從ALAssetRepresentation生成自定義縮略圖

Question

我的主要問題是我需要獲取ALAsset對象的縮略圖。

我嘗試了很多的解決方案，並尋找天堆棧溢出，所有的解決方案，我發現工作不適合我，由於這些約束：

• 我不能使用默認的縮略圖，因爲它是太少;
• 我無法使用fullScreen或fullResolution圖像，因爲屏幕上有很多圖像;
• 我不能使用的UIImage或UIImageView的調整大小，因爲這些負載 的全分辨率圖像
• 我無法加載在內存中的圖像，我與20Mpx圖像工作;
• 我需要創建一個200x200像素版本的原始資產加載到屏幕上;

這是我來的代碼的最後一次迭代：

#import <AssetsLibrary/ALAsset.h> 
#import <ImageIO/ImageIO.h> 

// ... 

ALAsset *asset; 

// ... 

ALAssetRepresentation *assetRepresentation = [asset defaultRepresentation]; 

NSDictionary *thumbnailOptions = [NSDictionary dictionaryWithObjectsAndKeys: 
    (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailWithTransform, 
    (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailFromImageAlways, 
    (id)[NSNumber numberWithFloat:200], kCGImageSourceThumbnailMaxPixelSize, 
    nil]; 

CGImageRef generatedThumbnail = [assetRepresentation CGImageWithOptions:thumbnailOptions]; 

UIImage *thumbnailImage = [UIImage imageWithCGImage:generatedThumbnail]; 

問題是，所得到的CGImageRef既不由取向改變，也沒有指定的最大像素大小的;

我也試圖找到利用CGImageSource調整的一種方式，但：

• 資產URL不能在CGImageSourceCreateWithURL:使用;
• 我不能從ALAsset或ALAssetRepresentation提取一個CGDataProviderRef與CGImageSourceCreateWithDataProvider:一起使用;
• CGImageSourceCreateWithData:要求我將fullResolution或全屏資產存儲在內存中才能正常工作。

我錯過了什麼嗎？

是否有另一種方法從ALAsset或ALAssetRepresentation獲取自定義縮略圖，我錯過了？

在此先感謝。

Answer 1

您可以使用CGImageSourceCreateThumbnailAtIndex從潛在的大圖像源創建一個小圖像。您可以使用ALAssetRepresentation的getBytes:fromOffset:length:error:方法從磁盤加載映像，並使用它創建CGImageSourceRef。

然後你只需要通過kCGImageSourceThumbnailMaxPixelSize和kCGImageSourceCreateThumbnailFromImageAlways選項CGImageSourceCreateThumbnailAtIndex與你所創建的圖像源，它會爲你創建一個更小的版本，而無需加載巨大的版本到內存中。

我寫了一個blog post和gist這一技術全面充實。

Answer 2

有通過Jesse Rusak提到與this方法的問題。您的應用程序將與以下堆棧崩潰，如果資產是太大：

0 CoreGraphics    0x2f602f1c x_malloc + 16 
1 libsystem_malloc.dylib 0x39fadd63 malloc + 52 
2 CoreGraphics    0x2f62413f CGDataProviderCopyData + 178 
3 ImageIO     0x302e27b7 CGImageReadCreateWithProvider + 156 
4 ImageIO     0x302e2699 CGImageSourceCreateWithDataProvider + 180 
... 

Link Register Analysis:

Symbol: malloc + 52

Description: We have determined that the link register (lr) is very likely to contain the return address of frame #0's calling function, and have inserted it into the crashing thread's backtrace as frame #1 to aid in analysis. This determination was made by applying a heuristic to determine whether the crashing function was likely to have created a new stack frame at the time of the crash.

Type: 1

這是很容易模仿的崩潰。讓我們用小塊讀取來自getAssetBytesCallback中ALAssetRepresentation的數據。塊的特定大小並不重要。唯一重要的是調用回調約20次。

static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) { 
    static int i = 0; ++i; 
    ALAssetRepresentation *rep = (__bridge id)info; 
    NSError *error = nil; 
    NSLog(@"%d: off:%lld len:%zu", i, position, count); 
    const size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:128 error:&error]; 
    return countRead; 
} 

這裏是日誌

2014-03-21 11:21:14.250 MRCloudApp[3461:1303] 20: off:2432 len:2156064

MRCloudApp(3461,0x701000) malloc: *** mach_vm_map(size=217636864) failed (error code=3)

*** error: can't allocate region

*** set a breakpoint in malloc_error_break to debug

我介紹了一個計數器來防止這種崩潰的尾部線條。您可以在下面看到我的解決辦法：

typedef struct { 
    void *assetRepresentation; 
    int decodingIterationCount; 
} ThumbnailDecodingContext; 
static const int kThumbnailDecodingContextMaxIterationCount = 16; 

static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) { 
    ThumbnailDecodingContext *decodingContext = (ThumbnailDecodingContext *)info; 
    ALAssetRepresentation *assetRepresentation = (__bridge ALAssetRepresentation *)decodingContext->assetRepresentation; 
    if (decodingContext->decodingIterationCount == kThumbnailDecodingContextMaxIterationCount) { 
     NSLog(@"WARNING: Image %@ is too large for thumbnail extraction.", [assetRepresentation url]); 
     return 0; 
    } 
    ++decodingContext->decodingIterationCount; 
    NSError *error = nil; 
    size_t countRead = [assetRepresentation getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error]; 
    if (countRead == 0 || error != nil) { 
     NSLog(@"ERROR: Failed to decode image %@: %@", [assetRepresentation url], error); 
     return 0; 
    } 
    return countRead; 
} 

- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(CGFloat)size { 
    NSParameterAssert(asset); 
    NSParameterAssert(size > 0); 
    ALAssetRepresentation *representation = [asset defaultRepresentation]; 
    if (!representation) { 
     return nil; 
    } 
    CGDataProviderDirectCallbacks callbacks = { 
     .version = 0, 
     .getBytePointer = NULL, 
     .releaseBytePointer = NULL, 
     .getBytesAtPosition = getAssetBytesCallback, 
     .releaseInfo = NULL 
    }; 
    ThumbnailDecodingContext decodingContext = { 
     .assetRepresentation = (__bridge void *)representation, 
     .decodingIterationCount = 0 
    }; 
    CGDataProviderRef provider = CGDataProviderCreateDirect((void *)&decodingContext, [representation size], &callbacks); 
    NSParameterAssert(provider); 
    if (!provider) { 
     return nil; 
    } 
    CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL); 
    NSParameterAssert(source); 
    if (!source) { 
     CGDataProviderRelease(provider); 
     return nil; 
    } 
    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{(NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES, 
                             (NSString *)kCGImageSourceThumbnailMaxPixelSize   : [NSNumber numberWithFloat:size], 
                             (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES}); 
    UIImage *image = nil; 
    if (imageRef) { 
     image = [UIImage imageWithCGImage:imageRef]; 
     CGImageRelease(imageRef); 
    } 
    CFRelease(source); 
    CGDataProviderRelease(provider); 
    return image; 
}