Drawing in Cocoa, wrong xy value

I'm trying to draw a bunch of rectangles in an NSView which is inside an NSScrollView. The code is working pretty well, but it seems like the computer is drawing at the wrong points.

When I use NSLog() to log the current x and y the values are right, but when I look at my view, there is extra space added before the drawing coordinate.

This is the code:

//
//  CoverFlowView.m
//  iWatch
//
//  Created by ief2 on 03/11/10.
//  
//

#import "CoverFlowView.h"

#pragma mark Defaults
#define COVER_HEIGHT2WIDTH(a) ((float)a / 1.4)
#define COVER_HEIGHT (200.0)
#define COVER_WIDTH (COVER_HEIGHT2WIDTH(COVER_HEIGHT))
#define COVER_SPACE_MIN (20.0)

#pragma mark Private Methods
@interface CoverFlowView (PrivateMethods)
- (CGFloat)coverSpaceForMoviesPerLine:(NSUInteger *)n;
- (void)frameDidChange;
- (void)updateFrame;
@end

NSColor *RandomColor() {
    float red = (float)(rand() % 255) / 254.0;
    float green = (float)(rand() % 255) / 254.0;
    float blue = (float)(rand() % 255) / 254.0;

    NSColor *theColor = [[NSColor colorWithDeviceRed:red 
                                               green:green 
                                                blue:blue 
                                               alpha:1.0] retain];
    return [theColor autorelease];
}

#pragma mark Implementation
@implementation CoverFlowView
#pragma mark Init and Dealloc
- (void)awakeFromNib {
    srand(time(NULL));
    NSMutableArray *colors = [[NSMutableArray alloc] initWithCapacity:100];
    register int i;
    for(i = 0; i < 100; i++) {
        [colors addObject:RandomColor()];
    }
    self.movies = colors;
    [colors autorelease];

    [[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(frameDidChange) 
                                                 name:NSViewFrameDidChangeNotification
                                               object:self];

}

- (id)initWithFrame:(NSRect)frame movies:(NSArray *)movs {

    self = [super initWithFrame:frame];
    if(self != nil) {
        self.movies = movs;

        // FIXME: Debug color array to movie
        NSMutableArray *colors = [[NSMutableArray alloc] initWithCapacity:100];
        register int i;
        srand((unsigned int)time(NULL));
        for(i = 0; i < 100; i++) {
            [colors addObject:RandomColor()];
        }
    }
    return self;
}

- (void)dealloc {
    [_movies release];

    [super dealloc];
}

#pragma mark Drawing
- (void)drawRect:(NSRect)dirtyRect {
    [[NSColor blackColor] setFill];
    NSRectFill(dirtyRect);

    NSUInteger moviesPerLine = 0;
    CGFloat coverSpace = 0;

    // Get the values
    coverSpace = [self coverSpaceForMoviesPerLine:&moviesPerLine];



    // if it's less than 1, break the drawing
    if(moviesPerLine < 1) {
        return;
    }

    // get the rows not to draw
    // p = top left point's y
   开发者_JAVA技巧 // h = movie cover height ->\
    // s = space between      -->The movies height and it's space above it
    // 
    //           p
    // f(x) = -------
    //         h + s
    double rows = ((dirtyRect.origin.y) / (COVER_HEIGHT + coverSpace));


    // get the first movie that has to be drawn
    // could be half-draw
    //
    // (NSUInteger)rows * moviesPerLine
    NSUInteger index = (NSUInteger)rows * moviesPerLine;
    if(!(index < [self.movies count]))
        return;


    // variable declaration
    CGFloat curY;
    CGFloat curX;

