正确使用NS_DESIGNATED_INITIALIZER

为什么会提到NS_DESIGNATED_INITIALIZER

最近在清理项目警告，遇到了“Method override for the designated initializer”的警告，全称为：

• Method override for the designated initializer of the superclass '-init' not found 或者
• Method override for the designated initializer of the superclass '-initWithNibName:bundle:' not found。

经排查是因为头文件中出现NS_DESIGNATED_INITIALIZER 声明的初始化方法，如下：

// .h
- (instancetype)initWithPurchaseService:(HTLPrepayPurchaseService *)service NS_DESIGNATED_INITIALIZER;

如果用了NS_DESIGNATED_INITIALIZER，那么警告中出现init还是initWithNibName或是其它，取决于你的继承的父类类型：

UIViewController

• Method override for the designated initializer of the superclass '-initWithNibName:bundle:' not found
• Method override for the designated initializer of the superclass '-initWithCoder:' not found

UIView

• Method override for the designated initializer of the superclass '-initWithFrame:' not found
• Method override for the designated initializer of the superclass '-initWithCoder:' not found

NSObject

• Method override for the designated initializer of the superclass '-init' not found

正确使用NS_DESIGNATED_INITIALIZER

为什么要用NS_DESIGNATED_INITIALIZER

Objective-C 中主要通过NS_DESIGNATED_INITIALIZER宏来实现指定构造器的。这里之所以要用这个宏，往往是想告诉调用者要用这个方法去初始化（构造）类对象。

怎样避免使用NS_DESIGNATED_INITIALIZER产生的警告

如果子类指定了新的初始化器，那么在这个初始化器内部必须调用父类的Designated Initializer。并且需要重写父类的Designated Initializer，将其指向子类新的初始化器。

如下：

// .h
- (instancetype)initWithName:(NSString *)name NS_DESIGNATED_INITIALIZER;

// .m
- (instancetype)init
{
    return  [self initWithName:@ "" ];
}

- (instancetype)initWithName:(NSString *)name
{
    self = [super init];
    if  (self) {
        // do something
    }
    return  self;
}

更好的做法

如果定义NS_DESIGNATED_INITIALIZER，大多是不想让调用者调用父类的初始化函数，只希望通过该类指定的初始化进行初始化，这时候就可以用NS_UNAVAILABLE宏。

如下：

// .h
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithName:(NSString *)name NS_DESIGNATED_INITIALIZER;

如果调用者使用init 初始化，编译器就会给出一个编译错误。使用NS_UNAVAILABLE后，就不需要在.m中重写父类初始化函数了。如果要允许调用者使用init就需要在.m中重写父类的初始化函数，如上提到的，否则就会报警告。

避免使用new

如果使用new来创建对象的话，即使init被声明为NS_UNAVAILABLE，也不会收到编译器的警告和错误提示了。

参考链接

• http://www.starfelix.com/blog/2014/04/13/zheng-que-bian-xie-designated-initializerde-ji-ge-yuan-ze/
• http://stackoverflow.com/questions/26185239/ios-designated-initializers-using-ns-designated-initializer
• https://craftbeercraftcode.com/2014/12/28/better-brewing-with-ns_unavailable/