Le flux du programme ne fonctionne pas

Question

Dans cet extrait de code, le flux du programme passe à en dehors de la boucle d'abord, puis passe à l'intérieur du bloc
resultBlock:^(ALAsset * asset). Le code imprime d'abord le NSLog en bas puis exécute le bloc à l'intérieur de la boucle. Qu'est-ce qu'il se passe ici?

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); 

Answer 1

Méthode assetForURL: resultBlock: failureBlock: exécute le chargement de l'équipement de manière asynchrone. C'est pourquoi nous avons d'abord exécuté le NSLog du bas puis Block. Si vous voulez qu'il soit exécuté de façon synchrone, faites comme ceci:

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: réponse améliorée de 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 
});