sendAsynchronousRequest方法會在塊外代碼前執行塊代碼

Question

當我使用sendAsynchronousRequest方法的時候這個方法內塊會在執行外部代碼後執行？

__block NSDictionary *dict; 
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) 
    { 
     NSInteger httpStatus = [((NSHTTPURLResponse *)response) statusCode]; 
     NSLog(@"httpStatus inside block:%d",httpStatus); 
     if ([data length]>0 && connectionError==nil) 
     { 
      dict=[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers|NSJSONReadingAllowFragments error:nil]; 
     } 
     else if (connectionError) 
     { 
      UIAlertView *alt=[[UIAlertView alloc] initWithTitle:@"Error" message:[connectionError localizedDescription] delegate:self cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil]; 
      [alt show]; 
     } 
    }]; 

return dict;//it will return null because it will run before execute inner block of sendAsynchronousRequest 

Answer 1

異步請求總是在網絡線程中運行。 你的[NSOperationQueue mainQueue]意味着block-callback將在mainQueue中進行安排。 如果您想等到請求完成，您可以使用同步請求，或使用信號量來阻止當前線程。

見

+ (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error; 

但要注意的是這種方式請求將阻止當前線程。使用這種方式時要小心。

Answer 2

請用正楷方法

首先定義一個塊

 typedef void (^OnComplate) (NSDictionary * dict); 
- (void)viewDidLoad{ 
    [self getDataWithBlokc:^(NSDictionary *dict) { 
     NSLog(@"%@",dict); 
    }]; 
} 

使用下面的方法

-(void)getDataWithBlokc:(OnComplate)block{ 
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) 
    { 
     NSInteger httpStatus = [((NSHTTPURLResponse *)response) statusCode]; 
     NSLog(@"httpStatus inside block:%d",httpStatus); 
     if ([data length]>0 && connectionError==nil) 
     { 
      NSDictionary* dict=[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers|NSJSONReadingAllowFragments error:nil]; 

      block(dict); 
     } 
     else if (connectionError) 
     { 
      UIAlertView *alt=[[UIAlertView alloc] initWithTitle:@"Error" message:[connectionError localizedDescription] delegate:self cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil]; 
      [alt show]; 
     } 
    }]; 
}