KVO

1. 可以重复添加同一observer，添加几次，但值改变时就会收到调用几次

    - (void)observeValueForKeyPath:(NSString *)keyPath
                          ofObject:(id)object
                            change:(NSDictionary<NSKeyValueChangeKey,id> *)change
                           context:(void *)context
   

   测试代码如下：

    - (void)viewDidLoad {
        [super viewDidLoad];
    
        self.nameLable = [[UILabel alloc] init];
        
        [self.nameLable addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew context:nil];
        
        self.nameLable.text = @"name0";
        
        [self.nameLable addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew context:nil];
        
        self.nameLable.text = @"name1";
        
        [self.nameLable removeObserver:self forKeyPath:@"text"];
        
        self.nameLable.text = @"name 2";
        
        [self.nameLable removeObserver:self forKeyPath:@"text"];
        
        self.nameLable.text = @"name 3";
    }
    
    - (void)observeValueForKeyPath:(NSString *)keyPath
                          ofObject:(id)object
                            change:(NSDictionary<NSKeyValueChangeKey,id> *)change
                           context:(void *)context {
        NSLog(@"%@",change);
    }
   

   输出结果

    2017-05-08 17:33:45.947 Test[56608:512908] {
        kind = 1;
        new = name0;
    }
    2017-05-08 17:33:45.948 Test[56608:512908] {
        kind = 1;
        new = name1;
    }
    2017-05-08 17:33:45.948 Test[56608:512908] {
        kind = 1;
        new = name1;
    }
    2017-05-08 17:33:45.948 Test[56608:512908] {
        kind = 1;
        new = "name 2";
    }
   

2. 如果对象已经被释放了，但是observer还没有被移除，会导致crash。

    Terminating app due to uncaught exception 'NSInternalInconsistencyException', 
    reason: 'An instance 0x7fbda040c020 of class UILabel was deallocated while key value observers were still registered with it.
    Current observation info: <NSKeyValueObservationInfo 0x60000022aa40> 
    (<NSKeyValueObservance 0x600000051640: Observer: 0x7fbda0406ca0, Key path: text, Options: <New: YES, Old: NO, Prior: NO> Context: 0x0, Property: 0x600000051670>
   

   如果没有添加但是直接移除，也会crash。

    Terminating app due to uncaught exception 'NSRangeException', 
    reason: 'Cannot remove an observer <ViewController 0x7feb6160a8d0> for the key path "text" from <UILabel 0x7feb615054d0> 
    because it is not registered as an observer.'
   

   虽然可以在移除的时候加入try catch来防止crash，但是异常还是会被第三方crash收集统计工具上报，真是头疼的事情。

保持屏幕常亮

加入以下代码可以在用户没有任何操作的情况下屏幕一直亮着，

 [[UIApplication sharedApplication] setIdleTimerDisabled:YES];

但是这个值会被系统可能被系统修改

@property(nonatomic,getter=isIdleTimerDisabled)       BOOL idleTimerDisabled;     // default is NO

一个重现的例子是

将idleTimerDisabled设置成yes后，调用系统的拍照，然后取消拍照，这个时候idleTimerDisabled会被设置成NO.
因为我们的应用需要保持屏幕常亮，因此需要在合适的地方加入kvo来监听这个值，如果被设置成NO，那就给它改成yes。
使用ijkplayer播放器，在播放的时候，ijkplayer会隔一段时间，大概15秒左右调用一下下面的语句来保持屏幕常亮

[[UIApplication sharedApplication] setIdleTimerDisabled:NO];
[[UIApplication sharedApplication] setIdleTimerDisabled:YES];

使用KVO以后，暂时没有发现屏幕在不应该变黑的时候黑掉。