后台子线程（非主线程）更新UI引起的警告

一、问题描述

-(void)sendAsynchronousRequest
{
    NSLog(@"%@",[NSThread currentThread]);
    [SVProgressHUD showWithStatus:@"正在登录....." maskType:SVProgressHUDMaskTypeBlack];
    NSString *urlString = [NSString stringWithFormat:@"http://120.25.226.186:32812/login?username=%@&pwd=%@&type=JSON",self.userName.text,self.userPWD.text];
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:urlString]];
    NSOperationQueue *operationQueue = [[NSOperationQueue alloc]init];
    [NSURLConnection sendAsynchronousRequest:request queue:operationQueue completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
        NSString *jsonString = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
        NSRange rangeStart = [jsonString rangeOfString:@"":""];
        NSUInteger start = rangeStart.location+3;
        NSRange rangeEnd = [jsonString rangeOfString:@""}"];
        NSUInteger end = rangeEnd.location;
        NSUInteger length = end-start;
        NSString *message =[jsonString substringWithRange:NSMakeRange(start, length)];
        if([jsonString containsString:@"success"])
        {
            NSLog(@"%@",[NSThread currentThread]);
            [SVProgressHUD showSuccessWithStatus:message maskType:SVProgressHUDMaskTypeBlack];
        }
        else
        {
　　　　　　　 NSLog(@"%@",[NSThread currentThread]);
            [SVProgressHUD showSuccessWithStatus:message maskType:SVProgressHUDMaskTypeBlack];
        }
    }];
}

运行时发生以下警告:

This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes.  This will cause an exception in a future release.

二、问题分析

此应用程序是由一个后台线程修改布局，从而导致崩溃，将导致在未来的版本异常。

更新UI必须在主线程。调用NSURLConnection的sendAsynchronousRequest: queue: completionHandler:方法会创建子线程执行请求，而在Block回调中调用的是主线程SVProgressHUD对象，从而导致错误。通过[NSThread currentThread]打印前后线程，前者在主线程：<NSThread: 0x7fbac9c074e0>{number = 1, name = main}，后者在子线程<NSThread: 0x7fbacc0208a0>{number = 3, name = (null)}。

三、问题解决

例如：GCD代码

dispatch_async(dispatch_get_main_queue(), ^{
　　// 更UI
});

 修改如下：

-(void)sendAsynchronousRequest
{
    NSLog(@"%@",[NSThread currentThread]);
    [SVProgressHUD showWithStatus:@"正在登录....." maskType:SVProgressHUDMaskTypeBlack];
    NSString *urlString = [NSString stringWithFormat:@"http://120.25.226.186:32812/login?username=%@&pwd=%@&type=JSON",self.userName.text,self.userPWD.text];
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:urlString]];
    NSOperationQueue *operationQueue = [[NSOperationQueue alloc]init];
    
    [NSURLConnection sendAsynchronousRequest:request queue:operationQueue completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
        NSString *jsonString = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
        NSRange rangeStart = [jsonString rangeOfString:@"":""];
        NSUInteger start = rangeStart.location+3;
        NSRange rangeEnd = [jsonString rangeOfString:@""}"];
        NSUInteger end = rangeEnd.location;
        NSUInteger length = end-start;
        NSString *message =[jsonString substringWithRange:NSMakeRange(start, length)];
        if([jsonString containsString:@"success"])
        {
            NSLog(@"%@",[NSThread currentThread]);
            dispatch_async(dispatch_get_main_queue(), ^{
                [SVProgressHUD showSuccessWithStatus:message maskType:SVProgressHUDMaskTypeBlack];
            });
        }
        else
        {
            NSLog(@"%@",[NSThread currentThread]);
            dispatch_async(dispatch_get_main_queue(), ^{
                [SVProgressHUD showSuccessWithStatus:message maskType:SVProgressHUDMaskTypeBlack];
            });
        }
    }];
}