如何将超类属性复制到子类？

Question

假设我有两个类，如下所示：

Car
{
   NSInteger wheels;
   NSInteger bumpers;
}

+ (Car *)carWithData:(NSDictionary *)carData;


Lexus : Car
{
   GPS *navigation;
}

+ (Lexus *)carWithData:(NSDictionary *)carData;

carWithData: 是一个简单的辅助方法，它创建一个 Car 实例，并用 carData 中的变量填充。雷克萨斯的版本还会设置导航数据。

如果不复制 Car 的代码，雷克萨斯的 carWithData 会是什么样子？

Answer 1

这是通过在 init 方法中调用 super 的 init... 实现来完成的：

//Car.m:
- (id)initWithData:(NSDictionary *)carData {
    self = [super init];
    if (self) {
        //setup generic car properties:
        self.wheels = [carData objectForKey:@"wheels"]; //example
        self.bumpers = [carData objectForKey:@"bumpers"]; //example
    }
    return self;
}

+ (id)carWithData:(NSDictionary *)carData {
    return [[[self alloc] initWithData:carData] autorelease];
}


//Lexus.m:
- (id)initWithData:(NSDictionary *)carData {
    //this call to super is where the car's generic properties get initialized:
    self = [super initWithWithData:carData];
    if (self) {
        //setup lexus car properties:
        self.navigation = [carData objectForKey:@"navigation"]; //example
    }
    return self;
}

//there is no need to override super's [carWithData:] method as it's only a wrapper anyway.

另请注意，initWith... 和 carWith ... 方法返回 id，而不是 Car 或 Lexus。
您的代码设置方式最终会出现转换问题，其中 [Lexus carWithData:dataDict] 确实返回类 Lexus 的对象，但编译器不知道关于它，因为它需要一辆汽车。

Answer 2

您不会使用不同的签名来定义方法，例如：

+ (Car *)carWithData:(NSDictionary *)carData;
+ (Lexus *)carWithData:(NSDictionary *)carData;

您应该使用

+ (id)carWithData:(NSDictionary *)carData;

子类的实现将如下所示

- (id)initWithData:(NSDictionary *)carData;
{
    self = [super initWithData:carData];
    if (self) {
         _navigation = [carData valueForKey:@"navigation"];
    }
    return self;
}

+ (id)carWithData:(NSDictionary *)carData;
{
    return [[[self alloc] initWithCarData:carData] autorelease];
}

Answer 3

这是我的解决方案：

// interface
-(id) initWithCarData:(NSDictionary *) carData;
+(Car *) carWithCarData:(NSDictionary *) carData;

// car implementation
-(id) initWithCarData:(NSDictionary *) carData
{
    if (self = [super init])
    {
          // initialize car data
    }

    return self;
}

+(Car *) carWithCarData:(NSDictionary *) carData
{
    // note that 'self' here is the current class, 
    // there is no need to overwrite this method in the subclass
    return [[self alloc] initWithCarData:carData];
}

// lexus implementation
-(id) initWithCarData:(NSDictionary *) carData
{
     // initialize the variables that the superclass recognizes
     if (self = [super initWithCarData:carData])
     {
         // initialize the lexus data
     }

     return self;
}

因此，当您调用 [Lexus carWithCarData:myData] 时，它最终会调用 Lexus 的 init 方法，而不是汽车的。

Answer 4

还有另一种更通用的方法，即使用 NSCoding 协议和 NSKeyedArchiver。

如果您想要复制到子类中的对象实现了 NSCoding 协议（许多基于 NS... 的类都是这种情况），则可以使用以下完全合法且安全的技巧：

// Assumptions:
// copyFrom is an object of ClassA
// We have a ClassB that is a subclass of ClassA
NSMutableData *data = [NSMutableData data];
NSKeyedArchiver *arch = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[copyFrom encodeWithCoder:arch]; // this archives its properties
[arch finishEncoding];
NSKeyedUnarchiver *ua = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
ClassB *ourCopy = [[ClassB alloc] initWithCoder:ua]; // this restores the properties into our new object