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@interface APPLCatListViewController ()
@property (nonatomic, strong) AAPLSegmentedDataSource *segmentedDataSource;
@property (nonatomic, strong) AAPLCatListDataSource *catsDataSource;
@property (nonatomic, strong) AAPLCatListDataSource *favoriteCatsDataSource;
@property (nonatomic, strong) NSIndexPath *selectedIndexPath;
@property (nonatomic, strong) id selectedDataSourceObserver;
@end

- (void)dealloc
{
    [self.segmentedDataSource aapl_removeObserver:self.selectedDataSourceObserver];
}

- (AAPLCatListDataSource *)newAllCatsDataSource
{
    AAPLCatListDataSource *dataSource = [[AAPLCatListDataSource alloc] init];
    dataSource.showingFavorites = NO;

    dataSource.title = NSLocalizedString(@"All", @"Title for available cats list");
    dataSource.noContentMessage = NSLocalizedString(@"All the big ...", @"The message to show when no cats are available");
    dataSource.noContentTitle = NSLocalizedString(@"No Cats", @"The title to show when no cats are available");
    dataSource.errorMessage = NSLocalizedString(@"A problem with the network ....", @"Message to show when unable to load cats");
    dataSource.errorTitle = NSLocalizedString(@"Unable To Load Cats", @"Title of message to show when unable to load cats");

    return dataSource;
}

@interface AAPLCatListDataSource : AAPLBasicDataSource
/// Is this list showing the favorites or all available cats?
@property (nonatomic) BOOL showingFavorites;
@end

- (void)setShowingFavorites:(BOOL)showingFavorites
{
    if (showingFavorites == _showingFavorites)
        return;

    _showingFavorites = showingFavorites;
    [self resetContent];
    [self setNeedsLoadContent];

    if (showingFavorites)
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(observeFavoriteToggledNotification:) name:AAPLCatFavoriteToggledNotificationName object:nil];
}

- (void)setNeedsLoadContent
{
    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(loadContent) object:nil];
    [self performSelector:@selector(loadContent) withObject:nil afterDelay:0];
}

- (void)loadContent
{
    // To be implemented by subclasses…
}

- (void)loadContent
{
    [self loadContentWithBlock:^(AAPLLoading *loading) {
        void (^handler)(NSArray *cats, NSError *error) = ^(NSArray *cats, NSError *error) {
            // Check to make certain a more recent call to load content hasn't superceded this one…
            if (!loading.current) {
                [loading ignore];
                return;
            }

            if (error) {
                [loading doneWithError:error];
                return;
            }

            if (cats.count)
                [loading updateWithContent:^(AAPLCatListDataSource *me) {
                    me.items = cats;
                }];
            else
                [loading updateWithNoContent:^(AAPLCatListDataSource *me) {
                    me.items = @[];
                }];
        };

        if (self.showingFavorites)
            [[AAPLDataAccessManager manager] fetchFavoriteCatListWithCompletionHandler:handler];
        else
            [[AAPLDataAccessManager manager] fetchCatListWithCompletionHandler:handler];
    }];
}

- (void)loadContentWithBlock:(AAPLLoadingBlock)block
{
    [self beginLoading];

    __weak typeof(&*self) weakself = self;

    AAPLLoading *loading = [AAPLLoading loadingWithCompletionHandler:^(NSString *newState, NSError *error, AAPLLoadingUpdateBlock update){
        if (!newState)
            return;

        [self endLoadingWithState:newState error:error update:^{
            AAPLDataSource *me = weakself;
            if (update && me)
                update(me);
        }];
    }];

    // Tell previous loading instance it's no longer current and remember this loading instance
    self.loadingInstance.current = NO;
    self.loadingInstance = loading;
    
    // Call the provided block to actually do the load
    block(loading);
}

+ (instancetype)loadingWithCompletionHandler:(void(^)(NSString *state, NSError *error, AAPLLoadingUpdateBlock update))handler
{
    NSParameterAssert(handler != nil);
    AAPLLoading *loading = [[self alloc] init];
    loading.block = handler;
    loading.current = YES;
    return loading;
}

- (void)_doneWithNewState:(NSString *)newState error:(NSError *)error update:(AAPLLoadingUpdateBlock)update
{
#if DEBUG
    if (!OSAtomicCompareAndSwap32(0, 1, &_complete))
        NSAssert(false, @"completion method called more than once");
#endif

    void (^block)(NSString *state, NSError *error, AAPLLoadingUpdateBlock update) = _block;

    dispatch_async(dispatch_get_main_queue(), ^{
        block(newState, error, update);
    });

    _block = nil;
}

[self endLoadingWithState:newState error:error update:^{
    AAPLDataSource *me = weakself;
    if (update && me)
        update(me);
}];

- (void)endLoadingWithState:(NSString *)state error:(NSError *)error update:(dispatch_block_t)update
{
    self.loadingError = error;
    self.loadingState = state;

    if (self.shouldDisplayPlaceholder) {
        if (update)
            [self enqueuePendingUpdateBlock:update];
    }
    else {
        [self notifyBatchUpdate:^{
            // Run pending updates
            [self executePendingUpdates];
            if (update)
                update();
        }];
    }

    self.loadingComplete = YES;
    [self notifyContentLoadedWithError:error];
}

@class AAPLCat;

@interface AAPLDataAccessManager : NSObject

+ (AAPLDataAccessManager *)manager;

- (void)fetchCatListWithCompletionHandler:(void(^)(NSArray *cats, NSError *error))handler;
- (void)fetchFavoriteCatListWithCompletionHandler:(void(^)(NSArray *cats, NSError *error))handler;
- (void)fetchDetailForCat:(AAPLCat *)cat completionHandler:(void(^)(AAPLCat *cat, NSError *error))handler;
- (void)fetchSightingsForCat:(AAPLCat *)cat completionHandler:(void(^)(NSArray *sightings, NSError *error))handler;

@end

[SegmentedDataSource setNeedsLoadContent]
                ↓
    [CatListDataSource loadContent]
                ↓
   [DataSource loadContentWithBlock:]
                ↓
创建 loading，设置 loading 完成后要做的事 → 拿到数据后放到 updateQueue 里，等全部拿到再执行 batchUpdate
                ↓
执行 loadContentBlock → 使用 DataAccessManager 去获取数据，拿到后交给 loading

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