Question

我在iPhone上使用大图像，因此将它们保存到磁盘可能需要几秒钟，有时还会在未完成保存时删除图像。我想中断保存过程并立即删除文件，而不是先等待它保存，但我找不到任何方法来中断队列或保存操作。

@interface YMSavedImage : NSObject {
    UIImage *image;
    NSString *path;
    dispatch_queue_t imageAccess;
    ...
}
@end

@implementation YMSavedImage

- (id)init
{
    ...
    imageAccess = dispatch_queue_create("imageAccessQueue", NULL);
    ...
}

- (void)save
{
    dispatch_async(imageAccess, ^(void) {
        [UIImagePNGRepresentation(image) writeToFile:path atomically:YES];  // This takes several seconds.
    });
}

- (void)delete
{
    dispatch_sync(imageAccess, ^(void) {
        if ([fileManager fileExistsAtPath:path]) {
            [fileManager removeItemAtPath:path error:NULL];
        }
    });
}

...

@end

Answer 1

writeToFile:atomically:想要一次性写入所有数据。最好抓住NSFileHandle并快速编写数据的子部分，在每次原子写入后，您可以检查用户是否请求取消保存。

我建议像：

- (void)save 
{
    dispatch_async(imageAccess, ^{

        // Create an empty file to populate
        [[NSFileManager defaultManager] createFileAtPath:_imageFilePath contents:nil attributes:nil];

        // Grab a handle on the file
        NSFileHandle *handle = [NSFileHandle fileHandleForWritingAtPath:_imageFilePath];

        NSData *imageData = UIImagePNGRepresentation(_image);

        // Store how long the image is
        NSUInteger totalLength = [imageData length];

        // Store how many chunks we should need with the set kDesiredChunkSize
        NSUInteger chunks = totalLength / kDesiredChunkSize;

        NSUInteger i = 0;

        // Walk through the image chunk by chunk and check if we are cancelled after each atomic operation
        for (i = 0; i < chunks; i++) {
            [handle seekToEndOfFile];
            [handle writeData:[imageData subdataWithRange:NSMakeRange(i * kDesiredChunkSize, kDesiredChunkSize)]];

            if (__userCancelledSave) { // The user asked us to cancel,
                [[NSFileManager defaultManager] removeItemAtPath:_imageFilePath error:nil]; // FIXME: Handle this error
                return;
            }
        }

        // Write any remaining data that's less than one chunk in size
        NSUInteger sizeDifference = totalLength - (chunks * kDesiredChunkSize);

        if (sizeDifference > 0) {
            [handle seekToEndOfFile];
            [handle writeData:[imageData subdataWithRange:NSMakeRange(chunks * kDesiredChunkSize, sizeDifference)]];
        }
    });
}

iOS中的中断队列或文件写入操作

1 个答案: