为什么NSOperation会禁用自动键值观察？
在使用自定义NSOperation子类时，我注意到[NSOperation automaticallyNotifiesObserversForKey]类方法禁用了自动键值观察(它至少为某些键路径返回NO )。正因为如此，NSOperation子类中的代码被手动调用willChangeValueForKey:和didChange…弄得乱七八糟，这在web上的许多代码示例中都可以看到。
为什么NSOperation要这么做？有了自动KVO支持，人们可以简单地声明操作生命周期标志(isExecuting等)的属性。并通过访问器触发KVO事件，即。以下代码：
[self willChangeValueForKey:@"isExecuting"];
executing = NO;
[self didChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
finished = YES;
[self didChangeValueForKey:@"isFinished"];…可以替换为以下内容：
[self setIsExecuting:NO];
[self setIsFinished:YES];在什么地方有陷阱吗？我刚刚重写了automaticallyNotifiesObserversForKey以返回YES，一切似乎都很正常。
回答 2
Stack Overflow用户
发布于 2011-01-21 12:57:58
最可能的解释是kvo密钥与标准约定不匹配。通常有像-isExecuting和-setExecuting:这样的方法，其中关键路径是@"executing"。在NSOperation的情况下，密钥路径改为@"isExecuting"。
另一种可能是大多数NSOperations实际上没有名为-setIsExecuting:的方法来更改该值。相反，它们将执行/完成标志建立在其他内部状态的基础上。在这种情况下，绝对需要使用显式的willChange/didChange通知。例如，如果我有一个包装NSURLConnection的NSOperation，我可能有两个ivar，一个名为data的保存下载的数据，一个名为connection的保存NSURLConnection，我可以像这样实现getter：
- (BOOL)isExecuting {
return (connection != nil);
}
- (BOOL)isFinished {
return (data != nil && connection == nil);
}现在我的-start方法可以使用
[self willChangeValueForKey:@"isExecuting"];
data = [[NSMutableData alloc] init]; // doesn't affect executing, but is used later
connection = [[NSURLConnection connectionWithRequest:request delegate:self] retain];
[self didChangeValueForKey:@"isExecuting"];开始执行，并且
[self willChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
[connection cancel];
[connection release];
connection = nil;
[self didChangeValueForKey:@"isFinished"];
[self didChangeValueForKey:@"isExecuting"];来完成。
Stack Overflow用户
发布于 2017-08-02 14:31:39
我不知道你为什么要说NSOperation不能使用自动KVO。但我只是尝试验证这一点，因此它可以使用KVO。
[self addObserver:self
forKeyPath:@"isReady"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
context:&ctxKVO_CSDownloadOperation];
[self addObserver:self
forKeyPath:@"isExecuting"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
context:&ctxKVO_CSDownloadOperation];
[self addObserver:self
forKeyPath:@"isFinished"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
context:&ctxKVO_CSDownloadOperation];
[self addObserver:self
forKeyPath:@"isCancelled"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
context:&ctxKVO_CSDownloadOperation];
...
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if (context == &ctxKVO_CSDownloadOperation) {
NSLog(@"KVO: %@", keyPath);
} else {
[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}
}结果是：
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isExecuting : 0
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] KVO: isExecuting
2017-08-02 14:29:58.831 CSDownloader[77366:5089399] isFinished : 0
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isFinished
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] isCancelled : 0
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isCancelled
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.832 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isExecuting : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isExecuting
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isFinished : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isFinished
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] isCancelled : 0
2017-08-02 14:29:58.833 CSDownloader[77366:5089399] KVO: isCancelled
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] isReady : 1
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] KVO: isReady
2017-08-02 14:29:58.834 CSDownloader[77366:5089399] isExecuting : 0所以我真的对这个问题和答案感到困惑。
https://stackoverflow.com/questions/3573236
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