iOS/Parse.com - 为 PFQuery 创建缓存

Question

我无法理解如何使用 Parse 缓存和检索对象。我正在尝试创建一个查询，并缓存对象，然后只检索缓存的对象，除非用户将对象保存到 PFObject，然后我想使用网络重新加载数据。

这是我的尝试，也是我查询 PFObjects 的方式。

    //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?

编辑：我只用几行代码就设法获得了所需的功能。

//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);
        }
    }];
}

本质上，我只是在需要查询重新加载数据时清除特定查询的缓存结果。我将查询的 cachePolicy 设置为 kPFCachePolicyCacheElseNetwork

这是进行缓存的最佳方式吗？似乎按预期工作。

Answer 1

看起来不错，但有点罗嗦。相反，如果不是查询本身，我会保留缓存策略的状态...

@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;
}

现在你的代码只需要一点点改变，而且更好一点，imo，因为它保存了清除缓存操作并明确表示你对缓存的意图。 （见 cmets 说“这里的差异”）...

//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);
        }
    }];
}