object-c学习笔记：类别的使用（category)

如何使用？

这里只是记录下如何使用的语法规则，至于类别的优缺点，使用场景，等等细节，sorry。。不知道 yet :)

基本上你只需要声明另外一个类，并且指定你需要扩展的类，简单的，看例子：

@interface Engine : NSObject
{
}
@end

@implementation Engine

- (NSString*) description
{
    return (@"engine");
}

@end

@interface Tier : NSObject
{
}
@end


@implementation Tier

- (NSString*) description
{
    return (@"tier");
}

@end

@interface Car : NSObject
{
    int tier_num;
    float engine_power;
    NSString* name;
    Engine* engine;
    Tier* tier;
}

@property (nonatomic) int tier_num;
@property (nonatomic) float engine_power;
@property (nonatomic, copy) NSString* name;
@property (nonatomic, retain) Engine* engine;
@property (nonatomic, retain) Tier* tier;

@end

@implementation Car    

@synthesize tier_num;
@synthesize engine_power;
@synthesize name;
@synthesize engine;
@synthesize tier;

- (id) init
{
    if (self = [super init]) {
        tier_num = 4;
        engine_power = 100;
        name = @"BMW";
    }
    return self;
}
@end

@interface Car (CarCanDescription)

- (void) print;

@end

@implementation Car (CarCanDescription)

- (void) print;
{
    NSLog(@"this is a car");
    NSLog(@"%@", engine);
    NSLog(@"%@", tier);
    NSLog(@"engine_power is %f", engine_power);
} 

@end


int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    
    Car* car = [[Car alloc] init];
    car.name = @"Mazd";
        
    Engine* newEngine = [[Engine alloc] init];
    car.engine = newEngine;
    Tier* tier = [[Tier alloc] init];
    car.tier = tier;
    [car print];
    
    // insert code here...
    
    [pool drain];
    return 0;
}

 

需要注意的是，当你在定义一个category时，不能指定新的实例变量，即使时空的括号，也会导致编译不过，道理也很简单，category只是改变类中函数列表，是不会去改变实例变量的，也就无法增加新的变量。