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【深入学习iOS开发(一)】Objective-C Reflection(Objective-C 反射机制)

Objective-C Reflection(Objective-C 反射机制)

 

NSObject 方法

除了个别例外,Cocoa 中大部分类都是 NSObject 的子类,因此大部分对象都继承了它所定义的方法。

NSObject 提供了一些简单的查询运行时系统信息的方法:

  • - (BOOL)isKindOfClass:(Class)aClass  
    Returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class.

  • - (BOOL)isMemberOfClass:(Class)aClass  
    Returns a Boolean value that indicates whether the receiver is an instance of a given class.

  • - (BOOL)respondsToSelector:(SEL)aSelector  
    Returns a Boolean value that indicates whether the receiver implements or inherits a method that can respond to a specified message.

  • - (BOOL)conformsToProtocol:(Protocol *)aProtocol  
    Returns a Boolean value that indicates whether the receiver conforms to a given protocol.

if ([@"Hello World" isKindOfClass:[NSObject class]]) {
    NSLog(@"YES");
} else {
    NSLog(@"NO");
}

if ([@"Hello World" isMemberOfClass:[NSObject class]]) {
    NSLog(@"YES");
} else {
    NSLog(@"NO");
}

if ([self respondsToSelector:@selector(test)]) {
    NSLog(@"YES");
} else {
    NSLog(@"NO");
}

if ([self conformsToProtocol:@protocol(NSObject)]) {
    NSLog(@"YES");
} else {
    NSLog(@"NO");
}

 

消息传递

在 Objective-C,直到运行时消息才会绑定到方法实现。

编译器把消息表达式(message expression)转换为调用 Objective-C 运行时函数 objc_msgSend

[receiver message] -> objc_msgSend(receiver, selector)

id objc_msgSend(id theReceiver, SEL theSelector, ...)

Sends a message with a simple return value to an instance of a class.

 

使用 objc_msgSend 函数,需要引入 Objective-C 运行时头文件:#import <objc/message.h>

[self test];
objc_msgSend(self, @selector(test));

 

通过定义在 NSObject 类中的方法 methodForSelector: ,你可以请求一个方法实现过程(procedure)的指针。

- (IMP)methodForSelector:(SEL)aSelector

Locates and returns the address of the receiver’s implementation of a method so it can be called as a function.

调用 IMP,头两个需要传递的参数,第一个是消息接收的对象,第二个是 selector。

IMP test = [self methodForSelector:@selector(test)];
test(self, @selector(test));

 

动态方法 

有些情况下,我们会想要提供动态的方法实现。

Objective-C 通过使用 @dynamic 声明属性(property)来告诉编译器,属性(property)相关的方法将会动态地提供。

你可以通过实现方法 resolveInstanceMethod: 和 resolveClassMetho: 为相应的一个实例和类方法来动态地提供一个 selector 实现。

  • + (BOOL)resolveInstanceMethod:(SEL)name
    Dynamically provides an implementation for a given selector for an instance method.

  • + (BOOL)resolveClassMethod:(SEL)name
    Dynamically provides an implementation for a given selector for a class method.

 

定义一个 User 类

在 .h 文件,定义属性 name

以往,会在相应的 .m 文件,使用 @synthesize 声明属性 name,这样编译器会自动生成相应的 setter 和 getter 方法

但是,当使用 @dynamic 声明属性 name 时,就必须自己提供相应的 setter 和 getter 方法

Override + (BOOL)resolveInstanceMethod:(SEL)name 方法

当类实例调用 - (void)setName:(NSString) aName 或  -(NSString *)name 方法时,就为其动态地提供方法实现

#import <Foundation/Foundation.h>

@interface User : NSObject {
    NSString *name;
}

@property (nonatomic, retain) NSString *name;

@end

 

#import "User.h"
#import <objc/runtime.h>

@implementation User

@dynamic name;

- (void)dynamicSetName:(NSString *) aName {
    if (name != nil) {
        [name release];
        name = nil;
    }
    [aName retain];
    name = aName;
}

- (NSString *)dynamicName {
    return name;
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    NSLog(@"Instance Method: %@", NSStringFromSelector(sel));
    if ([@"setName:" isEqualToString:NSStringFromSelector(sel)]) {
        class_addMethod([self class], sel, [self instanceMethodForSelector:@selector(dynamicSetName:)], "v@:");
        return YES;
    } else if ([@"name" isEqualToString:NSStringFromSelector(sel)]) {
        class_addMethod([self class], sel, [self instanceMethodForSelector:@selector(dynamicName)], "@");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

@end

 

参考:https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Introduction/Introduction.html

 

posted on 2013-05-05 15:50 Anthony Li 阅读(...) 评论(...) 编辑 收藏

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