iOS 报错:(子线程中更新UI)This application is modifying the autolayout engine from a background thread after

今天在写程序的时候，使用Xcode 运行工程时报出下面的错误错信息，我还觉得是什么呢，很久没遇到过这样的错误了。

**ProjectName[1512:778965] This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.**
** Stack:(**
** 0   CoreFoundation                      0x00000001843b61d8 <redacted> + 148** ** 1 libobjc.A.dylib 0x0000000182df055c objc_exception_throw + 56** ** 2 CoreFoundation 0x00000001843b6108 <redacted> + 0** ** 3 Foundation 0x0000000184f9dea4 <redacted> + 192** ** 4 Foundation 0x0000000184de53fc <redacted> + 36** ** 5 UIKit 0x000000018ab5c770 <redacted> + 72** ** 6 UIKit 0x000000018a20e1e8 <redacted> + 1140** ** 7 QuartzCore 0x00000001876ce188 <redacted> + 148** ** 8 QuartzCore 0x00000001876c2e64 <redacted> + 292** ** 9 QuartzCore 0x00000001876c2d24 <redacted> + 32** ** 10 QuartzCore 0x000000018763f7ec <redacted> + 252** ** 11 QuartzCore 0x0000000187666c58 <redacted> + 512** ** 12 QuartzCore 0x0000000187667124 <redacted> + 660** ** 13 libsystem_pthread.dylib 0x000000018344afbc <redacted> + 572** ** 14 libsystem_pthread.dylib 0x000000018344ace4 <redacted> + 200** ** 15 libsystem_pthread.dylib 0x000000018344a378 pthread_mutex_lock + 0** ** 16 libsystem_pthread.dylib 0x0000000183449da4 start_wqthread + 4** **)** 

从上面的报错信息能够看出，主线程在运行的时候子线程修改了主线程UI的布局约束，在iOS开发中，全部的有关界面UI的更新操做必须在主线程中完成。这样的错误很容易出如今使用block的时候，由于个人block就是在子线程中进行的，因此回顾了刚才本身写的代码，发现还真是粗心了。
解决的办法就是把有关UI更新的代码操做放到主线程中就能够了。
修改前:

  [self.healthMgr stepCount:^(double steps, NSError *error) {
            cell.stepsNumberLabel.text = [NSString stringWithFormat:@"%.0f 步数", steps];
    }];
    
    [self.healthMgr distance:^(double distance, NSError *error) {
            cell.distanceLabel.text = [NSString stringWithFormat:@"%.02f 千米", distance];
    }];
    
    [self.healthMgr floorsClimbed:^(double floors, NSError *error) {
            cell.floorsNumberLabel.text = [NSString stringWithFormat:@"%.0f 楼层", floors];
    }];

 

修改后：

[self.healthMgr stepCount:^(double steps, NSError *error) {
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.stepsNumberLabel.text = [NSString stringWithFormat:@"%.0f 步数", steps];
        });
    }];
    
    [self.healthMgr distance:^(double distance, NSError *error) {
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.distanceLabel.text = [NSString stringWithFormat:@"%.02f 千米", distance];
        });
    }];
    
    [self.healthMgr floorsClimbed:^(double floors, NSError *error) {
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.floorsNumberLabel.text = [NSString stringWithFormat:@"%.0f 楼层", floors];
        });
    }];

 

这时候，你可能会问block是在子线程执行的吗？
答：不必定。这个得看你执行block的时候，是哪一种线程了，要是在主线程执行block，那么你的block里边的线程就是主线程了。不然就是子线程。