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I'm developing a page view controller for showing PDF files and I want to start rendering a UIImage from a PDF page in a different thread and pass the rendered image to the main thread through notifications.

I started by creating a different method that renders my page in a UIImage and post a notification when the image is ready but at least for now I'm still doing it on the main thread.

- (void)generatePage:(int)pageNumber withSize:(CGSize)size {
    UIImage *resultingImage;

    CGFloat sizeRatio = size.width/size.height;

    CGPDFDocumentRef pdfRef = [self PDFDocumentRef];
    CGPDFPageRef myPageRef = CGPDFDocumentGetPage(pdfRef, pageNumber);

    CGRect pageRect = CGPDFPageGetBoxRect(myPageRef, kCGPDFMediaBox);
    CGFloat pdfHWRatio = pageRect.size.width/pageRect.size.height;

    CGFloat pdfScale;

    if (sizeRatio < pdfHWRatio) {
        pdfScale = size.width/pageRect.size.width;
    }
    else {
        pdfScale = size.height/pageRect.size.height;
    }
    pageRect.size = CGSizeMake(pageRect.size.width*pdfScale, pageRect.size.height*pdfScale);
    pageRect.origin = CGPointZero;


    UIGraphicsBeginImageContext(pageRect.size);

    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetRGBFillColor(context, 1.0,1.0,1.0,1.0);
    CGContextFillRect(context,pageRect);

    CGContextSaveGState(context);

    CGContextTranslateCTM(context, 0.0, pageRect.size.height);
    CGContextScaleCTM(context, pdfScale, -pdfScale);

    CGContextDrawPDFPage(context, myPageRef);
    CGContextRestoreGState(context);

    resultingImage = UIGraphicsGetImageFromCurrentImageContext();

    if (resultingImage) {
        [pagesCache setObject:resultingImage
                   forKey:[self generateCacheNameWithSize:size
                                            andPageNumber:pageNumber]];
        NSDictionary *userInfo = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:
                                                                  resultingImage,
                                                                  [NSNumber numberWithInt:pageNumber],
                                                                  nil]
                                                         forKeys:[NSArray arrayWithObjects:
                                                                  CatalogRenderedPageImageKey,
                                                                  CatalogRenderedPageNumberKey,
                                                                  nil]];

        NSNotification *note = [NSNotification notificationWithName:CatalogRenderPageNotification
                                                         object:self
                                                       userInfo:userInfo];
        [[NSNotificationCenter defaultCenter] postNotification:note];
    }


    UIGraphicsEndImageContext();

    CGPDFDocumentRelease(pdfRef);
}

I'm successfully getting the notification and the UIImage is correctly acquired (same object id) but the UIImageView is empty.

- (void) imageLoaded:(NSNotification *)note {
    NSDictionary *userInfo = note.userInfo;
    NSNumber *pageNumber = [userInfo objectForKey:CatalogRenderedPageNumberKey];

    if ([self.pageNumber isEqualToNumber:pageNumber]) {
        UIImage *image = [userInfo objectForKey:CatalogRenderedPageImageKey];
        self.image = image;
        self.imageView.image = _image;
        [[NSNotificationCenter defaultCenter] removeObserver:self name:CatalogRenderPageNotification object:self.catalog];
    }
}

What am I missing? I've checked the storage property in the UIImageView and it's being filled with the UIImage.

You may have noticed that I'm saving my rendered images in a cache and truth is that when I get the images from that cache I'm able to see them in the UIImageView so I dismiss the chance that the UIImage is being generated badly.

UPDATE: I started calling the generatePage in a different thread:

dispatch_async(dispatch_queue_create("renderingQueue", DISPATCH_QUEUE_CONCURRENT), ^{
            [self generatePage:pageNumber withSize:size];
        });

And posting the notification in the main thread as @phyx23 suggested. It now works but I still don't understand why it didn't work before.

I'm leaving it open so that anyone can leave their thoughts on why this would happen.

4

1 回答 1

1

我的猜测是，问题的原因是当您将创建的图像分配给图像视图时，您仍然处于图像上下文中。您应该先结束图像上下文，UIGraphicsEndImageContext然后发布通知。此外，当您开始在后台执行它时，请确保您在主线程上发布通知。

resultingImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
CGPDFDocumentRelease(pdfRef);

if (resultingImage) {
  dispatch_async(dispatch_get_main_queue(), ^{
    [pagesCache setObject:resultingImage
               forKey:[self generateCacheNameWithSize:size
                                        andPageNumber:pageNumber]];
    NSDictionary *userInfo = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:
                                                              resultingImage,
                                                              [NSNumber numberWithInt:pageNumber],
                                                              nil]
                                                     forKeys:[NSArray arrayWithObjects:
                                                              CatalogRenderedPageImageKey,
                                                              CatalogRenderedPageNumberKey,
                                                              nil]];

    NSNotification *note = [NSNotification notificationWithName:CatalogRenderPageNotification
                                                     object:self
                                                   userInfo:userInfo];
    [[NSNotificationCenter defaultCenter] postNotification:note];
  });
}
于 2012-08-26T11:49:39.670 回答