6.2 Example: storing text for an editor 示例：为编辑器存储文本

Let’s consider an example from a software design class in which students were asked to build simple GUI text editors. The editors had to display a file and allow users to point, click, and type to edit the file. The editors had to support multiple simultaneous views of the same file in different windows; they also had to support multi-level undo and redo for modifications to the file.

让我们考虑一个软件设计课程的示例，其中要求学生构建简单的 GUI 文本编辑器。编辑器必须显示一个文件，并允许用户指向，单击并键入以编辑该文件。编辑者必须在不同的窗口中支持同一文件的多个同时视图。他们还必须支持多级撤消和重做以修改文件。

Each of the student projects included a class that managed the underlying text of the file. The text classes typically provided methods for loading a file into memory, reading and modifying the text of the file, and writing the modified text back to a file.

每个学生项目都包括一个管理文件的基础文本的类。文本类通常提供以下方法：将文件加载到内存，读取和修改文件的文本以及将修改后的文本写回到文件。

Many of the student teams implemented special-purpose APIs for the text class. They knew that the class was going to be used in an interactive editor, so they thought about the features that the editor had to provide and tailored the API of the text class to those specific features. For example, if a user of the editor typed the backspace key, the editor deleted the character immediately to the left of the cursor; if the user typed the delete key, the editor deleted the character immediately to the right of the cursor. Knowing this, some of the teams created one method in the text class to support each of these specific features:

许多学生团队为文本课实现了专用的 API。他们知道该类将在交互式编辑器中使用，因此他们考虑了编辑器必须提供的功能，并针对这些特定功能定制了文本类的 API。例如，如果编辑者的用户键入了退格键，则编辑者会立即删除光标左侧的字符；如果用户键入删除键，则编辑器立即删除光标右侧的字符。知道这一点后，一些团队在文本类中创建了一个方法来支持以下每个特定功能：

  1. void backspace(Cursor cursor);
  3. void delete(Cursor cursor);

Each of these methods takes the cursor position as its argument; a special type Cursor represents this position. The editor also had to support a selection that could be copied or deleted. The students handled this by defining a Selection class and passing an object of this class to the text class during deletions:

这些方法中的每一个都以光标位置作为参数。特殊类型的光标表示此位置。编辑器还必须支持可以复制或删除的选择。学生通过定义选择类并在删除过程中将该类的对象传递给文本类来解决此问题：

  1. void deleteSelection(Selection selection);

The students probably thought that it would be easier to implement the user interface if the methods of the text class corresponded to the features visible to users. In reality, however, this specialization provided little benefit for the user interface code, and it created a high cognitive load for developers working on either the user interface or the text class. The text class ended up with a large number of shallow methods, each of which was only suitable for one user interface operation. Many of the methods, such as delete, were only invoked in a single place. As a result, a developer working on the user interface had to learn about a large number of methods for the text class.

学生们可能认为，如果文本类的方法与用户可见的功能相对应，则将更易于实现用户界面。但是，实际上，这种专业化对用户界面代码几乎没有好处，并且为使用用户界面或文本类的开发人员带来了很高的认知负担。文本类以大量浅层方法结束，每种浅层方法仅适用于一个用户界面操作。许多方法（例如 delete）仅在单个位置调用。结果，在用户界面上工作的开发人员必须学习大量有关文本类的方法。

This approach created information leakage between the user interface and the text class. Abstractions related to the user interface, such as the selection or the backspace key, were reflected in the text class; this increased the cognitive load for developers working on the text class. Each new user interface operation required a new method to be defined in the text class, so a developer working on the user interface was likely to end up working on the text class as well. One of the goals in class design is to allow each class to be developed independently, but the specialized approach tied the user interface and text classes together.

这种方法在用户界面和文本类之间造成了信息泄漏。与用户界面有关的抽象（例如选择或退格键）反映在文本类中；这增加了从事文本课的开发人员的认知负担。每个新的用户界面操作都需要在文本类中定义一个新方法，因此使用该用户界面的开发人员也可能最终也要使用该文本类。类设计的目标之一是允许每个类独立开发，但是专用方法将用户界面和文本类联系在一起。