ios - 使用游标加载 UICollectionView

Question

我有一个应用程序，其中包含从 CloudKit 中提取的图像 Collection View 。我有一个 CKManager 类，它执行所有与 CK 相关的方法。在 viewcontroller 中，我调用了 CKManager 中的一个方法来从 CK 中检索初始数据，这一切都很完美。我正在使用 CKQueryOperation，所以我可以分块提取数据，尽管到目前为止我设置 ckQueryOperation.resultsLimit = CKQueryOperationMaximumResults 只是为了测试。因此，当滚动 collectionview 时，图像/单元格不会在您滚动时“淡入”。我假设这是因为在渲染单元格之前已经检索了所有数据。目前大约有 50 条记录，加载速度相当快，但当我将结果限制设置为 25 条时，加载速度肯定更快。

我的问题是我不完全理解如何使用游标来执行此操作，即使我已经计划通过在我的代码中实现游标来实现它。我找到了这个 thread我大部分都理解，但它是在 Swift 中，它也没有回答我所有的问题。我根据该线程中 Edwin 的回答修改了我的代码，但我确定我在 Swift 到 OB-C 的转换中遗漏了一些东西。

下面是我在 CKManager 类中调用的代码。我可以从日志中看到它工作正常并且可以识别光标。我不明白的是我如何/何时再次调用它以从该光标点获取下一个结果 block ？如果 resultsLimit 没有像最初那样设置为最大值，我会得到指定的结果数量 (20)，并且不会检索剩余的结果。所以我不知道如何在光标停止的地方获得剩余的结果。我确实知道，因为我使用的是 collectionview，所以每次获得下一个结果 block 时，我都需要更新部分中的项目数。

非常感谢!

更新:更改了 loadCloudKitDataWithCompletionHandler 以添加对接受游标的新方法的调用 - loadCloudKitDataFromCursor:withCompletionHandler:。唯一缺少的是弄清楚在 ViewController 中的哪个位置处理从使用光标更新 numberOfItemsInSection 的方法返回的结果，然后重新加载 CollectionView。

来自 CKManager...

- (void)loadCloudKitDataFromCursor:(CKQueryCursor *)cursor withCompletionHandler:(void (^)(NSArray *, CKQueryCursor *, NSError *))completionHandler {
    NSMutableArray *cursorResultSet = [[NSMutableArray alloc] init];
    __block NSArray *results;

    if (cursor) { // make sure we have a cursor to continue from
        NSLog(@"INFO: Preparing to load records from cursor...");
        CKQueryOperation *cursorOperation = [[CKQueryOperation alloc] initWithCursor:cursor];
        cursorOperation.resultsLimit = 20;

        // processes for each record returned
        cursorOperation.recordFetchedBlock = ^(CKRecord *record) {
            NSLog(@"RecordFetchBlock returned from cursor CID record: %@", record.recordID.recordName);
            [cursorResultSet addObject:record];
        };
        // query has completed
        cursorOperation.queryCompletionBlock = ^(CKQueryCursor *cursor, NSError *error) {
            results = [cursorResultSet copy];
            [cursorResultSet removeAllObjects]; // get rid of the temp results array
            completionHandler(results, cursor, error);
            if (cursor) {
                NSLog(@"INFO: Calling self to fetch more data from cursor point...");
                [self loadCloudKitDataFromCursor:cursor withCompletionHandler:^(NSArray *results, CKQueryCursor *cursor, NSError *error) {
                    results = [cursorResultSet copy];
                    [cursorResultSet removeAllObjects]; // get rid of the temp results array
                    completionHandler(results, cursor, error);
                }];
            }
        };

        [self.publicDatabase addOperation:cursorOperation];
    }

}

- (void)loadCloudKitDataFromCursor:(CKQueryCursor *)cursor withCompletionHandler:(void (^)(NSArray *, CKQueryCursor *, NSError *))completionHandler {
    NSMutableArray *cursorResultSet = [[NSMutableArray alloc] init];
    __block NSArray *results;

    if (cursor) { // make sure we have a cursor to continue from
        NSLog(@"INFO: Preparing to load records from cursor...");
        CKQueryOperation *cursorOperation = [[CKQueryOperation alloc] initWithCursor:cursor];
        cursorOperation.resultsLimit = 20;

        // processes for each record returned
        cursorOperation.recordFetchedBlock = ^(CKRecord *record) {
            NSLog(@"RecordFetchBlock returned from cursor CID record: %@", record.recordID.recordName);
            [cursorResultSet addObject:record];
        };
        // query has completed
        cursorOperation.queryCompletionBlock = ^(CKQueryCursor *cursor, NSError *error) {
            results = [cursorResultSet copy];
            [cursorResultSet removeAllObjects]; // get rid of the temp results array
            completionHandler(results, cursor, error);
            if (cursor) {
                NSLog(@"INFO: Calling self to fetch more data from cursor point...");
                [self loadCloudKitDataFromCursor:cursor withCompletionHandler:^(NSArray *results, CKQueryCursor *cursor, NSError *error) {
                    results = [cursorResultSet copy];
                    [cursorResultSet removeAllObjects]; // get rid of the temp results array
                    completionHandler(results, cursor, error);
                }];
            }
        };

