委托是一个协议(介面)，定义了许多物件可以实作的方法，为了接收来自其他物件的特定讯息。
A delegate is a protocol (interface) that defines methods that an object implements in order to receive specific messages from other objects.
每个输出介面都可由物件上的用户端(即接收)来实作。
Each outgoing interface can be implemented by a client on an object (that is, a sink).
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