weak reference

基本解释

Weakly reachable: An object that is not strongly or softly reachable and can be accessed through a weak reference.
弱可及對象（weakly reachable）：不是強可及對象也不是軟可及對象，竝且能夠通過弱引用訪問的對象。
Weak references tell the garbage collector that it should collect an object if there are no references to it other than the weak reference.
弱引用告訴垃圾收集器如果某個對象除了弱引用之外沒有其他任何引用，則應該收集該對象。
Using a weak reference here keeps objects that are above the minimum around as long as possible, but makes them available for GC as required.
在此使用弱引用可以使除最低數量對象以外的對象盡可能久地保持活躍狀態，但是使它們可供 GC 根據需要使用。