The program flow going wrong

问题

In this code snippet the flow of program goes out of for loop first then only goes inside the block
resultBlock:^(ALAsset *asset). The code prints the NSLog at the bottom first then executes the block inside the loop. What's happening here?

 ALAssetsLibrary *lib=[ALAssetsLibrary new];
    _sizeOfSelectedImage=0;
        for (int i=0; i<assets.count; i++) {
            ALAsset *asset=assets[i];
            FileOP *fileMgr=[[FileOP alloc]init];
            NSString *baseDir=[fileMgr GetDocumentDirectory];

            //STORING FILE INTO LOCAL

            [lib assetForURL:asset.defaultRepresentation.url
                 resultBlock:^(ALAsset *asset){
                     ALAssetRepresentation *repr = [asset defaultRepresentation];
                     CGImageRef cgImg = [repr fullResolutionImage];
                     NSString *fname = repr.filename;
                     UIImage *img = [UIImage imageWithCGImage:cgImg];
                     NSData *data = UIImagePNGRepresentation(img);
                     [data writeToFile:[baseDir stringByAppendingPathComponent:fname]
                            atomically:YES];
                     //FOR LOCAL URL OF THE IMAGE
                     //NSString *imageURL = [baseDir stringByAppendingPathComponent:fname];
                     //UIImage *myImg =[UIImage imageWithContentsOfFile:imageURL];
                     //NSLog(@"%@ URL OF IMAGE ",imageURL);
                     NSLog(@"Image %d has %d size",i,data.length);
                     _sizeOfSelectedImage   +=data.length;
                     NSLog(@"%d is the size",_sizeOfSelectedImage);

                 }
                             failureBlock:^(NSError *error){

                }];
        }

        NSLog(@"COPIED %lu FILE INTO LOCAL MEMORY AND TOTAL SIZE COPIED IS %d ",(unsigned    long)assets.count,_sizeOfSelectedImage);

回答1:

Method assetForURL:resultBlock:failureBlock: will execute the load of asset asynchronously. That's why we first have the bottom NSLog executed and then in Block. If you want it to be executed synchronously, do it like this:

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0) ^{
        ALAssetsLibrary *lib=[ALAssetsLibrary new];  
        _sizeOfSelectedImage=0;  

        dispatch_group_t group = dispatch_group_create();  

        for (int i=0;i<assets.count;i++) {  
                ALAsset *asset=assets[i];  
                FileOP *fileMgr=[[FileOP alloc]init];  
                NSString *baseDir=[fileMgr GetDocumentDirectory];  

                //STORING FILE INTO LOCAL
                dispatch_group_enter(group);  
                [lib assetForURL:asset.defaultRepresentation.url
                     resultBlock:^(ALAsset *asset){
                         ALAssetRepresentation *repr = [asset defaultRepresentation];
                         CGImageRef cgImg = [repr fullResolutionImage];
                         NSString *fname = repr.filename;
                         UIImage *img = [UIImage imageWithCGImage:cgImg];
                         NSData *data = UIImagePNGRepresentation(img);
                         [data writeToFile:[baseDir stringByAppendingPathComponent:fname]
                                atomically:YES];
                         //FOR LOCAL URL OF THE IMAGE
                         //NSString *imageURL = [baseDir stringByAppendingPathComponent:fname];
                         //UIImage *myImg =[UIImage imageWithContentsOfFile:imageURL];
                         //NSLog(@"%@ URL OF IMAGE ",imageURL);
                         NSLog(@"Image %d has %d size",i,data.length);
                         _sizeOfSelectedImage   +=data.length;
                         NSLog(@"%d is the size",_sizeOfSelectedImage);
                        dispatch_group_leave(group);  
                     }
                     failureBlock:^(NSError *error){
                                dispatch_group_leave(group);  
                    }];
            }
            dispatch_group_wait(group, DISPATCH_TIME_FOREVER);  
            NSLog(@"COPIED %lu FILE INTO LOCAL MEMORY AND TOTAL SIZE COPIED IS %d ",(unsigned    long)assets.count,_sizeOfSelectedImage);
            dispatch_async(dispatch_get_main_queue(), ^{
                 // Do your call back on main thread here
            });
    });  

Edit1: Enhanced answer from Ken

ALAssetsLibrary *lib=[ALAssetsLibrary new];
_sizeOfSelectedImage=0;

dispatch_group_t group = dispatch_group_create();

for (int i=0;i<assets.count;i++) {
    ALAsset *asset=assets[i];
    FileOP *fileMgr=[[FileOP alloc]init];
    NSString *baseDir=[fileMgr GetDocumentDirectory];

    //STORING FILE INTO LOCAL
    dispatch_group_enter(group);
    [lib assetForURL:asset.defaultRepresentation.url
         resultBlock:^(ALAsset *asset){
             ALAssetRepresentation *repr = [asset defaultRepresentation];
             CGImageRef cgImg = [repr fullResolutionImage];
             NSString *fname = repr.filename;
             UIImage *img = [UIImage imageWithCGImage:cgImg];
             NSData *data = UIImagePNGRepresentation(img);
             [data writeToFile:[baseDir stringByAppendingPathComponent:fname]
                    atomically:YES];
             //FOR LOCAL URL OF THE IMAGE
             //NSString *imageURL = [baseDir stringByAppendingPathComponent:fname];
             //UIImage *myImg =[UIImage imageWithContentsOfFile:imageURL];
             //NSLog(@"%@ URL OF IMAGE ",imageURL);
             NSLog(@"Image %d has %d size",i,data.length);
             _sizeOfSelectedImage   +=data.length;
             NSLog(@"%d is the size",_sizeOfSelectedImage);
             dispatch_group_leave(group);
         }
        failureBlock:^(NSError *error){
            dispatch_group_leave(group);
        }];
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // Do your call back on main thread here
    NSLog(@"COPIED %lu FILE INTO LOCAL MEMORY AND TOTAL SIZE COPIED IS %d ",(unsigned    long)assets.count,_sizeOfSelectedImage);

    // Your code here
});

来源：https://stackoverflow.com/questions/24857150/the-program-flow-going-wrong