打字稿枚举默认值

Question

我刚刚开始使用 Aurelia 框架学习 Typescript。我已经使用 ViewEngineHooks http://davismj.me/blog/template-constants/在 Aurelia 的 Todo 应用程序中实现了 Matthew Davis 的博客 TypeScript Enums in Aurelia Templates 。

我想将默认枚举值设置为列表中的第二个值，并将默认值设置为列表中的第一个值。

另外，我想知道 {Todo, Priority} 还是仅 {Todo} 需要导入，如 todo-list.ts 所示

待办事项

// Pro Tip: By starting our enum at 1, we ensure that all values in the enum are truthy.
export enum Priority {
    High = 1,
    Medium,
    Low
}

export class Todo {
    @observable done;
    //*** Setting priority: Priority = 2 OR priority: Priority = Priority.Medium - Does not change the default from High / 1 ***
    //constructor(public list: TodoList, public description: string, public priority: Priority = 2, public editing: boolean = false) { 
    constructor(public list: TodoList, public description: string, public priority: Priority = Priority.Medium, public editing: boolean = false) { 
        this.list = list;
        this.description = description;
        //*** Setting this.priority = 2 OR this.priority = Priority.Medium - Does not change the default from High / 1 ;
        //this.priority = 2;
        this.priority = Priority.Medium;
        this.editing = false;
    }

待办事项列表.ts

//*** Is {Todo} OR {Todo, Priority} needed for importing? ***
//import {Todo} from './todo';
import {Todo, Priority} from './todo';
...
  add(description) {
    if (description) {
      //*** Setting (this, description, 2) OR (this, description, Priority.Medium) - Does not change the default from High / 1 ***
      //this.todos.push(new Todo(this, description, 2));
      this.todos.push(new Todo(this, description, Priority.Medium));
      this.invalidateView();
    }
  }

待办事项.html

<select id="priority" value.bind="type">
  <option value.bind="Priority[type]" repeat.for="type of Prioritys">${type}</option>
</select>

Answer 1

据我所知，在声明变量时，您无法设置枚举的默认值，就像数字或布尔值没有默认值一样。但是，您可以为函数参数定义默认值，就像在构造函数 ( priority: Priority = Priority.Medium) 中所做的那样，这样在调用构造函数时就不必提供该参数。

两个附加说明：不应该this.priority = priority; this.editing = editing;代替 吗this.priority = Priority.Medium; this.editing = false;？其次，如果你把public类属性放在参数前面，则会自动添加并分配类属性，因此您不需要构造函数的额外行。然而，对于更复杂的类，我可能会手动创建和分配属性。

关于您的第二个问题：您需要Priority在引用该枚举后立即导入，例如在编写Priority.Medium. Priority例如，当您比较两个不同类型的属性而不使用枚举名称（例如）时，您不必导入它this.todos[0].priority === this.todos[1].priority。