opaque和maintainState

opaque：是否是全屏不透明的，当一个组件opaque=true时，那么可以认为它之下的组件都不需要再绘制了。一般情况下，一个 Page 的opaque=true，不是全屏的 PopupWindow opaque=false。
maintainState：一个已经不可见(被上面的盖住完全看不到啦~)的组件，是否还需要保存状态。当maintainState=true时，会让这个组件继续活着，但不会再进行绘制操作了。如果maintainState=false，则不会继续维持这个 Widget 的生存状态了。

# class OverlayState
@override
  Widget build(BuildContext context) {
    // This list is filled backwards and then reversed below before
    // it is added to the tree.
    final List<Widget> children = <Widget>[];
    bool onstage = true;
    int onstageCount = 0;
    for (int i = _entries.length - 1; i >= 0; i -= 1) {
      final OverlayEntry entry = _entries[i];
      if (onstage) {
        onstageCount += 1;
        children.add(_OverlayEntryWidget(
          key: entry._key,
          entry: entry,
        ));
        if (entry.opaque)
          onstage = false;
      } else if (entry.maintainState) {
        children.add(_OverlayEntryWidget(
          key: entry._key,
          entry: entry,
          tickerEnabled: false,
        ));
      }
    }
///_Theatre父类是MultiChildRenderObjectWidget
    return _Theatre(
      skipCount: children.length - onstageCount,
      children: children.reversed.toList(growable: false),
    );
  }

Overlay rebuild的时候是会调用List<OverlayEntry> _entries内OverlayEntry的builder方法rebuild child widget的

OverlayState

OverlayState中页面存储在_entries中
final List<OverlayEntry> _entries = <OverlayEntry>[];
通过insert和_remove方法可以增删OverlayEntry

void insert(OverlayEntry entry, { OverlayEntry below, OverlayEntry above }) {
...
///OverlayEntry持有OverlayState
entry._overlay = this;
...
}
void _remove(OverlayEntry entry)

OverlayEntry可以自己将自身从OverlayState中删除

void remove() {
    ...
      overlay._remove(this);
   ...
  }

NavigatorState

NavigatorState使用了Overlay管理页面
传递给Overlay的OverlayEntry由各Route自己管理

///NavigatorState class
Iterable<OverlayEntry> get _allRouteOverlayEntries sync* {
    for (final _RouteEntry entry in _history)
      yield* entry.route.overlayEntries;
  }

@override
  Widget build(BuildContext context) {
   ...
   Overlay(
              key: _overlayKey,
              initialEntries: overlay == null ?  _allRouteOverlayEntries.toList(growable: false) : const <OverlayEntry>[],
            ),
...
  }

NavigatorState中存的是_RouteEntry

List<_RouteEntry> _history = <_RouteEntry>[];

_RouteEntry(
///Route
    this.route, {
      @required _RouteLifecycle initialState,
    })

_RouteEntry有个currentState值标识_RouteEntry当前状态，当为add和push状态时，调用entry的方法最终会调用Route的install方法，从而创建Route的OverlayEntry

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
    ...
    int index = _history.length - 1;
  ...
    final List<_RouteEntry> toBeDisposed = <_RouteEntry>[];
    while (index >= 0) {
      switch (entry.currentState) {
        case _RouteLifecycle.add:
          assert(rearrangeOverlay);
          entry.handleAdd(
            navigator: this,
            previousPresent: _getRouteBefore(index - 1, _RouteEntry.isPresentPredicate)?.route,
          );
          assert(entry.currentState == _RouteLifecycle.adding);
          continue;
        case _RouteLifecycle.adding:
          if (canRemoveOrAdd || next == null) {
            entry.didAdd(
              navigator: this,
              isNewFirst: next == null
            );
            assert(entry.currentState == _RouteLifecycle.idle);
            continue;
          }
          break;
        case _RouteLifecycle.push:
        case _RouteLifecycle.pushReplace:
        case _RouteLifecycle.replace:
          assert(rearrangeOverlay);
          entry.handlePush(
            navigator: this,
            previous: previous?.route,
            previousPresent: _getRouteBefore(index - 1, _RouteEntry.isPresentPredicate)?.route,
            isNewFirst: next == null,
          );
         ...
          break;
       ...
      }
    }

/// A route that displays widgets in the [Navigator]'s [Overlay].
abstract class OverlayRoute<T> extends Route<T> {
  /// Creates a route that knows how to interact with an [Overlay].
  OverlayRoute({
    RouteSettings settings,
  }) : super(settings: settings);

  /// Subclasses should override this getter to return the builders for the overlay.
  @factory
  Iterable<OverlayEntry> createOverlayEntries();

  @override
  List<OverlayEntry> get overlayEntries => _overlayEntries;
  final List<OverlayEntry> _overlayEntries = <OverlayEntry>[];

  @override
  void install() {
    assert(_overlayEntries.isEmpty);
    _overlayEntries.addAll(createOverlayEntries());
    super.install();
  }

ModalRoute会创建两个OverlayEntry，_modalBarrier为遮罩，_modalScope为page本身

abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T> {
...
@override
  Iterable<OverlayEntry> createOverlayEntries() sync* {
    yield _modalBarrier = OverlayEntry(builder: _buildModalBarrier);
    yield _modalScope = OverlayEntry(builder: _buildModalScope, maintainState: maintainState);
  }
...
}

Route有一个_popCompleter属性

final Completer<T> _popCompleter = Completer<T>();
当NavigatorState push一个Route的时候返回的就是_popCompleter.future,当pop的时候也是通过_popCompleter.complete(result ?? currentResult);传递返回值

MaterialApp

MaterialApp(
        routes: {'main': (context) => Counter()},
        onGenerateRoute: (setting) {
          // setting.name
          return MaterialPageRoute(builder: (_) => Counter());
        },
      ),

_MaterialAppState内使用了WidgetsApp，pushName的时候先判断routes是否有，routes没有再返回widget.onGenerateRoute(settings)

WidgetsApp(
      routes: widget.routes,
      onGenerateRoute: widget.onGenerateRoute,
)

Route<dynamic> _onGenerateRoute(RouteSettings settings) {
    final String name = settings.name;
    final WidgetBuilder pageContentBuilder = name == Navigator.defaultRouteName && widget.home != null
        ? (BuildContext context) => widget.home
        : widget.routes[name];

    if (pageContentBuilder != null) {
      final Route<dynamic> route = widget.pageRouteBuilder<dynamic>(
        settings,
        pageContentBuilder,
      );
      return route;
    }
    if (widget.onGenerateRoute != null)
      return widget.onGenerateRoute(settings);
    return null;
  }

Widget build(BuildContext context) {
    Widget navigator;
    if (_navigator != null) {
      navigator = Navigator(
        ...
        onGenerateRoute: _onGenerateRoute,
        ...
      );
    }
}

路由管理封装

https://github.com/LBXjixiangniao/flutter_summary/blob/master/lib/router/router.dart

flutter 路由及Overlay