blocks|key|1733467|text|您创建了一个新类，它将classA和classB的实例作为成员变量。然后，通过传递get/set方法来实现属性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1733468|@interface+ClassAB
{
++++ClassA+*objectA;
++++ClassB+*objectB;
}
@property+(nonatomic,strong)+id+propertyA;
@property+(nonatomic,strong)+id+propertyB;
@end

@implementation+ClassAB
-+(id)propertyA+{+return+objectA.propertyA;+}
-+(void)setPropertyA:(id)value+{+objectA.propertyA+=+value;+}
-+(id)propertyB+{+return+objectB.propertyB;+}
-+(void)setPropertyB:(id)value+{+objectB.propertyB+=+value;+}
@end|code-block|syntax|javascript|1733469|这就是合成的含义。一些语言有特殊的语法来做到这一点(例如，在Ruby中，你可以在另一个类/模块中包含一组方法)，但Objective-C不允许这样做。|1733470|在Objective-C中你可以做的一件事是捕获发送到你的对象的消息，这些消息没有关联的方法，并将它们转发给另一个对象。如果您正在编写一个将伪装成另一个类的类，或者如果有许多不同的消息要转发，而您不想手动将它们全部写出来，则此技巧非常有用。|1733471|使用消息转发的缺点是您放弃了一些控制，并且很难预测消息何时将由您的类或转发类处理。例如，如果一个超类实现了一个方法，那么该方法将被执行，转发代码将不会被调用。|1733472|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

You make a new class, which has instances of classA and classB as member variables. Then you implement the properties by passing through the get/set methods.

<pre><code>@interface ClassAB
{
 ClassA *objectA;
 ClassB *objectB;
}
@property (nonatomic,strong) id propertyA;
@property (nonatomic,strong) id propertyB;
@end

@implementation ClassAB
- (id)propertyA { return objectA.propertyA; }
- (void)setPropertyA:(id)value { objectA.propertyA = value; }
- (id)propertyB { return objectB.propertyB; }
- (void)setPropertyB:(id)value { objectB.propertyB = value; }
@end
</code></pre>

And that's what composition is. Some languages have special syntax to do this (e.g., in Ruby you can include a set of methods from one class/module in another), but Objective-C doesn't allow this.

One thing you can do in Objective-C is catch messages sent to your object which do not have an associated method, and forward them to another object. This trick is useful if you are writing a class that will pose as another class, or if there are many different messages to forward and you don't want to write them all out manually.

The downside to using message forwarding is that you give up some control and it can be a harder to predict when a message will be handled by your class or the forwarding class. For example, if a superclass implements a method, that method will be executed and the forwarding code will not be called.

blocks|key|1733487|text|假设按照您所说的那样实现了ClassA和ClassB，则可以很好地工作，并且很容易扩展。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1733488|@interface+ClassAB+:+NSObject

@property+int+a;
@property+int+b;

@property+ClassA+*aObject;
@property+ClassB+*bObject;

@end

@implementation+ClassAB

@dynamic+a,+b;
@synthesize+aObject,+bObject;

-(id)+forwardingTargetForSelector:(SEL)aSelector
{
++++if+([aObject+respondsToSelector:aSelector])
++++++++return+aObject;
++++else+if+([bObject+respondsToSelector:aSelector])
++++++++return+bObject;

++++return+nil;++++
}

@end

int+main(int+argc,+const+char+*+argv[])
{
++++@autoreleasepool+{
++++++++ClassA+*a+=+[ClassA+new];
++++++++ClassB+*b+=+[ClassB+new];

++++++++ClassAB+*ab+=+[ClassAB+new];
++++++++ab.aObject+=+a;
++++++++ab.bObject+=+b;

++++++++ab.a+=+10;
++++++++ab.b+=+20;

++++++++NSLog(@"%25i,+%25i",+a.a,+b.b);+//+outputs+10,+20
++++}
++++return+0;
}|code-block|syntax|javascript|1733489|entityMap^0|D|6|K|6|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

Assuming <code>ClassA</code> and <code>ClassB</code> are implemented as you said, this works very well, and is easily extendable.

<pre><code>@interface ClassAB : NSObject

@property int a;
@property int b;

@property ClassA *aObject;
@property ClassB *bObject;

@end

@implementation ClassAB

@dynamic a, b;
@synthesize aObject, bObject;

-(id) forwardingTargetForSelector:(SEL)aSelector
{
 if ([aObject respondsToSelector:aSelector])
 return aObject;
 else if ([bObject respondsToSelector:aSelector])
 return bObject;

 return nil; 
}

@end

int main(int argc, const char * argv[])
{
 @autoreleasepool {
 ClassA *a = [ClassA new];
 ClassB *b = [ClassB new];

 ClassAB *ab = [ClassAB new];
 ab.aObject = a;
 ab.bObject = b;

 ab.a = 10;
 ab.b = 20;

 NSLog(@"%i, %i", a.a, b.b); // outputs 10, 20
 }
 return 0;
}
</code></pre>

Let's say I have property <code>A</code> on <code>classA</code> and property <code>B</code> on <code>classB</code> and I want <code>classAB</code> to have both properties <code>A</code> and <code>B</code>. I still don't understand how to make this all work with composition.

I realize this can be done with inheritance, but I want to learn how to do this with composition. I've looked at examples and I still don't get how it all works.

How does class composition work in iOS?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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假设我在classA上有A属性，在classB上有B属性，我希望classAB同时有A和B属性。我仍然不明白如何让这一切都与组合一起工作。我知道继承可以做到这一点，但我想学习如何使用组合来做到这一点。我看过一些例子，但我仍然不明白它是如何工作的。

问在iOS中，类组合是如何工作的？
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问在iOS中，类组合是如何工作的？
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