介绍@dynamic的用法
Objective-C 2.0提供了属性(@property),可以让编译器自动生成setter和getter方法。如果不想编译器自作主张生成这些setter和getter方法,则使用@dynamic。举个简单例子,如下
#import <Foundation/Foundation.h> @interface Person : NSObject @property (copy) NSString *name; @end @implementation Person // @dynamic tells compiler don't generate setter and getter automatically @dynamic name; @end int main(int argc, const charchar * argv[]) { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; Person *a = [[Person alloc] init]; a.name = @"Hello"; // will crash here NSLog(@"%@", a.name); [a release]; [pool drain]; return 0; } // main
运行该程序,Xcode会报错“-[PersonsetName:]: unrecognized selector sent to instance 0x1001149d0”。如果将@dynamic注释掉,则一切Ok。
这里由于使用@dynamic,我们需要自己提供setter和getter方法。一般有两种方法:1)自己提供setter和getter方法,将编译器自动生成的setter和getter方法手动再写一遍;2)动态方法决议(DynamicMethod Resolution),在运行时提供setter和getter对应实现的C函数。
对于第一种方法,需要在类中显式提供实例变量,因为@dynamic不能像@synthesize那样向实现文件(.m)提供实例变量。
#import <Foundation/Foundation.h> @interface Person : NSObject { // must provide a ivar for our setter and getter NSString *_name; } @property (copy) NSString *name; @end @implementation Person // @dynamic tells compiler don't generate setter and getter automatically @dynamic name; // We provide setter and getter here - (void) setName:(NSString *)name { if (_name != name) { [_name release]; _name = [name copy]; } } - (NSString *) name { return _name; } @end // Person int main(int argc, const charchar * argv[]) { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; Person *a = [[Person alloc] init]; a.name = @"Hello"; // Ok, use our setter a.name = @"Hello, world"; NSLog(@"%@", a.name); // Ok, use our getter [a release]; [pool drain]; return 0; } // main
对于第二种方法,在运行时决定setter和getter对应实现的C函数,使用了NSObject提供的resolveInstanceMethod:方法。在C函数中不能直接使用实例变量,需要将ObjC对象self转成C中的结构体,因此在Person类同样需要显式声明实例变量而且访问级别是@public,为了隐藏该实例变量,将声明放在扩展(extension)中
#import <Foundation/Foundation.h> #import <objc/objc-runtime.h> // for class_addMethod() // ------------------------------------------------------ // A .h file @interface Person : NSObject @property (copy) NSString *name; - (void) hello; @end // ------------------------------------------------------ // A .m file // Use extension to override the access level of _name ivar @interface Person () { @public NSString *_name; } @end @implementation Person // @dynamic implies compiler to look for setName: and name method in runtime @dynamic name; // Only resolve unrecognized methods, and only load methods dynamically once + (BOOL) resolveInstanceMethod:(SEL)sel { // Capture setName: and name method if (sel == @selector(setName:)) { class_addMethod([self class], sel, (IMP)setName, "v@:@"); return YES; } else if (sel == @selector(name)) { class_addMethod([self class], sel, (IMP)getName, "@@:"); return YES; } return [super resolveInstanceMethod:sel]; } void setName(id self, SEL _cmd, NSString* name) { // Implement @property (copy) if (((Person *)self)->_name != name) { [((Person *)self)->_name release]; ((Person *)self)->_name = [name copy]; } } NSString* getName(id self, SEL _cmd) { return ((Person *)self)->_name; } - (void) hello { NSLog(@"Hello, world"); } @end // Person int main(int argc, const charchar * argv[]) { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; Person *a = [[Person alloc] init]; [a hello]; // never call resolveInstanceMethod a.name = @"hello1"; NSLog(@"%@", a.name); a.name = @"hello2"; NSLog(@"%@", a.name); [a release]; [pool drain]; return 0; } // main
总结以上,@dynamic的作用就是禁止编译器为@property产生setter和getter方法,有两种办法实现setter和getter方法:1)自己提供setter和getter方法;2)方法动态决议(DynamicMethod Resolution)。