sendAsynchronousRequest 导致 UI 冻结

downloadImages 是一个按钮，每当我按下它时，旋转器应该开始滚动，异步请求应该 ping Google（以确保存在连接），并且在收到响应后，我开始同步下载图像。

不知怎的，旋转器不会去，看起来好像请求是同步的而不是异步的。

- (IBAction)downloadImages:(id)sender {

    NSString *ping=@"http://www.google.com/";

    GlobalVars *globals = [GlobalVars sharedInstance];
    [self startSpinner:@"Please Wait."];
    NSURL *url = [[NSURL alloc] initWithString:ping];
    NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:5.0];
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
        if (data) {
            for(int i=globals.farmerList.count-1; i>=0;i--)
            {
            //Definitions
            NSString * documentsDirectoryPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

            //Get Image From URL
                NSString *urlString = [NSString stringWithFormat:@"https://myurl.com/%@",[[globals.farmerList objectAtIndex:i] objectForKey:@"Image"]];
            UIImage * imageFromURL = [self getImageFromURL:urlString];



            //Save Image to Directory
            [self saveImage:imageFromURL withFileName:[[globals.farmerList objectAtIndex:i] objectForKey:@"Image"] ofType:@"jpg" inDirectory:documentsDirectoryPath];
            }
            [self stopSpinner];

        }
    }];
}

旋转器代码：

//show loading activity.
- (void)startSpinner:(NSString *)message {
    //  Purchasing Spinner.
    if (!connectingAlerts) {
        connectingAlerts = [[UIAlertView alloc] initWithTitle:NSLocalizedString(message,@"")
                                                     message:nil
                                                    delegate:self
                                           cancelButtonTitle:nil
                                           otherButtonTitles:nil];
        connectingAlerts.tag = (NSUInteger)300;
        [connectingAlerts show];

        UIActivityIndicatorView *connectingIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
        connectingIndicator.frame = CGRectMake(139.0f-18.0f,50.0f,37.0f,37.0f);
        [connectingAlerts addSubview:connectingIndicator];
        [connectingIndicator startAnimating];

    }
}
//hide loading activity.
- (void)stopSpinner {
    if (connectingAlerts) {
        [connectingAlerts dismissWithClickedButtonIndex:0 animated:YES];
        connectingAlerts = nil;
    }
    // [self performSelector:@selector(showBadNews:) withObject:error afterDelay:0.1];
}

如题： getImageFromURL 代码

-(UIImage *) getImageFromURL:(NSString *)fileURL {
    UIImage * result;

    NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fileURL]];
    result = [UIImage imageWithData:data];

    return result;
}
-(void) saveImage:(UIImage *)image withFileName:(NSString *)imageName ofType:(NSString *)extension inDirectory:(NSString *)directoryPath {
    if ([[extension lowercaseString] isEqualToString:@"png"]) {
        [UIImagePNGRepresentation(image) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.%@", imageName, @"png"]] options:NSAtomicWrite error:nil];
    } else if ([[extension lowercaseString] isEqualToString:@"jpg"] || [[extension lowercaseString] isEqualToString:@"jpeg"]) {
        [UIImageJPEGRepresentation(image, 1.0) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.%@", imageName, @"jpg"]] options:NSAtomicWrite error:nil];
    } else {
        NSLog(@"Image Save Failed\nExtension: (%@) is not recognized, use (PNG/JPG)", extension);
    }
}

那是因为您正在创建一个异步操作，然后通过使用告诉它在主线程上执行[NSOperationQueue mainQueue];.

相反，创建 NSOpeartionQueue 的新实例并将其作为参数传递。

NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];