不能在异常情况下显示UIAlert

Question

我正在使用UITableView。如果没有网络连接，那么会在viewDidload中引发异常。我viewDidLoad中的功能是：

@try { 
       NSLog(@"Request URL = %@",URLString); 

       NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL 
                    URLWithString:URLString]]; 
       NSData *response = [NSURLConnection sendSynchronousRequest:request 
                returningResponse:nil error:nil]; 

       NSError *jsonParsingError = nil; 
       NSDictionary *tableData = [NSJSONSerialization JSONObjectWithData:response 
                     options:0 
                     error:&jsonParsingError]; 
       // Grab whole data with data field in JSON 
       // responseArray = [tableData objectForKey:@"data"]; 


       responseArray = [[NSMutableArray alloc]initWithArray:[tableData objectForKey:@"data"]]; 
       for(int i = 0; i < responseArray.count; i++) 
       { 
        NSArray * tempArray = responseArray[i]; 
        responseArray[i] = [tempArray mutableCopy]; 
       } 


       UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom]; 
       [btn setFrame:CGRectMake(280.0, 0.0, 40.0, 40.0)]; 
       [btn setImage:[UIImage imageNamed:@"sort_icon.png"] forState:UIControlStateNormal]; 
       [btn addTarget:self action:@selector(showActionSheet) forControlEvents:UIControlEventTouchUpInside]; 
       UIBarButtonItem *barbutton = [[UIBarButtonItem alloc]initWithCustomView:btn]; 
       self.navigationItem.rightBarButtonItem = barbutton; 

     } 
     @catch (NSException *exception) 
     { 
      exceptionOccured = YES; 
      NSLog(@"Exception Ocurred"); 
      UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Error in connectivity" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; 
      [alert show]; 
     } 

在的cellForRowAtIndexPath我这样做：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 
{ 
    @try { 

    NSMutableDictionary *tempDict = [[NSMutableDictionary alloc] init]; 
    tempDict = [responseArray objectAtIndex:indexPath.row]; 

    return cell; 

    } 
    @catch (NSException *exception) 
    { 
     NSLog(@"Error in CEll Create"); 
     NSLog(@"Draw Alert"); 
     UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Error in connectivity" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; 
     [alert show]; 
    } 
} 

和AlertViewDelegate功能我做

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex 
{ 

    [self.navigationController popViewControllerAnimated:YES]; 
} 

现在的问题是，它没有显示警报每当出现异常并重新抛出异常并显示错误时

Thread 1: EXC_BAD_ACCESS(code=2, address=0x2) 

任何帮助将不胜感激...

Answer 1

你应该避免在你的代码中抛出异常。

首先，您可以使用Reachability Class来确定是否有活动的Internet连接可用。

我会明确推荐使用NSURLConnectionDelegate协议来进行URL连接。所以你可以使用更好的异步编程风格。

Answer 2

问题在别的地方。当调用- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath时，意味着系统需要重新加载tableView。因此，您不应该检查该函数内的数据。当你输入这个函数时，你应该确保你的数据是正确的。

你必须做的是首先检查在一个特定的功能您的数据：

 -(void)CheckData(NSArray *responseArray) { 
      @try { 
       //Retrieve your data & check if its valid 
       self.dataArray = responseArray; 
       [self.tableView reloadData]; 
      } 
      @catch (NSException *exception) 
      { 
       NSLog(@"Error in check Data"); 
       NSLog(@"Draw Alert"); 
       UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Error  in connectivity" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; 
       [alert show]; 
      } 

然后，实现您的数据源代表：

-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section { 
      return [self.dataArray count]; 
     } 

    -(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { 
     static NSString *CellIdentifier = @"Cell"; 

     UITableViewCell *cell = (UITableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier]; 
     if(cell == nil){ 
      cell = [[[UITableViewCell alloc]initWithStyle:UITableViewStylePlain reuseIdentifier:CellIdentifier] autorelease] ; 
     } 

     //TODO: Init your cell using your self.dataArray content 

     return cell; 
    } 

Answer 3

我会推荐进口这种SystemConfiguration框架和使用一样。这将检查网络连接是否可用。以下是简单代码： -

#import <SystemConfiguration/SystemConfiguration.h> 
-(void)yourMethod 
{  
    SCNetworkConnectionFlags flags = 0; 
      if (yourhostname && [yourhostname length] > 0) 
      { 
       flags = 0; 
       BOOL found = NO; 
       SCNetworkReachabilityRef reachabilityRef = SCNetworkReachabilityCreateWithName(kCFAllocatorDefault, [yourhostname UTF8String]); 
       if (reachabilityRef) 
       { 
        found = SCNetworkReachabilityGetFlags(reachabilityRef, &flags) 
        && (flags & kSCNetworkFlagsReachable) 
        && !(flags & kSCNetworkFlagsConnectionRequired); 
        CFRelease(reachabilityRef); 
        reachabilityRef = NULL; 
       } 
       if (found) 
       {  
        NSLog(@"Connection available");   
        } 
        else 
       { 
        NSLog(@"Connection not available");   
        } 
}