Having trouble with graphics context

Question

I am working on multiple terminal screen app, for that I have a custom UIView subclass for the terminal views. Every time I need a new terminal screen, I prepare a new view.

This view class draws the text using a CGContextRef. The problem I am facing is that the context only draws the text of the last view that was created, e.g. if I have 3 terminals and drawing on first/second, it still draws on the third one.

My code so far:

-(void)drawRect:(CGRect)rect{  
    contxt = UIGraphicsGetCurrentContext();
}


-(void)setNeedsDisplayInRect:(CGRect)rect{

    UIGraphicsPushContext(contxt);

    //CGContextSaveGState(contxt);

    CGContextSetTextMatrix(contxt,CGAffineTransformIdentity);
    if (translated) {
        CGContextScaleCTM(contxt, 1, -1);
        translated = NO;
    }
    CGRect rectConvert = CGRectMake(rect.origin.x, rect.origin.y-screenWindowHeight, rect.size.width, rect.size.height);

    CGContextSetFillColorWithColor(contxt, bgColor.CGColor);
    CGContextFillRect(contxt, rectConvert);

    if (!isDeleteChar) {
        CGContextSetFillColorWithColor(contxt, fgColor.CGColor);
        [displayString drawInRect:rectConvert withFont:font lineBreakMode:NSLineBreakByWordWrapping alignment:NSTextAlignmentLeft];
    }
    if (ul == EXTENDED_5250_UNDERLINE) {
        CGContextSetFillColorWithColor(contxt, fgColor.CGColor);
        [@"_" drawInRect:rectConvert withFont:font lineBreakMode:NSLineBreakByWordWrapping alignment:NSTextAlignmentLeft];
    }

    //CGContextRestoreGState(contxt);
    UIGraphicsPopContext();
}

Finally I solved it by own using

[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate: [NSDate date]]; just after setNeedsDisplay.

Answer 1

First and foremost, you should not be doing drawing in the -setNeedsDisplayRect: method, all of your drawing code should be in drawRect: instead. This way, the main runloop can better organize redrawing of the views.

Second, I suspect the variables that you are using for your CGRect conversions are faulty and are drawing outside of the view bounds. You can test this premise by clipping the view's drawing (set layer.masksToBounds to YES for the views)

If this is the case, you can adjust them to be relative to the view (all drawing within the view should be relative to its bounds, not its frame). When drawing the view, assume a canvas that stretches the bounds property of the view, i.e origin at (0,0) and size of (width,height).

Now, it is worth also discussing what the rect property passed to drawRect: really is, it is not guaranteed to be the entire bounds of the view, so you should not assume that. It is the portion of the view that needs to be redrawn, however, common practice (for simpler views) is to ignore that parameter and just redraw the entire view. Once this becomes too expensive (or you need the drawing to be more optimal), you can look into doing partial redraws of your view.

All in all, it is difficult to diagnose the full problem without seeing the entire UIView subclass code.