Prior to Xcode 4.3, if you wanted to use a method before you declared its implementation, you were required to forward declare the method (as with a C function prototype). This would usually be done in a class continuation (AKA class extension, private category). There's a good example in this question: Private Methods in Objective-C, in Xcode 4.3 I no longer need to declare them in my implementation file ?.
As of 4.3, this is no longer required; DRYers rejoice.
But can this magic be turned off, returning to the pre-4.3 behavior?
Currently this is not possible. The parsing behavior for Objective-C was changed to no longer require forward declarations.
来源：https://stackoverflow.com/questions/9981533/in-xcode-4-3-can-i-require-forward-method-declarations-as-before