自定义类工厂方法

+ (id)person;
+ (id)person
{
    return  [[Person alloc]init];
}

+ (id)personWithAge:(int)age;
+ (id)personWithAge:(int)age
{
    Person *p = [[self alloc] init];
    [p setAge:age];
    return p;
}

其实苹果在书写工厂方法时也是按照这样的规划书写
    [NSArray array];
    [NSArray arrayWithArray:<#(NSArray *)#>];
    [NSDictionary dictionary];
    [NSDictionary dictionaryWithObject:<#(id)#> forKey:<#(id<NSCopying>)#>];
    [NSSet set];
    [NSSet setWithObject:<#(id)#>];

interface Person : NSObject
+ (id)person;
@end

@implementation Person
+ (id)person
{
    return  [[Person alloc]init];
}
@end

@interface Student : Person
@property NSString *name;
@end

@implementation Student

@end

int main(int argc, const char * argv[])
{
    Student *stu = [Student person];// [[Person alloc] init]
    [stu setName:@"lnj"]; // 报错, 因为Person中没有setName
}

@interface Person : NSObject
+ (id)person;
@end

@implementation Person
+ (id)person
{
//   return  [[Person alloc]init];
//     谁调用这个方法,self就代表谁
//    注意:以后写类方法创建初始化对象,写self不要直接写类名
    return  [[self alloc]init];
}
@end

@interface Student : Person
@property NSString *name;
@end

@implementation Student
@end

int main(int argc, const char * argv[])
{
    Student *stu = [Student person];// [[Student alloc] init]
    [stu setName:@"lnj"];
}