Question

我正在将照片从iPhone相册导入我的应用程序的文档文件夹。这是我的代码。

for (int j=0; j<[assetArray count]; j++) {

    ALAsset *assest = [assetArray objectAtIndex:j];
    CGImageRef imageRef = assest.defaultRepresentation.fullResolutionImage;
    UIImage *image = [UIImage imageWithCGImage:imageRef];
    NSData *imageData = UIImageJPEGRepresentation(image);
    [imageData writeToFile:documentsPath atomically:YES];
}

它的工作正常但是当我尝试导入更高分辨率的图像时需要更多时间。以最短时间导入的正确方法是什么？顺便说一句：我将image转换为NSData需要更多时间。

Answer 1

出于多种原因，将fullResolutionImage用于此任务是危险的。一些评论：

对于大图像，可能会出现内存问题。使用fullResolutionImage方法时。请注意，Photo-Library（至少在iPad上）也可以包含RAW图像。
从图像文件内部开始，性能不是最理想的.ImageIO首先创建一个CGImageRef然后转换为JPEG。这需要时间。
AssetLibrary还可以包含视频。在这种情况下，fullResolutionImage仅返回视频的previewImage，而不返回实际视频。
存储实际的资产对象没有问题，因为它们的内存很小。

将图像写出到文档目录的更好方法是使用ALAssetsRepresentation的getBytes方法。这应该是更快，更有效的记忆方式。它还为您提供原始图像文件（包括元数据），也适用于视频。

您重写的示例代码将如下所示：

//reading out the orginal images
    for (int j=0; j<[assetArray count]; j++) {

    ALAssetRepresentation *representation = [[assetArray objectAtIndex:j] defaultRepresentation];
    NSString* filename = [documentPath stringByAppendingPathComponent:[representation filename]];

    [[NSFileManager defaultManager] createFileAtPath:filename contents:nil attributes:nil];
    NSOutputStream *outPutStream = [NSOutputStream outputStreamToFileAtPath:filename append:YES];
    [outPutStream open];

    long long offset = 0;
    long long bytesRead = 0;

    NSError *error;
    uint8_t * buffer = malloc(131072);
    while (offset<[representation size] && [outPutStream hasSpaceAvailable]) {
        bytesRead = [representation getBytes:buffer fromOffset:offset length:131072 error:&error];
        [outPutStream write:buffer maxLength:bytesRead];
        offset = offset+bytesRead;
    }
    [outPutStream close];
    free(buffer);
}


//reading out the fullScreenImages and thumbnails
for (int j=0; j<[assetArray count]; j++) 
{
    @autoreleasepool
    {

        ALAsset *asset = [assetArray objectAtIndex:j];

         NSString *orgFilename = [representation filename];
         NSString *filenameFullScreen = [NSString stringWithFormat:@"%@_fullscreen.png",[orgFilename stringByDeletingPathExtension]]
         NSString* pathFullScreen = [documentPath stringByAppendingPathComponent:filenameFullScreen];

         CGImageRef imageRefFullScreen = [[asset defaultRepresentation] fullScreenImage];
         UIImage *imageFullScreen = [UIImage imageWithCGImage:imageRefFullScreen];
         NSData *imageDataFullScreen = UIImagePNGRepresentation(imageFullScreen);
         [imageDataFullScreen writeToFile:pathFullScreen atomically:YES];

         NSString *filenameThumb = [NSString stringWithFormat:@"%@_thumb.png",[orgFilename stringByDeletingPathExtension]]
         NSString* pathThumb = [documentPath stringByAppendingPathComponent:filenameThumb];

         CGImageRef imageRefThumb = [asset thumbnail];
         UIImage *imageThumb = [UIImage imageWithCGImage:imageRefThumb];
         NSData *imageDataThumb = UIImagePNGRepresentation(imageThumb);
         [imageDataThumb writeToFile:pathThumb atomically:YES];

    }
}

Answer 2

我正在使用ELCImagePicker，并且在使用套件从照片库一次导入多张照片时遇到同样的问题。我们无法减少导入所需的时间，但崩溃问题将得到解决。

for (int j=0; j<[assetArray count]; j++) 
{
    @autoreleasepool // This is compiler level feature so will only work on xcode 4.1 or above
    {
         ALAsset *assest = [assetArray objectAtIndex:j];
         CGImageRef imageRef = assest.defaultRepresentation.fullResolutionImage;
         UIImage *image = [UIImage imageWithCGImage:imageRef];
         NSData *imageData = UIImagePNGRepresentation(image);
         [imageData writeToFile:documentsPath atomically:YES];
    }
}

如果可能的话，尝试只在AssetArray中存储AssetURL而不是整个ALAsset对象，并从url一次创建ALAsset，这样可能有助于减少内存消耗。在这种情况下，您需要使用块

ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
{
    CGImageRef iref = [[myasset defaultRepresentation] fullResolutionImage];
    if (iref) //You have image so use it 
    {
    }
};

ALAssetsLibraryAccessFailureBlock failureblock  = ^(NSError *myerror)
{
    NSLog(@"Can't get image - %@",[myerror localizedDescription]);
};

ALAssetsLibrary* assetslibrary = [[[ALAssetsLibrary alloc] init] autorelease];
[assetslibrary assetForURL:imageURL resultBlock:resultblock failureBlock:failureblock];

导入的正确方法是什么？从iPhone相册保存照片？

2 个答案: