object-c學習筆記：類別的使用（category)

如何使用？

這裡只是記錄下如何使用的文法規則，至於類別的優缺點，使用情境，等等細節，sorry。。不知道 yet

基本上你只需要聲明另外一個類，並且指定你需要擴充的類，簡單的，看例子：

代碼

@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，當你在定義一個category時，不能指定新的執行個體變數，即使時空的括弧，也會導致編譯不過，道理也很簡單，category只是改變類中函數列表，是不會去改變執行個體變數的，也就無法增加新的變數。