stepper 概览
CDK 步进器（stepper）提供了一个基础，可以在此基础上构建具体的步进器变体。步进器是一种类似向导的工作流，可以把内容划分为一些逻辑步骤
CDK stepper provides a foundation upon which more concrete stepper varieties can be built. A stepper is a wizard-like workflow that divides content into logical steps
步进器的 CDK 基础版本主要用来管理哪一步处于活动状态。这包括处理键盘交互，并暴露一个用来在工作流程中前进或后退的 API。
The base CDK version of the stepper primarily manages which step is active. This includes handling keyboard interactions and exposing an API for advancing or rewinding through the workflow.
标记为 linear 的步进器要求用户在继续下一步之前完成前面的步骤。对于每一个步骤来说，其 stepControl 属性都可以设置为顶级的 AbstractControl，用于检查该步骤的有效性。
A stepper marked as linear requires the user to complete previous steps before proceeding.
For each step, the stepControl attribute can be set to the top level AbstractControl that
is used to check the validity of the step.
有两种可能的方式。一种是使用单一表单进行步进，另一种是使用不同的表单进行步进。
There are two possible approaches. One is using a single form for stepper, and the other is using a different form for each step.
或者，如果你不想使用 Angular 表单，也可以把 completed 属性传入每个步骤，这些步骤会阻止用户继续，直到这个属性变为 true。注意，如果同时设置了 completed 和 stepControl，则 stepControl 优先。
Alternatively, if you don't want to use the Angular forms, you can pass in the completed property
to each of the steps which won't allow the user to continue until it becomes true. Note that if
both completed and stepControl are set, the stepControl will take precedence.
在步进器中使用单一表单时，所有中间步骤的"下一个"/"上一个"按钮都必须设置为 type="button"，以防止在完成所有步骤之前提交表单。
When using a single form for the stepper, any intermediate next/previous buttons within the steps
must be set to type="button" in order to prevent submission of the form before all steps are
complete.
当每个步骤各自使用一个表单时，只要提交了一个表单，工作流就会向前推进。
When using a form for each step, the workflow is advanced whenever one of the forms is submitted.
如果线性步进器中的不要求某个步骤必须完成，那么可以在 linear 步进器中的这个 CdkStep 上设置 optional 属性。
If completion of a step in linear stepper is not required, then the optional attribute can be set
on CdkStep in a linear stepper.
默认情况下，步骤是可编辑的，这意味着用户可以返回之前已完成的步骤并重新编辑他们的响应。可以在 CdkStep 上设置 editable="true" 来修改默认行为。
By default, steps are editable, which means users can return to previously completed steps and
edit their responses. editable="false" can be set on CdkStep to change the default.
默认情况下，如果步骤有效（在线性步进器的情况下）并且用户已经与该步骤交互过，则该步骤的 completed 属性会返回 true。但是，用户也可以按需设置 completed 属性来覆盖默认的 completed 行为。
By default, the completed attribute of a step returns true if the step is valid (in case of
linear stepper) and the user has interacted with the step. The user, however, can also override
this default completed behavior by setting the completed attribute as needed.
有两个按钮指令可以支持不同步骤之间的导航： CdkStepperNext 和 CdkStepperPrevious。当放置在某个步骤中时，它们会自动添加点击处理器来分别推进或回退此工作流。
There are two button directives to support navigation between different steps:
CdkStepperNext and CdkStepperPrevious. When placed inside of a step, these will automatically
add click handlers to advance or rewind the workflow, respectively.
如果要将步进器重置为初始状态，可以使用 reset 方法。注意，重置它会在底部操作表控件上调用 reset 来清除这个值。
If you want to reset a stepper to its initial state, you can use the reset method. Note that
resetting it will call reset on the underlying form control which clears the value.
LEFT_ARROW：焦点移到上一步的标题LEFT_ARROW: Focuses the previous step header
RIGHT_ARROW：焦点移到下一步的标题RIGHT_ARROW: Focuses the next step header
ENTER、SPACE：选取具有焦点的当前步骤ENTER, SPACE: Selects the step that the focus is currently on
TAB：焦点移到下一个 tabbable 元素TAB: Focuses the next tabbable element
TAB+SHIFT：焦点移到上一个 tabbable 元素SHIFT+TAB: Focuses the previous tabbable element
出于无障碍性的目的，CDK 的 stepper 被视为标签视图，所以默认情况下它被赋予 role="tablist"。在步骤标题中可以单击，以选取 role="tab" 的步骤，并展开每个带有 role="tabpanel" 的步骤内容。步骤标题上的 aria-selected 属性和步骤内容上的 aria-expanded 属性是根据所选步骤的变化自动设置的。
The CDK stepper is treated as a tabbed view for accessibility purposes, so it is given
role="tablist" by default. The header of step that can be clicked to select the step
is given role="tab", and the content that can be expanded upon selection is given
role="tabpanel". aria-selected attribute of step header and aria-expanded attribute of
step content is automatically set based on step selection change.
步进器及其中的每个步骤都应该通过 aria-label 或者 aria-labelledby 给出一个有意义的标签。
The stepper and each step should be given a meaningful label via aria-label or aria-labelledby.