超级中的dispatch_once？

Question

If I call dispatch_once inside an instance method of a super class, will it execute once for each subclass? I'm guessing not because the Xcode Grand Central Dispatch (GCD) Reference says it "executes a block object once and only once for the lifetime of an application."

Answer 1

Your reading of the documentation is correct. That block will execute only once, provided you are using the same once token.

Answer 2

Your reading of the documentation is almost correct: dispatch_once() will execute only once for the lifetime of a given dispatch_once_t variable. Since most applications declare this variable as a static global, the documentation covers this pattern. If you have a token which is unique for each class instance and starts out properly zero-initialized, then it will execute for each instance.