Discussion 解説
This property returns a proxy for the receiving object if the receiver adopts the NSDiscardable protocol and still has content that has not been discarded.
The proxy calls begin on the receiver to keep the content available as long as the proxy lives, and calls end when the proxy is deallocated.
The wrapper object is otherwise a subclass of NSProxy and forwards messages to the original receiver object as an NSProxy does.
This method can be used to hide an NSDiscardable object's content volatility by creating an object that responds to the same messages but holds the contents of the original receiver available as long as the created proxy lives. Thus hidden, the NSDiscardable object (by way of the proxy) can be given out to unsuspecting recipients of the object who would otherwise not know they might have to call begin and end around particular usages (specific to each NSDiscardable object) of the NSDiscardable object.