第一个方法只是设置要访问的目标对象。

The first method just sets a target object to be accessed.

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这三个方法都接受转换环境对象，可以查询它们来访问源和目标对象。

Each of these methods accepts a transformation context object that can be queried to gain access to the source and target objects.

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确保目标对象及其子类可以访问工厂方法。

Assure that the factory method is accessible to the target object and its subclasses.

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