        [self.publicDatabase addOperation:cursorOperation];
    }

}

从调用 CKManager 获取数据的 ViewController 方法内部...

dispatch_async(queue, ^{
        [self.ckManager loadCloudKitDataWithCompletionHandler:^(NSArray *results, CKQueryCursor *cursor, NSError *error) {
            if (!error) {
                if ([results count] > 0) {
                    self.numberOfItemsInSection = [results count];
                    NSLog(@"INFO: Success querying the cloud for %lu results!!!", (unsigned long)[results count]);
                    [self loadRecipeDataFromCloudKit]; // fetch the recipe images from CloudKit
                    // parse the records in the results array
                    for (CKRecord *record in results) {
                        ImageData *imageData = [[ImageData alloc] init];
                        CKAsset *imageAsset = record[IMAGE];
                        imageData.imageURL = imageAsset.fileURL;
                        imageData.imageName = record[IMAGE_NAME];
                        imageData.imageDescription = record[IMAGE_DESCRIPTION];
                        imageData.userID = record[USER_ID];
                        imageData.imageBelongsToCurrentUser = [record[IMAGE_BELONGS_TO_USER] boolValue];
                        imageData.recipe = [record[RECIPE] boolValue];
                        imageData.liked = [record[LIKED] boolValue]; // 0 = No, 1 = Yes
                        imageData.recordID = record.recordID.recordName;
                        // check to see if the recordID of the current CID is userActivityDictionary. If so, it's in the user's private
                        // data so set liked value = YES
                        if ([self.imageLoadManager lookupRecordIDInUserData:imageData.recordID]) {
                            imageData.liked = YES;
                        }
                        // add the CID object to the array
                        [self.imageLoadManager.imageDataArray addObject:imageData];

                        // cache the image with the string representation of the absolute URL as the cache key
                        if (imageData.imageURL) { // make sure there's an image URL to cache
                            if (self.imageCache) {
                                [self.imageCache storeImage:[UIImage imageWithContentsOfFile:imageData.imageURL.path] forKey:imageData.imageURL.absoluteString toDisk:YES];
                            }
                        } else {
                            NSLog(@"WARN: CID imageURL is nil...cannot cache.");
                            dispatch_async(dispatch_get_main_queue(), ^{
                                //[self alertWithTitle:@"Yikes!" andMessage:@"There was an error trying to load the images from the Cloud. Please try again."];
                                UIAlertView *reloadAlert = [[UIAlertView alloc] initWithTitle:YIKES_TITLE message:ERROR_LOADING_CK_DATA_MSG delegate:nil cancelButtonTitle:CANCEL_BUTTON otherButtonTitles:TRY_AGAIN_BUTTON, nil];
                                reloadAlert.delegate = self;
                                [reloadAlert show];
                            });
                        }
                    }
                    // update the UI on the main queue
                    dispatch_async(dispatch_get_main_queue(), ^{
                        // enable buttons once data has loaded...
                        self.userBarButtonItem.enabled = YES;
                        self.cameraBarButton.enabled = YES;
                        self.reloadBarButton.enabled = YES;

                        if (self.userBarButtonSelected) {
                            self.userBarButtonSelected = !self.userBarButtonSelected;
                            [self.userBarButtonItem setImage:[UIImage imageNamed:USER_MALE_25]];
                        }
                        [self updateUI]; // reload the collectionview after getting all the data from CK
                    });
                }
                // load the keys to be used for cache look up
                [self getCIDCacheKeys];
            } else {
                NSLog(@"Error: there was an error fetching cloud data... %@", error.localizedDescription);
                dispatch_async(dispatch_get_main_queue(), ^{
                    //[self alertWithTitle:@"Yikes!" andMessage:@"There was an error trying to load the images from the Cloud. Please try again."];
                    UIAlertView *reloadAlert = [[UIAlertView alloc] initWithTitle:YIKES_TITLE message:ERROR_LOADING_CK_DATA_MSG delegate:nil cancelButtonTitle:CANCEL_BUTTON otherButtonTitles:TRY_AGAIN_BUTTON, nil];
                    reloadAlert.delegate = self;
                    [reloadAlert show];
                });
            }
        }];
    }

Answer 1

你很接近。对于 newOperation，您还必须设置 recordFetchedBlock 和 queryCompletionBlock。当您将新操作分配给操作并执行它时，您将不会丢失引用并且您的代码将继续运行。 替换你的一行 [self.publicDatabase addOperation:newOperation];与:

newOperation.recordFetchedBlock = operation.recordFetchedBlock
newOperation.queryCompletionBlock = operation.queryCompletionBlock
operation = newOperation
[self.publicDatabase addOperation:operation];