前言

上一篇 Flutter路由管理和页面参数的传递（获取&返回） 文章中我们讲述了这么用代码实现 Flutter 中页面参数的传递，这一篇我们用源码分析一下 Navigator 为什么可以进行页面参数传递。

从页面跳转入口的代码进行分析：

Navigator.of(context).pushNamed('/route1');

Navigator 的获取

Navigator 对应的 State 是 NavigatorState ，所以实际上我们需要获取的是 NavigatorState 。

class Navigator extends StatefulWidget {
  /******部分代码省略*****/
  static NavigatorState of(
    BuildContext context, {
        bool rootNavigator = false,
        bool nullOk = false,
    }) {
    final NavigatorState navigator = rootNavigator
        ? context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>())
        : context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
    assert(() {
        if (navigator == null && !nullOk) {
        throw FlutterError(
            'Navigator operation requested with a context that does not include a Navigator.\n'
            'The context used to push or pop routes from the Navigator must be that of a '
            'widget that is a descendant of a Navigator widget.'
        );
        }
        return true;
    }());
    return navigator;
    }
}

我们从源看到 NavigatorState 的获取实际是获取的 context.ancestorStateOfType 。

abstract class Element extends DiagnosticableTree implements BuildContext {
  /******部分代码省略*****/
  @override
  State ancestorStateOfType(TypeMatcher matcher) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    while (ancestor != null) {
      //从当前的Element节点一直向上寻找到匹配的StatefulElement
      if (ancestor is StatefulElement && matcher.check(ancestor.state))
        break;
      ancestor = ancestor._parent;
    }
    final StatefulElement statefulAncestor = ancestor;
    //返回匹配的StatefulElement的state
    return statefulAncestor?.state;
  }
}

循环遍历向上寻找 Navigator 的 state ，这里就是 NavigatorState 。

Navigator的生成

Navigator 的 Widget 是是什么时候添加到视图树中的呢？我们从 Flutter 应用程序的入口开始一步一步跟进代码的执行：

void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(...);
  }
}

MaterialApp 传入 routes 和 onGenerateRoute 等参数，MaterialApp 的 state 是 _MaterialAppState 它构建的是 WidgetsApp 类型的 Widget ，同时 routes 和 onGenerateRoute 等参数也进行了透传。

class MaterialApp extends StatefulWidget {
  const MaterialApp({
    Key key,
    this.navigatorKey,
    this.home,
    this.routes = const <String, WidgetBuilder>{},
    this.initialRoute,
    this.onGenerateRoute,
    this.onUnknownRoute,
    /******部分代码省略*****/
  })
  /******部分代码省略*****/
  @override
  _MaterialAppState createState() => _MaterialAppState();
}
class _MaterialAppState extends State<MaterialApp> {
  /******部分代码省略*****/
  @override
  Widget build(BuildContext context) {
    Widget result = WidgetsApp(
      key: GlobalObjectKey(this),
      navigatorKey: widget.navigatorKey,
      navigatorObservers: _navigatorObservers,
        pageRouteBuilder: <T>(RouteSettings settings, WidgetBuilder builder) =>
            MaterialPageRoute<T>(settings: settings, builder: builder),
      home: widget.home,
      routes: widget.routes,
      initialRoute: widget.initialRoute,
      onGenerateRoute: widget.onGenerateRoute,
      onUnknownRoute: widget.onUnknownRoute,
      /******部分代码省略*****/
    );
}

我们再看看 WidgetsApp 对应的 State 的 _WidgetsAppState 。在_WidgetsAppState 的 Widget build(BuildContext context) 方法中我们找到了管理路由的 Navigator 的构造时机。

class WidgetsApp extends StatefulWidget {
  WidgetsApp({ // can't be const because the asserts use methods on Iterable :-(
    Key key,
    this.navigatorKey,
    this.onGenerateRoute,
    this.onUnknownRoute,
    this.navigatorObservers = const <NavigatorObserver>[],
    this.initialRoute,
    this.pageRouteBuilder,
    this.home,
    this.routes = const <String, WidgetBuilder>{},
    /******部分代码省略*****/
  );
  @override
  _WidgetsAppState createState() => _WidgetsAppState();
}
class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserver {
  @override
  Widget build(BuildContext context) {
    Widget navigator;
    if (_navigator != null) {
      navigator = Navigator(
        key: _navigator,
        // If window.defaultRouteName isn't '/', we should assume it was set
        // intentionally via `setInitialRoute`, and should override whatever
        // is in [widget.initialRoute].
        initialRoute: WidgetsBinding.instance.window.defaultRouteName != Navigator.defaultRouteName
            ? WidgetsBinding.instance.window.defaultRouteName
            : widget.initialRoute ?? WidgetsBinding.instance.window.defaultRouteName,
        onGenerateRoute: _onGenerateRoute,
        onUnknownRoute: _onUnknownRoute,
        observers: widget.navigatorObservers,
      );
    }
    Widget result;
    if (widget.builder != null) {
      result = Builder(
        builder: (BuildContext context) {
          return widget.builder(context, navigator);
        },
      );
    } else {
      assert(navigator != null);
      result = navigator;
    }
    /******部分代码省略*****/
    /**上面经过多次的操作之后，navigator变为result的某个子孙节点上的child**/
    Widget title;
    if (widget.onGenerateTitle != null) {
      title = Builder(
        // This Builder exists to provide a context below the Localizations widget.
        // The onGenerateTitle callback can refer to Localizations via its context
        // parameter.
        builder: (BuildContext context) {
          final String title = widget.onGenerateTitle(context);
          assert(title != null, 'onGenerateTitle must return a non-null String');
          return Title(
            title: title,
            color: widget.color,
            child: result,
          );
        },
      );
    } else {
      title = Title(
        title: widget.title,
        color: widget.color,
        child: result,
      );
    }
    /******部分代码省略*****/
    /**上面经过多次的操作之后，result变为title的某个子孙节点上的child**/
    return MediaQuery(
      data: MediaQueryData.fromWindow(WidgetsBinding.instance.window),
      child: Localizations(
        locale: appLocale,
        delegates: _localizationsDelegates.toList(),
        //将title作为child视图，也就是说navigator变为其中的某个子孙节点视图
        child: title,
      ),
    );
  }
}

