使異步塊在其他代碼之前運行

Question

我試圖從ALAssetLibrary中獲取視頻，以便我可以用它做任何事情。我使用的塊來做到這一點：

NSMutableArray *assets = [[NSMutableArray alloc] init]; 

library = [[ALAssetsLibrary alloc] init]; 

NSLog(@"library allocated"); 

// Enumerate just the photos and videos group by using ALAssetsGroupSavedPhotos. 

[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) { 

    NSLog(@"Begin enmeration"); 

    [group setAssetsFilter:[ALAssetsFilter allVideos]]; 

    NSLog(@"Filter by videos"); 

    [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:[group numberOfAssets]-1] 

          options:0 

         usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop) { 

          NSLog(@"Asset retrieved"); 

          if (alAsset) { 

           ALAssetRepresentation *representation = [alAsset defaultRepresentation]; 

           NSURL *url = [representation url]; 

           AVAsset *recentVideo = [AVURLAsset URLAssetWithURL:url options:nil]; 

           [assets addObject:recentVideo]; 

           NSLog(@"Asset added to array"); 

          } 
         }]; 
} 

AVMutableComposition *composition = [[AVMutableComposition alloc] init]; 

NSLog(@"creating source"); 
AVURLAsset* sourceAsset = [assets objectAtIndex:0]; 

當我運行的代碼塊得到跳過和程序崩潰時，我嘗試訪問該元素的數組中，因爲它不存在。我被告知，這是因爲這些塊是異步的，但我不知道如何讓它們在所有其他操作之前運行。 performSelectorOnMainThread聽起來像它可能會這樣做，但我無法找到任何解釋我將如何去做這件事。

Answer 1

如果你想

AVMutableComposition *composition = [[AVMutableComposition alloc] init]; 

NSLog(@"creating source"); 
AVURLAsset* sourceAsset = [assets objectAtIndex:0]; 

的group枚舉後發生的，然後把它的第一個塊內，但在枚舉後：

AVMutableComposition *composition = [[AVMutableComposition alloc] init]; 
__block AVURLAsset* sourceAsset = nil; 
[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) { 

    // snip... 

    [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:[group numberOfAssets]-1] 
          options:0 
         usingBlock:^{ 
          // snip... 
         }]; 
    sourceAsset = [assets objectAtIndex:0]; 
    // Now do other things that depend on sourceAsset being set 
}]; 

的__block關鍵字允許指針被設置爲塊內的新對象;否則變量不能重新分配。