ios - 为什么 View 即使在我的线程上也没有更新？

Question

我试图了解为什么在设置某些值时 View 没有更新。问题看起来微不足道，但我花了很多时间来找到问题 - 没有结果。

当返回 block 中的最后一个值时， View 将更新为此最后报告的进度值。

slider 上的设置值在主线程上 - 在 dispatch_async() 中执行更改后没有任何变化在主线程上。

这是完整的代码:

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self performSelector:@selector(doUpdate) withObject:nil afterDelay:5.0f];
}


- (void)doUpdate {
    __weak typeof(self.slider) weakSlider = self.slider;
    [self doSomethingAndReportProgress:^(CGFloat progress) {
        NSLog(@"3. reported progress: %f", progress);
        weakSlider.value = progress; /// slider has range 0 - 100
        NSLog(@"4. weakSlider.value: %f", weakSlider.value);
    }];
}

- (void)doSomethingAndReportProgress:(void(^)(CGFloat))block {
    [self doSomethingOtherAndReportProgress:^(CGFloat progress) {
        block(progress);
        NSLog(@"2. reported progress: %f", progress);
    }];
}

- (void)doSomethingOtherAndReportProgress:(void(^)(CGFloat))block {
    for (int i = 0; i < 10000; i++) {
        CGFloat progress = i / 100.0;
        block(progress);
        NSLog(@"1. reported progress: %f", progress);
    }
}

@end

来自控制台的日志报废:

2014-04-23 09:48:03.991 UIUpdateInBlock[87087:60b] 3. reported progress: 2.050000
2014-04-23 09:48:03.991 UIUpdateInBlock[87087:60b] 4. weakSlider.value: 2.050000
2014-04-23 09:48:03.991 UIUpdateInBlock[87087:60b] 2. reported progress: 2.050000
2014-04-23 09:48:03.992 UIUpdateInBlock[87087:60b] 1. reported progress: 2.050000
....
2014-04-23 09:48:04.421 UIUpdateInBlock[87087:60b] 3. reported progress: 3.950000
2014-04-23 09:48:04.422 UIUpdateInBlock[87087:60b] 4. weakSlider.value: 3.950000
2014-04-23 09:48:04.423 UIUpdateInBlock[87087:60b] 2. reported progress: 3.950000
2014-04-23 09:48:04.423 UIUpdateInBlock[87087:60b] 1. reported progress: 3.950000
....

为什么日志的顺序是:3, 4, 2 ,1 而不是 1, 2, 3, 4？

先感谢您。

Answer 1

因为你把你的NSLog调用后block() ，只要把它们放在前面，你就会得到正确的结果；)