在构建的 MediaQuery 就存在我们需要的 Navigator 。

Navigator.png

这张图是程序运行时候使用（DevTools）进行的页面元素分析，也证明了 Navigator 是在页面的 Widget 元素路径上的。

pushNamed方法解析

@optionalTypeArgs
static Future<T> pushNamed<T extends Object>(
  BuildContext context,
  String routeName, {
  Object arguments,
  }) {
  return Navigator.of(context).pushNamed<T>(routeName, arguments: arguments);
}
@optionalTypeArgs
Future<T> pushNamed<T extends Object>(
  String routeName, {
  Object arguments,
}) {
  return push<T>(_routeNamed<T>(routeName, arguments: arguments));
}
Route<T> _routeNamed<T>(String name, { @required Object arguments, bool allowNull = false }) {
  assert(!_debugLocked);
  assert(name != null);
  final RouteSettings settings = RouteSettings(
    name: name,
    isInitialRoute: _history.isEmpty,
    arguments: arguments,
  );
  Route<T> route = widget.onGenerateRoute(settings);
  if (route == null && !allowNull) {
    assert(() {
      if (widget.onUnknownRoute == null) {
        throw FlutterError(...);
      }
      return true;
    }());
    route = widget.onUnknownRoute(settings);
    assert(() {
      if (route == null) {
        throw FlutterError(...);
      }
      return true;
    }());
  }
  return route;
}

我们看到是调用了 widget.onGenerateRoute(settings) 生成路由， 这里的 onGenerateRoute 在 Navigator 在构造的时候传入的 onGenerateRoute 。

onGenerateRoute

Navigator 在构造的时候如果我们细心就会发现 onGenerateRoute 现在改为了 _onGenerateRoute 。

也就是 _WidgetsAppState 的 _onGenerateRoute 方法实现：

Route<dynamic> _onGenerateRoute(RouteSettings settings) {
  final String name = settings.name;
  //从widget注册的路由中获取name对应的WidgetBuilder
  final WidgetBuilder pageContentBuilder = name == Navigator.defaultRouteName && widget.home != null
      ? (BuildContext context) => widget.home
      : widget.routes[name];
    //如果pageContentBuilder不为空，那么和RouteSettings一起执行widget.pageRouteBuilder构造一个route
  if (pageContentBuilder != null) {
    assert(widget.pageRouteBuilder != null,
      'The default onGenerateRoute handler for WidgetsApp must have a '
      'pageRouteBuilder set if the home or routes properties are set.');
    final Route<dynamic> route = widget.pageRouteBuilder<dynamic>(
      settings,
      pageContentBuilder,
    );
    assert(route != null,
      'The pageRouteBuilder for WidgetsApp must return a valid non-null Route.');
    return route;
  }
  //如果pageContentBuilder为空，那么执行widget.onGenerateRoute的方法
  if (widget.onGenerateRoute != null)
    return widget.onGenerateRoute(settings);
  return null;
}

widget.pageRouteBuilder 的方法，我们在生成 WidgetsApp可以看到是：

pageRouteBuilder: <T>(RouteSettings settings, WidgetBuilder builder) =>
            MaterialPageRoute<T>(settings: settings, builder: builder)

所以最终我们通过在 MaterialApp 注册 routes 生成了一个 MaterialPageRoute 用来进行页面跳转。

最后如果 routes 为空的话，我们执行 widget.onGenerateRoute 。这个解释了在 Flutter路由管理和页面参数的传递（获取&返回） 这篇文章末尾说的 onGenerateRoute 方式进行的参数传递，必须不能进行 routers 的注册。

文章到这里就全部讲述完啦，若有其他需要交流的可以留言哦_！！

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