objective-c - ALAsset 上的 defaultRepresentation fullScreenImage 不返回全屏图像

Question

在我的应用程序中，我将图像作为 Assets 保存到相册中。我还想检索它们并全屏显示它们。我使用以下代码:

ALAsset *lastPicture = [scrollArray objectAtIndex:iAsset]; 

ALAssetRepresentation *defaultRep = [lastPicture defaultRepresentation];

UIImage *image = [UIImage imageWithCGImage:[defaultRep fullScreenImage] 
                                          scale:[defaultRep scale] orientation:
    (UIImageOrientation)[defaultRep orientation]];

问题是返回的图像是nil。我在 ALAssetRepresentation 引用资料中读到，当图像不适合时，它会返回 nil。

我将此图像放入具有 iPad 屏幕大小的 UIImageView。我想知道你是否可以帮我解决这个问题？

提前致谢。

Answer 1

我不喜欢 fullScreenImage 或 fullResolutionImage。我发现，当您对队列中的多个 Assets 执行此操作时，即使您立即释放 UIImage，内存使用量也会急剧增加，而这是不应该的。此外，当使用 fullScreenImage 或 fullResolutionImage 时，返回的 UIImage 仍然是压缩的，这意味着它将在第一次绘制之前解压缩，因此在主线程上会阻塞你的 UI。

我更喜欢使用这种方法。

-(UIImage *)fullSizeImageForAssetRepresentation:(ALAssetRepresentation *)assetRepresentation
{
    UIImage *result = nil;
    NSData *data = nil;

    uint8_t *buffer = (uint8_t *)malloc(sizeof(uint8_t)*[assetRepresentation size]);
    if (buffer != NULL) {
        NSError *error = nil;
        NSUInteger bytesRead = [assetRepresentation getBytes:buffer fromOffset:0 length:[assetRepresentation size] error:&error];
        data = [NSData dataWithBytes:buffer length:bytesRead];

        free(buffer);
    }

    if ([data length])
    {
        CGImageSourceRef sourceRef = CGImageSourceCreateWithData((__bridge CFDataRef)data, nil);

        NSMutableDictionary *options = [NSMutableDictionary dictionary];

        [options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceShouldAllowFloat];
        [options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceCreateThumbnailFromImageAlways];
        [options setObject:(id)[NSNumber numberWithFloat:640.0f] forKey:(id)kCGImageSourceThumbnailMaxPixelSize];
        //[options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceCreateThumbnailWithTransform];

        CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(sourceRef, 0, (__bridge CFDictionaryRef)options);

        if (imageRef) {
            result = [UIImage imageWithCGImage:imageRef scale:[assetRepresentation scale] orientation:(UIImageOrientation)[assetRepresentation orientation]];
            CGImageRelease(imageRef);
        }

        if (sourceRef)
            CFRelease(sourceRef);
    }

    return result;
}

你可以这样使用它:

// Get the full image in a background thread
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

    UIImage* image = [self fullSizeImageForAssetRepresentation:asset.defaultRepresentation];
    dispatch_async(dispatch_get_main_queue(), ^{

    // Do something with the UIImage
    });
});