# 11.有状态Widget初始化重写&链式调用&排序

### 前言

前文我们通过联系人页面聊到了断言的使用，本文将继续结合该场景了机更多 `Flutter` 开发小技巧。

### 初始化方法重写

对于一个 StatefulWidget 我们如果需要在其初始化的时候进行一些操作的话，可以通过重写 `initState()` 来达到效果。

```
class _ContactsPageState extends State<ContactsPage> {

  @override
  void initState() {
    super.initState();

    // 添加数据
    _contacts
        .addAll(friendsData);

    _systems
        .addAll(friendsHeaderData);
  }

  ...
}
```

### 链式调用

在 `iOS` 开发中，比如使用 `SnapKit` 进行UI控件约束布局的时候，我们经常会用到这样的`链式调用`，非常爽。

```
make.leading.trailing.top.equalToSuperview()
```

其实在 Flutter 开发中我们也可以使用类似的写法：

```
@override
  void initState() {
    super.initState();

    // 链式调用 多次添加元素
    _contacts
      ..addAll(friendsData)
      ..addAll(friendsData);

    _systems.addAll(friendsHeaderData);
  }
```

### 排序

在上面可以看到 `..addAll(friendsData)` 这样连续进行了调用。那么是否可以做的更多呢？

通常联系人列表我们会进行按字母排序，这里通过链式调用进行尝试：

```
@override
  void initState() {
    super.initState();
    _contacts
      ..addAll(friendsData)
      ..addAll(friendsData)
      ..sort((a, b) {
        if (a.indexLetter == null || b.indexLetter == null) {
          return 0;
        }
        return a.indexLetter!.compareTo(b.indexLetter!);
      });
    _systems.addAll(friendsHeaderData);
  }
```

#### 和 Swift sort 的不同

这里你可能会发现，这里的 `sort` 函数和 `Swift` 的略有不同，内部回调的返回值不是 `bool` 而是 `int` 。

结合函数头文件注释我们可以了解更多：

````
  /// Sorts this list according to the order specified by the [compare] function.
  ///
  /// The [compare] function must act as a [Comparator].
  /// ```dart
  /// var numbers = ['two', 'three', 'four'];
  /// // Sort from shortest to longest.
  /// numbers.sort((a, b) => a.length.compareTo(b.length));
  /// print(numbers);  // [two, four, three]
  /// ```
  /// The default [List] implementations use [Comparable.compare] if
  /// [compare] is omitted.
  /// ```dart
  /// List<int> nums = [13, 2, -11];
  /// nums.sort();
  /// print(nums);  // [-11, 2, 13]
  /// ```
  /// In that case, the elements of the list must be [Comparable] to
  /// each other.
  ///
  /// A [Comparator] may compare objects as equal (return zero), even if they
  /// are distinct objects.
  /// The sort function is not guaranteed to be stable, so distinct objects
  /// that compare as equal may occur in any order in the result:
  /// ```dart
  /// var numbers = ['one', 'two', 'three', 'four'];
  /// numbers.sort((a, b) => a.length.compareTo(b.length));
  /// print(numbers);  // [one, two, four, three] OR [two, one, four, three]
  /// ```
  void sort([int compare(E a, E b)?]);
````

#### 简略写法

这里的排序函数可以在条件比较简单的情况下可以简写，提高编码效率

**未简写**

```
sort((a, b) {
  if (a.indexLetter == null || b.indexLetter == null) {
    return 0;
  }
  return a.indexLetter!.compareTo(b.indexLetter!);
});
```

**简写**

```
sort( (a, b) => a.indexLetter.compareTo(b.indexLetter) );
```

> 头文件注释中有这样简写说明，平时多阅读注释，可以帮助我们写出更有语言特色风格的代码哦～


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