[英]AFJSONRequestOperation - interrupt operation queue in case of failure
[英]Interrupt queue or file write operation in iOS
我在iPhone上处理大图像,因此将它们保存到磁盘可能要花费几秒钟的时间,有时还需要在尚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,而不必等待首先保存文件,但是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
writeToFile:atomically:
将要writeToFile:atomically:
将所有数据写入文件。 您最好抓住NSFileHandle
并快速写入数据的子部分,在每次原子写入之后,您可以检查用户是否已请求取消保存。
我建议类似的东西:
- (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)]];
}
});
}
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