Question

I have to develop an application which takes PDF as input, that PDF document has 200 pages. I am using UIScrollView to swipe left and right. On each swipe I am drawing a PDF document. The code is as under :

- (id)initWithFrame:(CGRect)frame content:(NSString *)aPDFPage type:(NSString *)contentType
{
    if ((self = [super initWithFrame:frame])) 
    {
        // Initialization code
        self.backgroundColor = [UIColor whiteColor];
        pageRef = [[NSString alloc]initWithString:aPDFPage];
        pageTypeRef = [[NSString alloc]initWithString:contentType];
    }
    return self;
}


- (void)drawRect:(CGRect)rect 
{
    ctx = UIGraphicsGetCurrentContext();
    [self drawPDF];
}

-(void)drawPDF
{
    NSString *pathToPdfDoc = [[NSBundle mainBundle] pathForResource:self.pageRef ofType:self.pageTypeRef];
    NSURL *pdfUrl = [NSURL fileURLWithPath:pathToPdfDoc];

    document = CGPDFDocumentCreateWithURL((CFURLRef)pdfUrl);

    page = CGPDFDocumentGetPage(document, 1);
    CGContextTranslateCTM(ctx, 0.0, [self bounds].size.height);
    CGContextScaleCTM(ctx, 1.0, -1.0);

    CGAffineTransform transform = aspectFit(CGPDFPageGetBoxRect(page, kCGPDFTrimBox),CGContextGetClipBoundingBox(ctx));

    CGContextConcatCTM(ctx, transform);

    CGContextSetInterpolationQuality(ctx, kCGInterpolationLow); 
    CGContextSetRenderingIntent(ctx, kCGRenderingIntentDefault);

    CGContextDrawPDFPage(ctx, page);
    CGPDFDocumentRelease(document);
}


-(void)dealloc 
{
    [pageRef release];
    [pageTypeRef release];
    [super dealloc];
}

This works fine. But if I swipe very fast, more often subsequent pages does not load instantly. Screen becomes white.

How to solve this, please guide.

Regards,
Ranjan

Answer 1

one of the obvious issues is that drawRect may be called regularly, and may only request that you draw some of the view’s rect.

in your implementation, you:

• read the pdf from disk. this takes a lot of time, especially since drawRect may be called at a high frequency.

• read the pdf. this takes some time — it can be avoided from occurring during drawRect.

• draw one page. draw only what you need to draw, where possible.

• dispose of the pdf. you should hold on to the pdf document while the pdf view is visible, rather than reading it from disk every time you need to draw.