWhite:0.85 alpha:0.5] set];
        [linePath fill];

        //increment the values for the next day
        currentDate = [NSDate dateWithTimeInterval:86400 sinceDate:currentDate]; // add one day to the current date
        pos = pos + (86400.0/ [zoomFactor floatValue]);
    }
}

In my NSTableView subclass I define a property for this object. Then in awakeFromNib I create an instance of this class:

- (void)awakeFromNib {
    self.extras = [[TNLChartDrawingExtras alloc] init];
}

In drawRect: I send this message:

- (void)drawRect:(NSRect)dirtyRect {
    // more code here...

    [self.extras drawDividersInRect:viewBounds startingAtDate:chart.startDate withZoomFactor:self.zoomFactor];
}

The code is executed but the lines it is supposed to draw don’t appear. If I put the code from drawDividersInRect:... in the drawRect: method, it works fine.

Answer 1

My original solution (described in the question) may have worked if I had continued to debug it. However, I think the more important question is what is the right way to approach this problem. Here I solve it by adding category on NSView to the project:

I’m trying to add custom drawing code to both NSTableView and NSTableCellView. Both are subclasses of NSView so I created a category of NSView and added my custom drawing method there. Now I can call my drawing method from both subclasses.