/**     运行时 ：runtime    可以访问隐藏的一些属性 **在这个代码中 修改 文本框占位符 的颜色   */+ (void)initialize{    [self getIvars];    [self getProperties];}//获取所有属性+ (void)getProperties{    unsigned int count = 0;        objc_property_t *properties = class_copyPropertyList([UITextField class], &count);    for (int i = 0; i < count; i++) {        objc_property_t property = properties[i];        NSLog(@"%s  <---->  %s",property_getName(property), property_getAttributes(property));    }    }//获取所有成员变量列表+ (void)getIvars{    unsigned int count = 0;    //拷贝出所有的成员变量列表  这是一个数组 可以访问隐藏的属性   Ivar *ivars = class_copyIvarList([UITextField class], &count);    for (int i = 0; i < count; i ++) {//        Ivar ivar = *(ivars + i);        Ivar ivar = ivars[i]; //等同上一句代码        NSLog(@"%s",ivar_getName(ivar));    }    //释放内存 因为带有 copy    free(ivars);}//视图加载出来时- (void)awakeFromNib{//    UILabel *placeHolderLabel = [self valueForKey:@"_placeholderLabel"];//    placeHolderLabel.textColor = [UIColor redColor];    //通过 kvc 赋值//    [self setValue:[UIColor orangeColor] forKeyPath:@"_placeholderLabel.textColor"];//设置光标颜色和文字颜色一致        self.tintColor = self.textColor;    [self resignFirstResponder];    }//文本框成为第一响应者 和放弃 时 设置文本框的 占位符的颜色 - (BOOL)becomeFirstResponder{    [self setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];    return [super becomeFirstResponder];}- (BOOL)resignFirstResponder{    [self setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];    return [super resignFirstResponder];}//外界可以通过访问这个属性 进行赋值- (void)setPlaceHolderColor:(UIColor *)placeHolderColor{    _placeHolderColor = placeHolderColor;    [self setValue:placeHolderColor forKeyPath:@"_placeholderLabel.textColor"];}