const表示不能修改 static表示作用域限定在本文件中
// EOCAnimatedView.h
#import <UIKit/UIKit.h>
@interface EOCAnimatedView : UIView
- (void)animate;
@end
// EOCAnimatedView.m
#import "EOCAnimatedView.h"
static const NSTimeInterval kAnimationDuration = 0.3;
@implementation EOCAnimatedView
- (void)animate {
[UIView animateWithDuration:kAnimationDuration
animations:^(){
// Perform animations
}];
}
@end
It is important that the variable is declared as both static and const. The const qualifier means that the compiler will throw an error if you try to alter the value. In this scenario, that’s exactly what is required. The value shouldn’t be allowed to change. The staticqualifier means that the variable is local to the translation unit in which it is defined. A translation unit is the input the compiler receives to generate one object file. In the case of Objective-C, this usually means that there is one translation unit per class: every implementation (.m) file. So in the preceding example, kAnimationDuration will be declared locally to the object file generated from EOCAnimatedView.m. If the variable were not declared static, the compiler would create an external symbol for it. If another translation unit also declared a variable with the same name, the linker would throw an error with a message similar to this:
duplicate symbol _kAnimationDuration in:
EOCAnimatedView.o
EOCOtherView.o