ios - 为什么在检索 json 数据和显示 viewcontroller 之间有很长的延迟

Question

我在 JSON 数据检索和 UITableViewController 启动之间存在长时间延迟的问题。

下面的方法使用从 UITableViewControllers 初始化程序调用的硬编码查询，并在 2 秒内检索和显示数据。

- (void)productsQuery
{
NSString *requestString = @"http://192.168.2.10/testQueries.php?Product_Description=tea";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];

NSURLSessionDataTask *dataTask =
[self.session dataTaskWithRequest:request
                completionHandler:
                    ^(NSData *data, NSURLResponse *response, NSError *error) {

                        NSArray *returnedItems =
                            [NSJSONSerialization JSONObjectWithData:data
                                                            options:NSJSONReadingMutableContainers
                                                              error:&error];

                        for (int i = 0; i < [returnedItems count]; i++) {

                            NSDictionary *item = [returnedItems objectAtIndex:i];

                            NSNumber *nBay = [item objectForKey:@"Bay_Number"];
                            NSNumber *nShelf = [item objectForKey:@"Shelf_Number"];
                            NSNumber *coordX = [item objectForKey:@"CoordinateX"];
                            NSNumber *coordY = [item objectForKey:@"CoordinateY"];

                            TNWProduct *product =[[TNWProduct alloc]
                                initWithProductDescription:[item     objectForKey:@"Product_Description"]
                        aisleNumber:[item objectForKey:@"Aisle_Number"]
                                bay:[nBay intValue]
                              shelf:[nShelf intValue]
                   nonAisleLocation:[item objectForKey:@"Location_Description"]
                        coordinateX:[coordX intValue]
                        coordinateY:[coordY intValue]];

                        [self.productList addObject:product];
                        }

                        NSLog(@"%@", self.productList);

                        dispatch_async(dispatch_get_main_queue(), ^{
                            [self.tableView reloadData];
                        });
                    }];

    [dataTask resume];
}

该方法随后被调整并移动到 UIView Controller ，以便用户可以输入查询。

JSON 数据仍会在 2 秒内被检索并添加到 NSMutableArray _productList 中，正如它在 NSLog 调用的控制台中显示的那样，但在启动 ProductListViewController 之前，它似乎在 5-15 秒内什么都不做。

@interface TNWSearchViewController () <UITextFieldDelegate>

@property (weak, nonatomic) NSString *userQuery;
@property (weak, nonatomic) IBOutlet UIToolbar *toolbar;
@property (weak, nonatomic) IBOutlet UITextView *informationMessages;
@property (nonatomic) NSURLSession *session;
@property (nonatomic, strong) NSMutableArray *productList;
@property (nonatomic, strong) NSArray *returnedItems;

@end

@implementation TNWSearchViewController
.
.
.
.

- (void)productQuery:(NSString *)query
{
if ([_productList count] > 0 ) {
    [_productList removeAllObjects];
}

NSMutableString *requestString = [@"http://192.168.2.10/testQueries.php?Product_Description=" mutableCopy];

[requestString appendString:query];

NSString *escapedRequestString = [requestString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];

NSLog(@"%@", escapedRequestString);

NSURL *url = [NSURL URLWithString:escapedRequestString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];

NSURLSessionDataTask *dataTask =
[self.session dataTaskWithRequest:request
                completionHandler:
 ^(NSData *data, NSURLResponse *response, NSError *error) {

     _returnedItems =
        [NSJSONSerialization JSONObjectWithData:data
                                        options:NSJSONReadingMutableContainers
                                          error:&error];
     //NSLog(@"Returned items = %@", _returnedItems);
     for (int i = 0; i < [_returnedItems count]; i++) {

         NSDictionary *item = [_returnedItems objectAtIndex:i];
         //NSLog(@"Item = %@", item);
         NSNumber *nBay = [item objectForKey:@"Bay_Number"];
         NSNumber *nShelf = [item objectForKey:@"Shelf_Number"];
         NSNumber *coordX = [item objectForKey:@"CoordinateX"];
         NSNumber *coordY = [item objectForKey:@"CoordinateY"];

         TNWProduct *product =[[TNWProduct alloc]
                               initWithProductDescription:[item objectForKey:@"Product_Description"]
                               aisleNumber:[item objectForKey:@"Aisle_Number"]
                               bay:[nBay intValue]
                               shelf:[nShelf intValue]
                               nonAisleLocation:[item objectForKey:@"Location_Description"]
                               coordinateX:[coordX intValue]
                               coordinateY:[coordY intValue]];
         NSLog(@"%@", product);
         [_productList addObject:product];

     }

     NSLog(@"Product list = %@", self.productList);

     if ( [_productList count] > 0 ) {

         TNWProductListViewController *plvc =
         [[TNWProductListViewController alloc] initWithStyle:UITableViewStylePlain];

         plvc.productList = [self.productList mutableCopy];

         [self.navigationController pushViewController:plvc animated:YES];

     } else {
         _informationMessages.text = @"No matches found";
     }
 }];

[dataTask resume];
}

将代码块从 for 循环和 if/else 语句中移出 [dataTask resume]导致应用程序按预期加载 UITableView，但无法再访问 _returnedItems 中的数据。

协助表示赞赏。

for (int i = 0; i < [_returnedItems count]; i++) {
.
.
.
} else {
     _informationMessages.text = @"No matches found";
}

Answer 1

将 ViewController 的创建和调用移至 dispatch_async block ，如下所示解决了该问题。

谢谢福尼克斯。

dispatch_async(dispatch_get_main_queue(), ^{
    if ( [[[TNWProductList productsStore] allProducts] count] > 0 ) {

        TNWProductListViewController *plvc =
        [[TNWProductListViewController alloc] initWithStyle:UITableViewStylePlain];


        [self.navigationController pushViewController:plvc animated:YES];

    } else {
        _informationMessages.text = @"No matches found";
    }