iOS/Parse.com - 爲PFQuery創建緩存

Question

我無法理解如何使用Parse緩存和檢索對象。我試圖創建一個查詢，並緩存對象，然後只檢索緩存的對象，除非用戶保存對象到PFObject，然後我想使用網絡來重新加載數據。

這是我的嘗試以及我如何查詢PFObjects。

//Gets the playlists from the desired user 
    +(void)getPlaylistsForUser:(PFUser *)user withCompletion:(void(^)(NSMutableArray *playlists))completion 
    { 
     PFQuery *userQuery = [PFQuery queryWithClassName:@"Playlists"]; 
     userQuery.cachePolicy = kPFCachePolicyCacheElseNetwork; 
     [userQuery whereKey:@"userId" equalTo:[user objectId]]; 
     [userQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) { 
      if (!error){ 
       NSMutableArray *playlists = [[NSMutableArray alloc]init]; 
       playlists = [objects mutableCopy]; 
        completion(playlists); 
       } 
     }]; 
    } 

How would I go about notifying the query that there needs to be a network call instead of returning cached results? 

編輯：我設法得到所需的功能只有幾行代碼。

//Gets the playlists from the desired user 
+(void)getPlaylistsForUser:(PFUser *)user withCompletion:(void(^)(NSMutableArray *playlists))completion 
{ 
    PFQuery *userQuery = [PFQuery queryWithClassName:@"Playlists"]; 
    [userQuery whereKey:@"userId" equalTo:[user objectId]]; 
    userQuery.cachePolicy = kPFCachePolicyCacheElseNetwork; 
    [userQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) { 
     if (!error){ 
      NSMutableArray *playlists = [[NSMutableArray alloc]init]; 
      playlists = [objects mutableCopy]; 
       completion(playlists); 
      } 
    }]; 
} 
+ (void)saveTrack:(NSDictionary *)track toPlaylist:(PFObject *)playlist withCompletion:(void(^)(BOOL success))completion 
{ 
    [playlist addObject:track forKey:@"playlist"]; 
    [playlist saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) { 
     if (error) 
     { 
      NSLog(@"Error creating/saving playlist"); 
     } 
     else{ 
      PFUser *current = [PFUser currentUser]; 
      PFQuery *playlistQuery = [PFQuery queryWithClassName:@"Playlists"]; 
      [playlistQuery whereKey:@"userId" equalTo:[current objectId]]; 
      [playlistQuery clearCachedResult]; 

      completion((BOOL)YES); 
     } 
    }]; 
} 

本質上，我只是在我需要查詢重新加載數據時清除特定查詢的緩存結果。我設置查詢的cachePolicy爲kPFCachePolicyCacheElseNetwork

這是緩存的最佳方式嗎？似乎按預期工作。

Answer 1

看起來很好，但有點羅嗦。我想，而是保持狀態的緩存策略如果不是查詢本身...

@property (assign, nonatomic) PFCachePolicy cachePolicy; 
// initialize this to kPFCachePolicyIgnoreCache (==0 so it will be by default) 

// return a playlist query 
+ (PFQuery *)playlistQueryForUser:(PFUser *)user { 
    PFQuery *query = [PFQuery queryWithClassName:@"Playlists"]; 
    query.cachePolicy = self.cachePolicy; 
    [query whereKey:@"userId" equalTo:[user objectId]]; 
    return query; 
} 

現在你的代碼只是需要一點點的變化，是一個更好一點，海事組織，因爲它節省了清除緩存操作並明確表示您的緩存意圖。 （請參閱評論說「差異在這裏」）...

//Gets the playlists from the desired user 
+(void)getPlaylistsForUser:(PFUser *)user withCompletion:(void(^)(NSMutableArray *playlists))completion 
{ 
    // diff here... 
    PFQuery *query = [self playlistQueryForUser:user]; 
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) { 
     if (!error){ 
      NSMutableArray *playlists = [[NSMutableArray alloc]init]; 
      playlists = [objects mutableCopy]; 
      completion(playlists); 

      // diff here... 
      self.cachePolicy = kPFCachePolicyCacheThenNetwork; 
     } 
    }]; 
} 

+ (void)saveTrack:(NSDictionary *)track toPlaylist:(PFObject *)playlist withCompletion:(void(^)(BOOL success))completion 
{ 
    [playlist addObject:track forKey:@"playlist"]; 
    [playlist saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) { 
     if (error) { 
      NSLog(@"Error creating/saving playlist"); 
     } else { 
      // diff here: no need to rebuild the query, just change policy... 
      self.cachePolicy = kPFCachePolicyIgnoreCache; 
      completion((BOOL)YES); 
     } 
    }]; 
}