Objective-C中属性的简单使用（存取方法）

我们知道写oc代码时可能会创建很多的类和对象，我们要修改对象的值时就使用oc的setter方法（设置属性值）和getter方法（读取属性值），代码量会很大，这就增加了写代码的时间。先来看看使用存取方法的一小段程序：

#import <Foundation/Foundation.h>@interface Tire : NSObject@end@implementation Tire-(NSString *)description{    return (@"I am a tire.I last a while");}@end@interface Engine : NSObject@end@implementation Engine-(NSString *)description{    return (@"I am an engine.Vrooom!");}@end@interface Car : NSObject{    Engine *engine;    Tire *tires[4];}//Engine类的getter方法-(Engine *)engine;//Engine类的setter方法-(void)setEngine:(Engine *)newEngine;//Tire类的getter方法-(Tire *)tireAtIndex:(int)index;//Tire类的setter方法-(void)setTire:(Tire *)tire atIndex:(int)index;-(void)print;@end@implementation Car-(Engine *)engine{    return (engine);}-(void)setEngine:(Engine *)newEngine{    engine = newEngine;}-(Tire *)tireAtIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in tireAtIndex:",index);        exit(1);    }    return (tires[index]);}-(void)setTire:(Tire *)tire atIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in setTire:atIndex:",index);        exit(1);    }    tires[index] = tire;}-(void)print{    NSLog(@"%@",engine);    NSLog(@"%@",tires[0]);    NSLog(@"%@",tires[1]);    NSLog(@"%@",tires[2]);    NSLog(@"%@",tires[3]);}@endint main(int argc, const char * argv[]) {    Car *car = [Car new];    Engine *engine = [Engine new];    [car setEngine:engine];    for(int i = 0;i < 4;i++)    {        Tire *tire = [Tire new];        [car setTire:tire atIndex:i];    }    [car print];    return 0;}

还好我们还可以简化这些代码，在接口声明中我们使用@property关键字，在实现中我们使用@synthesize关键字。代码如下：

#import <Foundation/Foundation.h>@interface Tire : NSObject@end@implementation Tire-(NSString *)description{    return (@"I am a tire.I last a while");}@end@interface Engine : NSObject@end@implementation Engine-(NSString *)description{    return (@"I am an engine.Vrooom!");}@end@interface Car : NSObject{    Engine *engine;    Tire *tires[4];}@property Engine *engine;//Tire类的getter方法-(Tire *)tireAtIndex:(int)index;//Tire类的setter方法-(void)setTire:(Tire *)tire atIndex:(int)index;-(void)print;@end@implementation Car@synthesize engine;-(Tire *)tireAtIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in tireAtIndex:",index);        exit(1);    }    return (tires[index]);}-(void)setTire:(Tire *)tire atIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in setTire:atIndex:",index);        exit(1);    }    tires[index] = tire;}-(void)print{    NSLog(@"%@",engine);    NSLog(@"%@",tires[0]);    NSLog(@"%@",tires[1]);    NSLog(@"%@",tires[2]);    NSLog(@"%@",tires[3]);}@endint main(int argc, const char * argv[]) {    Car *car = [Car new];    //通过属性来设置Car对象里的成员，而不需要发送setter消息来设置（即不需要显示调用setter方法）,而且这里我们使用了点运算符    car.engine = [Engine new];    for(int i = 0;i < 4;i++)    {        Tire *tire = [Tire new];        [car setTire:tire atIndex:i];    }    [car print];    return 0;}

看上面的代码是不是简化了？@property是一种新的编译器功能，它意味着声明了一个新对象的属性。@property预编译指令的作用是自动声明属性的setter和getter方法，@synthesize也是一种新的编译器功能，它表示“创建了该属性的访问代码”。所有的属性都是基于变量的，所以你在合成setter和setter方法的时候，编译器会自动创建与属性名称相同的实例变量。假设你有一个子类，并且想要从子类直接通过属性来访问变量。在这种情况下，变量就必须放在头文件中。如果变量只属于当前类，则可以把它们放在.m文件里（并且要删除原interface代码中的声明语句）。还有，如果我们没有指定实例变量，编译器会自动帮我们创建。如下：

#import <Foundation/Foundation.h>@interface Tire : NSObject@end@implementation Tire-(NSString *)description{    return (@"I am a tire.I last a while");}@end@interface Engine : NSObject@end@implementation Engine-(NSString *)description{    return (@"I am an engine.Vrooom!");}@end@interface Car : NSObject{    //Engine *engine;       //删除了声明的实例变量，但不会有任何影响    Tire *tires[4];}@property Engine *engine;//Tire类的getter方法-(Tire *)tireAtIndex:(int)index;//Tire类的setter方法-(void)setTire:(Tire *)tire atIndex:(int)index;-(void)print;@end@implementation Car@synthesize engine;-(Tire *)tireAtIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in tireAtIndex:",index);        exit(1);    }    return (tires[index]);}-(void)setTire:(Tire *)tire atIndex:(int)index{    if(index < 0 || index > 3)    {        NSLog(@"bad index (%d) in setTire:atIndex:",index);        exit(1);    }    tires[index] = tire;}-(void)print{    NSLog(@"%@",engine);    NSLog(@"%@",tires[0]);    NSLog(@"%@",tires[1]);    NSLog(@"%@",tires[2]);    NSLog(@"%@",tires[3]);}@endint main(int argc, const char * argv[]) {    Car *car = [Car new];    //通过属性来设置Car对象里的成员，而不需要发送setter消息来设置（即不需要显示调用setter方法）,而且这里我们使用了点运算符    car.engine = [Engine new];    for(int i = 0;i < 4;i++)    {        Tire *tire = [Tire new];        [car setTire:tire atIndex:i];    }    [car print];    return 0;}

以上代码中还巧妙的使用了点表达式，点表达式可以用来设置和访问对象的属性。上述几个程序运行的结果都一样，读者可以试着编译运行一下，博主编译后运行结果截图如下：

最后，谢谢各位读者的浏览。