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介绍@dynamic的用法

高洛峰

发布： 2016-12-13 09:18:51

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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&#39;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

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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&#39;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

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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

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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)。