NSOperation KVO完成

Question

我试图继承NSOperation，并从中读取一些样本， 他们说：当任务结束时，使用NSOperation的KVO完成操作， 代码在这里：

[self willChangeValueForKey:@"isFinished"];
[self willChangeValueForKey:@"isExecuting"]
finished = YES;
executing = NO;
[self didChangeValueForKey:@"isFinished"];
[self didChangeValueForKey:@"isExecuting"];

然后isFinished被称为

- (BOOL) isFinished{
    return(finished);
}

谁能解释一下这个给我？为什么isFinished被调用，isFinished会完成操作吗？正如我所说，KVO手动需要[self didChangeValueForKey：@“isExecuting”];我没有看到像addobserver：和observeValueForKeyPath：

这样的代码

我写

 -(void)call
{
     [self willChangeValueForKey:@"isVip"];
     [self didChangeValueForKey:@"isVip"];
}

-(void)isVip
{
    NSLog(@"Im vip");
}

执行[self call];

时，不会调用

isVip

Answer 1

NSOperationQueue实现将观察您的操作的“isFinished”属性（使用KVO），因此它知道何时从队列中删除它。在被告知其价值发生变化后，内部Apple代码很可能会调用isFinished。

Answer 2

添加到quellish的答案，这是你如何覆盖执行，完成，取消。

//.m
@interface MyOperation ()  //class extension, make these otherwise read-only properties read-write, we must synthesize
@property(atomic, assign, readwrite, getter=isExecuting) BOOL executing;
@property(atomic, assign, readwrite, getter=isFinished) BOOL finished;
@property(atomic, assign, readwrite, getter=isCancelled) BOOL cancelled;
@end

@implementation CoreLocationOperation
@synthesize executing, finished, cancelled;

+ (BOOL)automaticallyNotifiesObserversForKey {
  return YES;
}

+ (NSSet *)keyPathsForValuesAffectingIsCancelled {
  NSSet *result = [NSSet setWithObject:@"cancelled"];
  return result;
}

+ (NSSet *)keyPathsForValuesAffectingIsExecuting {
  NSSet *result = [NSSet setWithObject:@"executing"];
      return result;
}

 + (NSSet *)keyPathsForValuesAffectingIsFinished {
    NSSet *result = [NSSet setWithObject:@"finished"];
   return result;
 }


- (void)start {
 //..
  //You can use self.executing = YES; (note we can change executing which would otherwise be read-only because we synthesized our own ivar.
  [self setExecuting:YES];
...
}

- (void)cancel {
//..
  //super will change the properties executing/finished for us or we can do it manually
  [super cancel];
...

}
    @end

我认为这比

更清楚

[self willChangeValueForKey:_NSURLOperationIsFinished];
    [self setIsFinished:YES];
    [self didChangeValueForKey:_NSURLOperationIsFinished];

Answer 3

首先，您不需要进行手动KVO通知。对于NSOperation子类，KVO通知应自动发送，除非您的班级通过实施+automaticallyNotifiesObserversForKey或+automaticallyNotifiesObserversOf<Key>选择退出NO来退出自动KVO通知。

NSOperation子类除非添加到NSOperationQueue，否则不是很有用。将操作添加到队列时，队列使用KVO来观察指示状态更改的属性，例如finished，executing，cancelled等。请注意，这些不是{ {1}}，isFinished或isExecuting - 这些是这些属性的合成get访问器的名称。

在您的问题中，您包含以下代码：

isCancelled

您在此处所做的是为获取访问者发送手动KVO通知，而不是正在观察的属性。相反，这将完成你似乎想要做的事情：

[self willChangeValueForKey:@"isFinished"];
[self willChangeValueForKey:@"isExecuting"]
finished = YES;
executing = NO;
[self didChangeValueForKey:@"isFinished"];
[self didChangeValueForKey:@"isExecuting"];

不是直接访问实例变量，而是使用访问器方法。这将正确发送这些属性的自动更改通知。

如果您对KVO 真正偏执并希望发送get访问者密钥路径（例如[self setFinished:YES]; [self setExecuting:NO]; ）的通知，请将您的属性注册为关键路径的依赖项：

isFinished

Registering dependencies is part of KVO compliance.