如何正确使用dispatch_sync？

Question

我正在从一个自定义的NSObject类获取twitter时间线数据，并且此类具有调用API并解析数据的所有系数。 我从我的视图控制器调用这个类，它有一个表视图，并需要使用来自twitter的数据填充tableview。但由于dispatch_sync的一些问题，我的视图控制器调用twitter类，并且在数组填充数据（我用来填充tableview）之前，控件返回到视图控制器。

下面是一些代码：

#import <UIKit/UIKit.h> 
#import "SecondViewController.h" 
#import "SpinnerController.h" 
#import "proAlertView.h" 
#import "SBJson.h" 
#import <Accounts/Accounts.h> 
#import <Twitter/Twitter.h> 

@interface TwitterDataLoad : NSObject<UIAlertViewDelegate> 
{ 
    NSMutableData * receivedData; 
    NSArray * results; 
    NSArray * cellContent; 
    NSMutableArray * totalContent; 
    SpinnerController * spinner; 
    proAlertView * av; 
    NSString *json_string; 
    NSDictionary * jsonResults; 
    SBJsonParser * parser; 
    NSMutableArray * dataForTable; 

} 
@property(nonatomic, strong) ACAccount * account; 
@property(nonatomic, strong) NSArray * accounts; 
@property(nonatomic, strong) ACAccountStore * accountStore; 
@property (nonatomic ,retain) SecondViewController * tbView; 

- (void)loadData; 

@end 

#import "TwitterDataLoad.h" 


@interface TwitterDataLoad() 

@end 

@implementation TwitterDataLoad 
@synthesize tbView; 


-(id) init { 
    self = [super init]; 
    if (self) { 
     [self loadData]; 
    } 
    return self; 
} 

- (void)loadData 
{ 
    dataForTable = [[NSMutableArray alloc]init]; 
    //Twitter new code 

    if (_accounts == nil) { 
     if (_accountStore == nil) { 
      _accountStore = [[ACAccountStore alloc] init]; 
     } 
     ACAccountType *accountTypeTwitter = 
     [_accountStore 
     accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter]; 
     [_accountStore requestAccessToAccountsWithType:accountTypeTwitter 
            withCompletionHandler:^(BOOL granted, NSError *error) { 
             if(granted) { 
              dispatch_sync(dispatch_get_main_queue(), ^{ 
               _accounts = [_accountStore 
                    accountsWithAccountType:accountTypeTwitter]; 

               [self sendRequest]; 
              }); 
             } 
            }]; 
    } 
} 

-(void) sendRequest { 

    totalContent = [[NSMutableArray alloc]init]; 
    _account = [_accounts objectAtIndex:0]; 

    TWRequest *postRequest = [[TWRequest alloc] 
           initWithURL: 
           [NSURL URLWithString:@"https://api.twitter.com/1/statuses/user_timeline.json?screen_name=test&count=20"] 
           parameters:nil 
           requestMethod:TWRequestMethodGET]; 

    av = [[proAlertView alloc]initWithTitle:nil message:@"Getting latest news..." delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil]; 
    UIActivityIndicatorView * indicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge]; 
    indicator.frame = CGRectMake(120, 55, 35, 35); 
    [av addSubview:indicator]; 
    [indicator startAnimating]; 
    [av setBackgroundColor:[UIColor clearColor] withStrokeColor:[UIColor blackColor]]; 
    [av show]; 


    [postRequest setAccount:_account]; 
    [postRequest performRequestWithHandler:^(NSData *responseData, 
              NSHTTPURLResponse *urlResponse, 
              NSError *error) { 
     if ([urlResponse statusCode] == 200) { 
      NSError *jsonError = nil; 
      results = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&jsonError]; 

      dispatch_sync(dispatch_get_main_queue(), ^{ 
       [av dismissAlert]; 
       [self parseJson]; 
      }); 
     } 
     else { 
      [self showMessage]; 
     } 
    }]; 

} 

-(void)showMessage { 

    av = [[proAlertView alloc]initWithTitle:@"Connection Problem" message:@"Please confirm the device is connected to the Internet." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil]; 
    [av setBackgroundColor:[UIColor clearColor] withStrokeColor:[UIColor blackColor]]; 
    [av show]; 
} 

-(void)parseJson { 

    NSMutableArray * complete = [[NSMutableArray alloc]init]; 
    for (NSDictionary * tweets in results) 
     { 
     NSString * status = [tweets objectForKey:@"text"]; 
     NSString * date = [tweets objectForKey:@"created_at"]; 
     NSDictionary * user = [tweets objectForKey:@"user"]; 
     NSString * artistName = [user objectForKey:@"name"]; 
     NSString * screenName = [user objectForKey:@"screen_name"]; 
     NSString * artistImage = [user objectForKey:@"profile_image_url"]; 
     cellContent = [NSArray arrayWithObjects:status,date,artistName,screenName,artistImage, nil]; 
     [complete addObject:cellContent]; 
     } 

     SecondViewController * tmpView = [[SecondViewController alloc]initWithNibName:@"SecondViewController_iPhone" bundle:nil]; 
      tmpView.dataToDisplay = complete; 
} 

这个类被称为是这样的：

- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
    TwitterDataLoad * data = [[TwitterDataLoad alloc] init]; 
    [data loadData]; 
    NSArray * temp = dataToDisplay; 
} 

我知道我在一个错误的方式返回值，但我想通过loadData消息返回它本身的视图控制器，但它也没有工作，所以我只是想这个。请不要在意这一点。

感谢

Answer 1

好吧，我想出了这个问题。 dispatch_sync是做什么是应该做的问题是与其他两个语句在其下dispatch_sync是被称为（其它块）

[_accountStore requestAccessToAccountsWithType:accountTypeTwitter 
             withCompletionHandler:^(BOOL granted, NSError *error) 

所以这是一个异步调用和调度同步被调用的程序时控制返回到主队列，因为我们将它定义为：dispatch_sync（dispatch_get_main_queue（）） 并且该方法在控制返回主队列之前返回，因此该数组没有得到任何值。 所以为了解决这个问题，我在我的调用类中写了一个块，一旦这个数据调用结束，就会返回调用。给我发电子邮件，如果你想要整个代码@ ghostsmitm @ gmail.com

Answer 2

请立即拨打您UITableView一旦数据已经加载reloadData。您可以在致电loadData后执行此操作。