Dart Typedefs

在 Dart 中，函数也是对象，就像字符和数字对象一样。 使用 typedef ，或者 function-type alias为函数起一个别名， 别名可以用来声明字段及返回值类型。 当函数类型分配给变量时，typedef会保留类型信息

请考虑以下代码，代码中未使用 typedef:

class SortedCollection {
  Function compare;
  SortedCollection(int f(Object a, Object b)) {
    compare = f;
  }
}
// Initial, broken implementation. // broken ？
int sort(Object a, Object b) => 0;
void main() {
  SortedCollection coll = SortedCollection(sort);
  // 虽然知道 compare 是函数，
  // 但是函数是什么类型 ？
  assert(coll.compare is Function);
}

当把 f 赋值给 compare 的时候，类型信息丢失了。 f 的类型是 (Object, Object) → int (这里 → 代表返回值类型)， 但是 compare 得到的类型是 Function 。如果我们使用显式的名字并保留类型信息， 这样开发者和工具都可以使用这些信息：

typedef Compare = int Function(Object a, Object b);

class SortedCollection {
  Compare compare;

  SortedCollection(this.compare);
}

// Initial, broken implementation.
int sort(Object a, Object b) => 0;

void main() {
  SortedCollection coll = SortedCollection(sort);
  assert(coll.compare is Function);
  assert(coll.compare is Compare);
}

⚠️注意: 目前，typedefs 只能使用在函数类型上， 我们希望将来这种情况有所改变

由于 typedefs 只是别名， 他们还提供了一种方式来判断任意函数的类型。例如：

typedef Compare<T> = int Function(T a, T b);
int sort(int a, int b) => a - b;
void main() {
  assert(sort is Compare<int>); // True!
}

typedef VoidCallback = void Function();

 const FlatButton({
    Key? key,
    required VoidCallback? onPressed,
    VoidCallback? onLongPress,
    ValueChanged<bool>? onHighlightChanged,
    MouseCursor? mouseCursor,
    ButtonTextTheme? textTheme,
    Color? textColor,
    Color? disabledTextColor,
    Color? color,
    Color? disabledColor,
    Color? focusColor,
    Color? hoverColor,
    Color? highlightColor,
    Color? splashColor,
    Brightness? colorBrightness,
    EdgeInsetsGeometry? padding,
    VisualDensity? visualDensity,
    ShapeBorder? shape,
    Clip clipBehavior = Clip.none,
    FocusNode? focusNode,
    bool autofocus = false,
    MaterialTapTargetSize? materialTapTargetSize,
    required Widget child,
    double? height,
    double? minWidth,
  }) : assert(clipBehavior != null),
       assert(autofocus != null),
       super(
         key: key,
         height: height,
         minWidth: minWidth,
         onPressed: onPressed,
         onLongPress: onLongPress,
         onHighlightChanged: onHighlightChanged,
         mouseCursor: mouseCursor,
         textTheme: textTheme,
         textColor: textColor,
         disabledTextColor: disabledTextColor,
         color: color,
         disabledColor: disabledColor,
         focusColor: focusColor,
         hoverColor: hoverColor,
         highlightColor: highlightColor,
         splashColor: splashColor,
         colorBrightness: colorBrightness,
         padding: padding,
         visualDensity: visualDensity,
         shape: shape,
         clipBehavior: clipBehavior,
         focusNode: focusNode,
         autofocus: autofocus,
         materialTapTargetSize: materialTapTargetSize,
         child: child,
      );