NSOperation KVO isFinished

Question

我试图继承一个 NSOperation，并从中读取一些示例， 他们说：当任务完成时，使用 NSOperation 的 KVO 来完成操作， 代码在这里：

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

然后 isFinished 被调用

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

谁能给我解释一下？为什么 isFinished 被调用，isFinished 会完成操作吗？据我了解，KVO 是否需要手动 [self didChangeValueForKey:@"isExecuting"];而且我没有看到像 addobserver: 和 observeValueForKeyPath: 这样的代码

我写

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

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

do [self call]时不调用isVip；

Answer 1

NSOperationQueue 实现将观察您的操作的“isFinished”属性（使用 KVO），因此它知道何时将其从队列中删除。 isFinished 很可能在被告知其值发生变化后被 Apple 内部代码调用。

Answer 2

除了平息答案之外，这就是您将覆盖执行、完成、取消的方式。

//.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

我认为这比拥有更清楚

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

Answer 3

首先，您不需要手动进行 KVO 通知。对于NSOperation 子类，应自动发送 KVO 通知，除非您的类通过实现 +automaticallyNotifiesObserversForKey 或 +automaticallyNotifiesObserversOf<Key> 来返回 NO 选择退出自动 KVO 通知。

NSOperation 子类不是很有用，除非它们被添加到NSOperationQueue。当一个操作加入队列时，队列使用KVO观察表示状态变化的属性，如finished、executing、cancelled等。注意这些不是isFinished、isExecuting , 或 isCancelled - 这些是这些属性的综合 get 访问器的名称。

在您的问题中包含以下代码：

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

您在这里所做的是为 get 访问器发送手动 KVO 通知，而不是正在观察的属性。相反，这将完成您似乎正在尝试做的事情：

[self setFinished:YES];
[self setExecuting:NO];

不要直接访问实例变量，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，并且想要发送获取访问器密钥路径（例如 isFinished）的通知，请将您的属性注册为密钥路径的依赖项：

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

Registering dependencies is part of KVO compliance.