模拟静态变量及静态类继承

在Objective-C中是没有静态类变量一说的，不过我们可以模拟出来，但如果类对象之间还有继承关系，这样就一点复杂的了。我们这里假设一个Coupon类对象需要保存当前还剩多少有优惠券，还有一个继承于Coupon类对象的NewCoupon类对象，但它也保存有自已可用的优惠券的数量，我们如何保证他们的静态类变量相互独立，请看下面的例子。

Coupon.h

#import <Foundation/Foundation.h>

@interface Coupon : NSObject {
    
}

+(int) numberCouponsLeft;
+(void) resetCoupon;

@end

Coupon.m

#import "Coupon.h"
#define INITIAL_COUPON_ALLOCATION 100

//设置静态变量
static int availableCoupons = INITIAL_COUPON_ALLOCATION;

//在实现类中定义私有静态访问器
@interface Coupon()
+(int) availableCoupons;
+(void) setAvailableCoupons:(int) inAvailableCoupons;
@end

@implementation Coupon

+(int) availableCoupons
{
    return availableCoupons;
}

+(void)setAvailableCoupons:(int)inAvailableCoupons
{
    availableCoupons = inAvailableCoupons;
}

//为了支持静态类的继承，重点看init
-(id) init
{
    if((self = [super init]))
    {
        //[self class] 表示用作接收者的那个对象，如果是继承类，指的是该继承类对象。
        if([[self class] availableCoupons] == 0)
        {
            //如果有效的Coupon为0,则消毁该类对象
            [self release];
            return nil;
        }
        
        //当前有效的Coupon减1
        [[self class] setAvailableCoupons: [[self class] availableCoupons]-1];
    }
    return self;
}

+(int) numberCouponsLeft
{
    return [self availableCoupons];
}

+(void) resetCoupon
{
    [self setAvailableCoupons:INITIAL_COUPON_ALLOCATION];
}

@end

NewCoupon.h

 

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


@interface NewCoupon : Coupon {
    
}

@end

NewCoupon.m

 

#import "NewCoupon.h"
#define INITIAL_New_COUPON_ALLOCATION 200

//在继承类中设置新的静态变量
static int availableCoupons = INITIAL_New_COUPON_ALLOCATION;

@interface NewCoupon() 
+(int)availableCoupons;
+(void) setAvailableCoupons: (int)inAvailableCoupons;
@end

@implementation NewCoupon

+(int)availableCoupons
{
    return availableCoupons;
}

+(void) setAvailableCoupons:(int)inAvailableCoupons
{
    availableCoupons = inAvailableCoupons;
}

+(void) resetCoupon
{
    [self setAvailableCoupons:INITIAL_New_COUPON_ALLOCATION];
}

@end

main.m

#import <Foundation/Foundation.h>
#import "Coupon.h"
#import "NewCoupon.h"

int main (int argc, const char * argv[])
{

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // insert code here...
    //NSLog(@"Hello, World!");
    
    [[Coupon alloc] init];
    NSLog(@"%d", [Coupon numberCouponsLeft]);
    [Coupon resetCoupon];
    NSLog(@"%d", [Coupon numberCouponsLeft]);
    
    [[NewCoupon alloc] init];
    NSLog(@"%d", [NewCoupon numberCouponsLeft]);
    [NewCoupon resetCoupon];
    NSLog(@"%d", [NewCoupon numberCouponsLeft]);

    [pool drain];
    return 0;
}

返回结果：

[Switching to process 3200 thread 0x0]
2011-05-03 10:15:08.505 demo04[3200:903] 99
2011-05-03 10:15:08.508 demo04[3200:903] 100
2011-05-03 10:15:08.508 demo04[3200:903] 199
2011-05-03 10:15:08.509 demo04[3200:903] 200