The reference must exist in the test and precede the conditionalblock.
这个引用必须存在于测试中并且在条件块前。
2
In most cases, a conditionalblock issues user input actions depending on the value of a reference.
大多数情况下，一个条件块依靠一个引用的值发出用户输入动作。
3
By arranging for objects of different types to implement common methods, we move the dispatch decision into the objects and out of an explicit conditionalblock.