[otherOperation addDependency:myOperation];

未验证：留着慢慢看 http://blog.csdn.net/proteas/article/details/7226173

近期将xcode升级到了4.2，SDK是 iOS5。在 iOS 5 下，以前可以正常工作的 NSOperation，会崩溃。崩溃的原因是：取消队列中的操作，但是该操作还没有开始。

解决这个问题的方法是：

在 start 方法中判断操作是否已经取消，如果取消，结束操作，没有取消，再执行操作。

在 cancel 方法中判断操作是否正在执行，如果在执行，结束操作，如果没有，修改操作的isCancelled状态。

头文件：

实现文件：

NSOperation class
The NSOperation class is an abstract class you use to encapsulate the code and data associated with a single task.Because it is abstract, you do not use this class directly but instead subclass or use one of the system-defined subclasses (NSInvocationOperation orNSBlockOperation) to perform the actual task.An operation object is a single-shot object—that is, it executes its task once and cannot be used to execute it again.You typically execute operations by adding them to an operation queue (an instance of the NSOperationQueue class).An operation queue executes its operations either directly, by running them on secondary threads, or indirectly using the libdispatch libraryIf you do not want to use an operation queue, you can execute an operation yourself by calling its start method directly from your code. Executing operations manually does put more of a burden on your code, because starting an operation that is not in the ready state triggers an exception. The isReady method reports on the operation’s readiness.  key-value coding (KVC) and key-value observing (KVO)
Concurrent(并发) versus Non-Concurrent
If you plan on executing an operation object manually, instead of adding it to a queue, you can design your operation to execute in a concurrent or non-concurrent manner. Operation objects are non-concurrent by default. In a non-concurrent operation, the operation’s task is performed synchronously—that is, the operation object does not create a separate thread on which to run the task. Thus, when you call the start method of a non-concurrent operation directly from your code, the operation executes immediately in the current thread. By the time the start method of such an object returns control to the caller, the task itself is complete.
In contrast to a non-concurrent operation, which runs synchronously, a concurrent operation runs asynchronously. In other words, when you call the start method of a concurrent operation, that method could return before the corresponding task is completed. This might happen because the operation object created a new thread to execute the task or because the operation called an asynchronous function. It does not actually matter if the operation is ongoing when control returns to the caller, only that it could be ongoing.
If you always plan to use queues to execute your operations, it is simpler to define them as non-concurrent. If you execute operations manually, though, you might want to define your operation objects as concurrent to ensure that they always execute asynchronously. Defining a concurrent operation requires more work, because you have to monitor the ongoing state of your task and report changes in that state using KVO notifications. But defining concurrent operations can be useful in cases where you want to ensure that a manually executed operation does not block the calling thread.
For non-concurrent operations, you typically override only one method:
main
Into this method, you place the code needed to perform the given task. Of course, you should also define a custom initialization method to make it easier to create instances of your custom class. You might also want to define getter and setter methods to access the data from the operation. However, if you do define custom getter and setter methods, you must make sure those methods can be called safely from multiple threads.
If you are creating a concurrent operation, you need to override the following methods at a minimum:
start
isConcurrent
isExecuting
isFinished  - (void)start
Begins the execution of the operation.
The default implementation of this method updates the execution state of the operation and calls the receiver’s main method. This method also performs several checks to ensure that the operation can actually run.  - (void)main
Performs the receiver’s non-concurrent task.
The default implementation of this method does nothing. You should override this method to perform the desired task. In your implementation, do not invoke super.If you are implementing a concurrent operation, you are not required to override this method but may do so if you plan to call it from your custom start method.  NSOperationQueue
The NSOperationQueue class regulates the execution of a set of NSOperation objects. After being added to a queue, an operation remains in that queue until it is explicitly canceled or finishes executing its task.You cannot directly remove an operation from a queue after it has been added. An operation remains in its queue until it reports that it is finished with its task.