CGDataProviderCopyData 在内存中堆积导致崩溃

Question

好的，我正在从服务器上下载一堆大图像（5mb），然后将这些图像拼接在一起，并从一个字节数组中渲染整个图像。但是，我意识到每个图像的数据都没有被释放，因此会导致内存警告和我的应用程序崩溃。我认为由于我的显式 (__bridge_transfer NSData *) 强制转换，ARC 会负责释放对象，但事实证明它仍然是一个问题。在仪器中，大约 1mb 的名为“CGDataProviderCopyData”的对象会堆积起来，并且不会因为每个正在拼接到整个图像中的文件而被丢弃。有什么想法或任何人可以引导我朝着正确的方向前进吗？非常感谢。

 // Create  array to add all files into total image
NSMutableArray *byteArray = [[NSMutableArray alloc] initWithCapacity:(imageHeight * imageWidth)];

// Iterate through each file in files array
for (NSString *file in array)
{        
    // Set baseURL for individual file path
    NSString *baseURL = [NSString stringWithFormat:@"http://xx.225.xxx.xxx%@",[imageInfo objectForKey:@"BaseURL"]];

    // Specify imagePath by appending baseURL to file name
    NSString *imagePath = [NSString stringWithFormat:@"%@%@", baseURL, file];

    // Change NSString --> NSURL --> NSData
    NSURL *imageUrl = [NSURL URLWithString:imagePath];
    NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];

    // Create image from imageData
    UIImage *image = [UIImage imageWithData:imageData];
    CGImageRef cgimage = image.CGImage;

    size_t width  = CGImageGetWidth(cgimage);
    size_t height = CGImageGetHeight(cgimage);

    size_t bpr = CGImageGetBytesPerRow(cgimage);
    size_t bpp = CGImageGetBitsPerPixel(cgimage);
    size_t bpc = CGImageGetBitsPerComponent(cgimage);
    size_t bytes_per_pixel = bpp / bpc;

    // Get CGDataProviderRef from cgimage
    CGDataProviderRef provider = CGImageGetDataProvider(cgimage);

    // This is the object that is not being released
    NSData *data = (__bridge_transfer NSData *)CGDataProviderCopyData(provider);      //Using (__bridge_transfer NSData *) casts the provider to type NSData and gives ownership to ARC, but still not discarded

    const UInt8 *bytes = (Byte *)[data bytes];

    // Log which file is currently being iterated through 
    NSLog(@"---Stitching png file to total image: %@", file);

    // Populate byte array with channel data from each pixel
    for(size_t row = 0; row < height; row++)
    {
        for(size_t col = 0; col < width; col++)
        {
            const UInt8* pixel =
            &bytes[row * bpr + col * bytes_per_pixel];

            for(unsigned short i = 0; i < 4; i+=4)
            {
                __unused unsigned short red = pixel[i];         // red channel - unused
                unsigned short green = pixel[i+1];              // green channel
                unsigned short blue = pixel[i+2];               // blue channel
                __unused unsigned short alpha = pixel[i+3];     // alpha channel - unused

                // Create dicom intensity value from intensity = [(g *250) + b]
                unsigned short dicomInt = ((green * 256) + blue);

                //Convert unsigned short intensity value to NSNumber so can store in array as object
                NSNumber *DICOMvalue = [NSNumber numberWithInt:dicomInt];

                // Add to image array (total image)
                [byteArray addObject:DICOMvalue];
            }
        }
    }
    data = nil;
}
return byteArray;

通过 Xcode 运行“分析”也不会显示任何明显的泄漏。

Answer 1

我拿了这段代码，几乎是一字不差地做了一些调查。使用 CFDataRef/NSData，我能够看到您在 NSDatas 没有消失的情况下看到的问题，并且我能够通过将使用 NSData 的代码部分包装在 @autoreleasepool 范围内来解决它，就像这样：

 // Create  array to add all files into total image
NSMutableArray *byteArray = [[NSMutableArray alloc] initWithCapacity:(imageHeight * imageWidth)];

// Iterate through each file in files array
for (NSString *file in array)
{        
    // Set baseURL for individual file path
    NSString *baseURL = [NSString stringWithFormat:@"http://xx.225.xxx.xxx%@",[imageInfo objectForKey:@"BaseURL"]];

    // Specify imagePath by appending baseURL to file name
    NSString *imagePath = [NSString stringWithFormat:@"%@%@", baseURL, file];

    // Change NSString --> NSURL --> NSData
    NSURL *imageUrl = [NSURL URLWithString:imagePath];
    NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];

    // Create image from imageData
    UIImage *image = [UIImage imageWithData:imageData];
    CGImageRef cgimage = image.CGImage;

    size_t width  = CGImageGetWidth(cgimage);
    size_t height = CGImageGetHeight(cgimage);

    size_t bpr = CGImageGetBytesPerRow(cgimage);
    size_t bpp = CGImageGetBitsPerPixel(cgimage);
    size_t bpc = CGImageGetBitsPerComponent(cgimage);
    size_t bytes_per_pixel = bpp / bpc;

    // Get CGDataProviderRef from cgimage
    CGDataProviderRef provider = CGImageGetDataProvider(cgimage);

    @autoreleasepool
    {
        // This is the object that is not being released
        NSData *data = (__bridge_transfer NSData *)CGDataProviderCopyData(provider);      //Using (__bridge_transfer NSData *) casts the provider to type NSData and gives ownership to ARC, but still not discarded

        const UInt8 *bytes = (Byte *)[data bytes];

        // Log which file is currently being iterated through 
        NSLog(@"---Stitching png file to total image: %@", file);

        // Populate byte array with channel data from each pixel
        for(size_t row = 0; row < height; row++)
        {
            for(size_t col = 0; col < width; col++)
            {
                const UInt8* pixel =
                &bytes[row * bpr + col * bytes_per_pixel];

                for(unsigned short i = 0; i < 4; i+=4)
                {
                    __unused unsigned short red = pixel[i];         // red channel - unused
                    unsigned short green = pixel[i+1];              // green channel
                    unsigned short blue = pixel[i+2];               // blue channel
                    __unused unsigned short alpha = pixel[i+3];     // alpha channel - unused

                    // Create dicom intensity value from intensity = [(g *250) + b]
                    unsigned short dicomInt = ((green * 256) + blue);

                    //Convert unsigned short intensity value to NSNumber so can store in array as object
                    NSNumber *DICOMvalue = [NSNumber numberWithInt:dicomInt];

                    // Add to image array (total image)
                    [byteArray addObject:DICOMvalue];
                }
            }
        }
        data = nil;
    }
}
return byteArray;

在添加 @autoreleasepool 之后，我注释掉了您创建 NSNumber 并将它们放入数组中的部分，我能够在 Instruments 的 Allocations 模板中看到确实 CFData现在，随着循环的每一轮，物体都会被释放。

我注释掉创建 NSNumbers 并将它们放入数组的部分的原因是，使用那里的代码，你最终将添加 width * height * 4 NSNumbers 到 byteArray。这意味着即使 NSData 被正确释放，您的堆使用量也会增加width * height * 4 * <at least 4 bytes, maybe more> 无论如何。也许这就是你需要做的，但这确实让我更难看到 NSDatas 发生了什么，因为它们的大小与 NSNumbers 数组相比相形见绌。

希望对您有所帮助。