    // start drawing until the line after max
    // start the loop
    do {
        NSUInteger rowIndex;
        // get the y for the movie
        // x = movie index (start from 1)
        // m = movies per row
        // h = movie height
        // s = movie space
        //
        //         [x - (x % m)]
        // f(x) = (|-----------|) * (h + s) + s
        //         [     m     ]
        // BUT: if rows == 2, rows -= 1
        rowIndex = ((index + 1) - ((index + 1) % moviesPerLine)) / moviesPerLine;
        ((index + 1) % moviesPerLine == 0) ? (rowIndex -= 1) : (rowIndex = rowIndex);
        curY = rowIndex * (COVER_HEIGHT + coverSpace) + coverSpace;


        // get the x fot the movie
        // x = movie index (start from 1)
        // m = movies per line
        // w = movie width
        // s = cover space
        //
        //              
        // f(x) = ((x - (x - (x % m))) - 1) * (w + s) + s
        // BUT: if row == 0, row = m
        //
        rowIndex = (index+1) - (((index+1) - ((index + 1) % moviesPerLine)));
        (rowIndex == 0) ? (rowIndex = moviesPerLine) : (rowIndex = rowIndex);
        curX = (rowIndex - 1) * (COVER_WIDTH + coverSpace) + coverSpace;
        NSLog(@"%s index: %3u || x: %5.0f || y: %5.0f", __FUNCTION__, index, curX, curY);

        // Start the drawing
        NSRect bezierPathRect = NSMakeRect(curX + coverSpace, curY, COVER_WIDTH, COVER_HEIGHT);
        NSBezierPath *path = [NSBezierPath bezierPathWithRect:bezierPathRect];
        [path setLineWidth:2.0];
        [[NSColor whiteColor] setStroke];
        [(NSColor *)[self.movies objectAtIndex:index] setFill];
        [path fill];
        [path stroke];

        // add up values
        index++;
        if(!(index < [self.movies count]))
            return;

    } while(curY - (COVER_HEIGHT + coverSpace) < dirtyRect.origin.y + dirtyRect.size.height);
}

- (BOOL)isFlipped {
    return YES;
}

#pragma mark Private Methods
- (void)frameDidChange {

}

- (CGFloat)coverSpaceForMoviesPerLine:(NSUInteger *)n {
    // x = number of covers
    // w = view width
    // m = cover width
    // y = the space
    //
    //        w - (mx)
    // f(x) = --------
    //         x + 1

    CGFloat m = COVER_WIDTH;
    CGFloat w = [self bounds].size.width;

    register NSUInteger x = 1;
    CGFloat space;
    while(1) {
        space = ((w-(m*x)) / ( x + 1.0));
        if(space < COVER_SPACE_MIN) {
            x--;
            space = ((w - (m*x)) / (x + 1.0));
            break;
        }
        x++;
    }

    if(n) *n = x;
    return space;
}

#pragma mark Properties
@synthesize movies=_movies;
- (void)setMovies:(NSArray *)ar {

    if(ar != _movies) {
        [_movies release];
        _movies = [ar retain];

        // Update frame size
        NSUInteger m;
        CGFloat space = [self coverSpaceForMoviesPerLine:&m];
        NSUInteger lines = [ar count] / m;
        CGFloat height = (COVER_HEIGHT + space) * ((CGFloat)lines) + space;
        CGFloat width = [self frame].size.width;
        NSRect frame = [self frame];
        frame.size = NSMakeSize(width, height);
        [self setFrame:frame];
        [self setNeedsDisplay:YES];
    }
}
@end

and here you can find a screen shot of the running application:

If someone can tell me what I'm doing wrong, I'd really appreciate it!

Thank you, ief2

EDIT: Don't mind the wrong titles, I made mistake giving the files a name :-)

Seems to me you’re calculating the X coordinate the wrong way. You have twice the spacing at the beginning of the line and no space at the end of the line. So if you subtract one spacing from curX you get the right result:

curX = (rowIndex - 1) * (COVER_WIDTH + coverSpace);

Also your code is more complicated than it needs to be. For example, you don’t really need a loop in coverSpaceForMoviesPerLine: and the loop in drawRect: can be simplified (and made more efficient at the same time) too.