objective-C中的Class(类类型),Selector(选择器SEL),函数指针(IMP)

今天在园子里看到了一篇牛文“Objective-C 2.0 with Cocoa Foundation--- 5，Class类型，选择器Selector以及函数指针 ”，讲得十分精彩，忍不住把它的代码加上注释整理于此，以便日后查看。

个人体会：obj-C中的“Class类型变量”比c#中的Object基类还要灵活，可以用它生成任何类型的实例（但是它又不是NSObject）。而选择器SEL与函数指针IMP，如果非要跟c#扯上关系的话，这二个结合起来，就点类似c#中的反射+委托，可以根据一个方法名称字符串，直接调用方法。

"牛"的基类 Cattle.h

#import <Foundation/Foundation.h>

@interface Cattle : NSObject {
	int legsCount;
}
- (void)saySomething;
- (void)setLegsCount:(int) count;
@end

 Cattle.m

#import "Cattle.h"

@implementation Cattle

-(void) saySomething
{
	NSLog(@"Hello, I am a cattle, I have %d legs.", legsCount);
}

-(void) setLegsCount:(int) count
{
	legsCount = count;
}
@end

子类“公牛" Bull.h

#import <Foundation/Foundation.h>
#import "Cattle.h"

@interface Bull : Cattle {
	NSString *skinColor;
}
- (void)saySomething;
- (NSString*) getSkinColor;
- (void) setSkinColor:(NSString *) color;
@end

Bull.m

#import "Bull.h"


@implementation Bull

-(void) saySomething
{
	NSLog(@"Hello, I am a %@ bull, I have %d legs.", [self getSkinColor],legsCount);
}

-(NSString*) getSkinColor
{
	return skinColor;
}

- (void) setSkinColor:(NSString *) color
{
	skinColor = color;
}
@end

代理类DoProxy.h (关键的代码都在这里)

#import <Foundation/Foundation.h>

//定义几个字符串常量
#define	SET_SKIN_COLOR @"setSkinColor:"
#define	BULL_CLASS @"Bull"
#define	CATTLE_CLASS @"Cattle"


@interface DoProxy : NSObject {
	BOOL notFirstRun;
	
	id cattle[3];
	//定义二个选择器
	SEL say;
	SEL skin;
	
	//定义一个函数指针(传统C语言的处理方式)
	void(*setSkinColor_Func)(id,SEL,NSString*);
	
	//定义一个IMP方式的函数指针(obj-C中推荐的方式)
	IMP say_Func;
	
	//定义一个类
	Class bullClass;
}

-(void) doWithCattleId:(id) aCattle colorParam:(NSString*) color;
-(void) setAllIVars;
-(void) SELFuncs;
-(void) functionPointers;

@end

DoProxy.m

#import "DoProxy.h"
#import "Cattle.h"
#import "Bull.h"

@implementation DoProxy


//初始化所有变量
- (void) setAllIVars
{	
	cattle[0] = [Cattle new];
	
	bullClass = NSClassFromString(BULL_CLASS);
	//即cattle[1],cattle[2]都是Bull类的实例
	cattle[1] = [bullClass new];
	cattle[2] = [bullClass new];
	
	say = @selector(saySomething);
	skin = NSSelectorFromString(SET_SKIN_COLOR);
}


//初始化id
- (void) doWithCattleId:(id) aCattle colorParam:(NSString*) color
{
	//第一次运行的时候
	if(notFirstRun == NO)
	{
		NSString *myName = NSStringFromSelector(_cmd);//取得当前正在执行的方法的名字
		NSLog(@"Running in the method of %@", myName);
		notFirstRun = YES;//修改初次运行标志位
	}
	
	NSString *cattleParamClassName = [aCattle className];//取得aCattle的"类名称"
	
	//如果aCattle是Bull或Cattle类的实例
	if([cattleParamClassName isEqualToString:BULL_CLASS] || [cattleParamClassName isEqualToString:CATTLE_CLASS])
	{
		[aCattle setLegsCount:4];//设置牛的4条腿
		if([aCattle respondsToSelector:skin])//如果aCattle对应的是类中,有定义方法"setSkinColor"
		{
			[aCattle performSelector:skin withObject:color];//则调用setSkinColor方法
		}
		else
		{
			NSLog(@"Hi, I am a %@, have not setSkinColor!", cattleParamClassName);//否则输出相应的提示信息
		}
		[aCattle performSelector:say];//最后执行saySomething方法(这二个方法在Bull与Cattle类中都有,所以肯定能运行)
	}
	else //如果aCattle即不是Bull类也不是Cattle类的实例
	{
		NSString *yourClassName = [aCattle className];
		NSLog(@"Hi, you are a %@, but I like cattle or bull!", yourClassName);//显示这个"异类"的相关信息
	}
}

//初始化选择器以及相应函数
- (void) SELFuncs
{
	[self doWithCattleId:cattle[0] colorParam:@"brown"];
	[self doWithCattleId:cattle[1] colorParam:@"red"];
	[self doWithCattleId:cattle[2] colorParam:@"black"];
	[self doWithCattleId:self colorParam:@"haha"];//这里故意传入一个异类self(即DoProxy本身),DoProxy当然不是Bull或Cattle
}

//函数指针测试
- (void) functionPointers
{
	//取得函数指针的第一种方式
	setSkinColor_Func=(void (*)(id, SEL, NSString*)) [cattle[1] methodForSelector:skin];
	//上面的语句其实等效于下面这种方法
	//IMP setSkinColor_Func = [cattle[1] methodForSelector:skin];
	
	//用第二种方法取得saySomething的函数指针
	say_Func = [cattle[1] methodForSelector:say];
	
	//用函数指针的形式调用setSkinColor
	setSkinColor_Func(cattle[1],skin,@"verbose");
	
	NSLog(@"Running as a function pointer will be more efficiency!");
	
	//调用saySomething方法
	say_Func(cattle[1],say); 
}

@end

测试主函数main()

#import <Foundation/Foundation.h>
#import "DoProxy.h"

int main (int argc, const char * argv[]) {
	NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	DoProxy *doProxy = [DoProxy new];
	
	[doProxy setAllIVars];
	[doProxy SELFuncs];
	[doProxy functionPointers];
	
	[doProxy release];
	[pool drain];
	return 0;
}

运行结果：

2011-02-28 21:40:33.240 HelloSelector[630:a0f] Running in the method of doWithCattleId:colorParam:
2011-02-28 21:40:33.245 HelloSelector[630:a0f] Hi, I am a Cattle, have not setSkinColor!
2011-02-28 21:40:33.247 HelloSelector[630:a0f] Hello, I am a cattle, I have 4 legs.
2011-02-28 21:40:33.248 HelloSelector[630:a0f] Hello, I am a red bull, I have 4 legs.
2011-02-28 21:40:33.250 HelloSelector[630:a0f] Hello, I am a black bull, I have 4 legs.
2011-02-28 21:40:33.251 HelloSelector[630:a0f] Hi, you are a DoProxy, but I like cattle or bull!
2011-02-28 21:40:33.252 HelloSelector[630:a0f] Running as a function pointer will be more efficiency!
2011-02-28 21:40:33.254 HelloSelector[630:a0f] Hello, I am a verbose bull, I have 4 legs.