For a given event, you use conditions to determine the new valid state.
对于一个给定事件，使用各种条件来决定新的有效状态。
Always construct objects in a valid state.
总是构建状态有效的对象。
The Save button is disabled until the form is in a valid state.
表单验证通过之前，保存按钮是禁用的。
The Save button is disabled until the form is in a valid state.
表单验证通过之前，保存按钮是禁用的。
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