docs(API): 翻译 ng-for-of

aio/content/translations/cn/api-plan.md

@@ -1,5 +1,7 @@
 ## 优先级
 
+按照下列优先级进行翻译：
+
 URL|浏览量|占比
 ---|---|----
 common/http/HttpClient | 7,263 | 2.26

packages/common/src/directives/ng_for_of.ts

@@ -26,21 +26,47 @@ export class NgForOfContext<T> {
  * The `NgForOf` directive instantiates a template once per item from an iterable. The context
  * for each instantiated template inherits from the outer context with the given loop variable
  * set to the current item from the iterable.
+ * <p>`NgForOf` 指令会为可迭代对象中的每一个条目实例化一个模板。实例化时的上下文环境来自其外部环境，它以当前正在迭代的条目作为循环变量。</p>
  *
  * ### Local Variables
  *
+ * ### 局部变量
+ *
  * `NgForOf` provides several exported values that can be aliased to local variables:
  *
+ * `NgForOf` 导出了一系列值，可以指定别名后作为局部变量使用：
+ *
  * - `$implicit: T`: The value of the individual items in the iterable (`ngForOf`).
+ *
+ *   `$implicit: T`：迭代目标（绑定到`ngForOf`）中每个条目的值。
+ *
  * - `ngForOf: NgIterable<T>`: The value of the iterable expression. Useful when the expression is
  * more complex then a property access, for example when using the async pipe (`userStreams |
  * async`).
+ *
+ *   `ngForOf: NgIterable<T>`：迭代表达式的值。当表达式不局限于访问某个属性时，这会非常有用，比如在使用 `async` 管道时（`userStreams |
+ * async`）。
+ *
  * - `index: number`: The index of the current item in the iterable.
+ *
+ *   `index: number`：可迭代对象中当前条目的索引。
+ *
  * - `first: boolean`: True when the item is the first item in the iterable.
+ *
+ *   `first: boolean`：如果当前条目是可迭代对象中的第一个条目则为 `true`。
+ *
  * - `last: boolean`: True when the item is the last item in the iterable.
+ *
+ *   `last: boolean`：如果当前条目是可迭代对象中的最后一个条目则为 `true`。
+ *
  * - `even: boolean`: True when the item has an even index in the iterable.
+ *
+ *   `even: boolean`：如果当前条目在可迭代对象中的索引号为偶数则为 `true`。
+ *
  * - `odd: boolean`: True when the item has an odd index in the iterable.
  *
+ *   `odd: boolean`：如果当前条目在可迭代对象中的索引号为奇数则为 `true`。
+ *
  * ```
  * <li *ngFor="let user of userObservable | async as users; index as i; first as isFirst">
  *    {{i}}/{{users.length}}. {{user}} <span *ngIf="isFirst">default</span>
@@ -49,19 +75,38 @@ export class NgForOfContext<T> {
  *
  * ### Change Propagation
  *
+ * ### 变更的传导机制
+ *
  * When the contents of the iterator changes, `NgForOf` makes the corresponding changes to the DOM:
  *
+ * 当迭代器的内容变化时，`NgForOf` 会对 DOM 做出相应的修改：
+ *
  * * When an item is added, a new instance of the template is added to the DOM.
+ *
+ *   当新增条目时，就会往 DOM 中添加一个模板实例。
+ *
  * * When an item is removed, its template instance is removed from the DOM.
+ *
+ *   当移除条目时，其对应的模板实例也会被从 DOM 中移除。
+ *
  * * When items are reordered, their respective templates are reordered in the DOM.
+ *
+ *   当条目集被重新排序时，他们对应的模板实例也会在 DOM 中重新排序。
+ *
  * * Otherwise, the DOM element for that item will remain the same.
  *
+ *   否则，条目对应的 DOM 元素就会保持不变。
+ *
  * Angular uses object identity to track insertions and deletions within the iterator and reproduce
  * those changes in the DOM. This has important implications for animations and any stateful
  * controls (such as `<input>` elements which accept user input) that are present. Inserted rows can
  * be animated in, deleted rows can be animated out, and unchanged rows retain any unsaved state
  * such as user input.
  *
+ * Angular 使用对象标识符（对象引用）来跟踪迭代器中的添加和删除操作，并把它们同步到 DOM 中。
+ * 这对于动画和有状态的控件（如用来接收用户输入的 `<input>` 元素）具有重要意义。添加的行可以带着动画效果进来，删除的行也可以带着动画效果离开。
+ * 而未变化的行则会保留那些尚未保存的状态，比如用户的输入。
+ *
  * It is possible for the identities of elements in the iterator to change while the data does not.
  * This can happen, for example, if the iterator produced from an RPC to the server, and that
  * RPC is re-run. Even if the data hasn't changed, the second response will produce objects with
@@ -69,16 +114,28 @@ export class NgForOfContext<T> {
  * elements were deleted and all new elements inserted). This is an expensive operation and should
  * be avoided if possible.
  *
+ * 即使数据没有变化，迭代器中的元素标识符也可能会发生变化。比如，如果迭代器处理的目标是通过 RPC 从服务器取来的，
+ * 而 RPC 又重新执行了一次。那么即使数据没有变化，第二次的响应体还是会生成一些具有不同标识符的对象。Angular 将会清除整个 DOM，
+ * 并重建它（就仿佛把所有老的元素都删除，并插入所有新元素）。这是很昂贵的操作，应该尽力避免。
+ *
  * To customize the default tracking algorithm, `NgForOf` supports `trackBy` option.
  * `trackBy` takes a function which has two arguments: `index` and `item`.
  * If `trackBy` is given, Angular tracks changes by the return value of the function.
  *
+ * 要想自定义默认的跟踪算法，`NgForOf` 支持 `trackBy` 选项。
+ * `trackBy` 接受一个带两个参数（`index` 和 `item`）的函数。
+ * 如果给出了 `trackBy`，Angular 就会使用该函数的返回值来跟踪变化。
+ *
  * ### Syntax
  *
+ * ### 语法
+ *
  * - `<li *ngFor="let item of items; index as i; trackBy: trackByFn">...</li>`
  *
  * With `<ng-template>` element:
  *
+ * 具有一个 `<ng-template>` 元素：
+ *
  * ```
  * <ng-template ngFor let-item [ngForOf]="items" let-i="index" [ngForTrackBy]="trackByFn">
  *   <li>...</li>
@@ -87,9 +144,12 @@ export class NgForOfContext<T> {
  *
  * ### Example
  *
+ * ### 范例
+ *
  * See a [live demo](http://plnkr.co/edit/KVuXxDp0qinGDyo307QW?p=preview) for a more detailed
  * example.
  *
+ * 参见[在线例子](http://plnkr.co/edit/KVuXxDp0qinGDyo307QW?p=preview)了解详情。
  *
  */
 @Directive({selector: '[ngFor][ngForOf]'})