Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
将对象组合成树形结构以表示“部分-整体”的层次结构，使得用户对单个对象和组合对象的使用具有一致性。

The classes and objects participating in this pattern are:

• declares the interface for objects in the composition.
• implements default behavior for the interface common to all classes, as appropriate.
• declares an interface for accessing and managing its child components.
• (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.

• represents leaf objects in the composition. A leaf has no children.
• defines behavior for primitive objects in the composition.

• defines behavior for components having children.
• stores child components.
• implements child-related operations in the Component interface.

• manipulates objects in the composition through the Component interface.