https://flutter.institute/sharing-data-inheritedmodel-vs-inheritedwidget/
其工作原理为被其嵌套的子widget可以通过InheritedModel.inheritFrom获取到InheritedModel或者InheritedWidget

https://loveky.github.io/2018/07/18/how-flutter-inheritedwidget-works/

InheritedWidget.png

Snip20190503_1.png

Element源码

this依赖于ancestor

@Deprecated(
    'Use dependOnInheritedElement instead. '
    'This feature was deprecated after v1.12.1.'
  )
  @override
  InheritedWidget inheritFromElement(InheritedElement ancestor, { Object aspect }) {
    return dependOnInheritedElement(ancestor, aspect: aspect);
  }

  @override
  InheritedWidget dependOnInheritedElement(InheritedElement ancestor, { Object aspect }) {
    assert(ancestor != null);
    _dependencies ??= HashSet<InheritedElement>();
    _dependencies.add(ancestor);
    ancestor.updateDependencies(this, aspect);
    return ancestor.widget;
  }

this依赖于指定类型widget的Element（ancestor）,并返回ancestor（也就是指定widget类型的element）的widget

@Deprecated(
    'Use dependOnInheritedWidgetOfExactType instead. '
    'This feature was deprecated after v1.12.1.'
  )
  @override
  InheritedWidget inheritFromWidgetOfExactType(Type targetType, { Object aspect }) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    final InheritedElement ancestor = _inheritedWidgets == null ? null : _inheritedWidgets[targetType];
    if (ancestor != null) {
      assert(ancestor is InheritedElement);
      return inheritFromElement(ancestor, aspect: aspect);
    }
    _hadUnsatisfiedDependencies = true;
    return null;
  }

  @override
  T dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({Object aspect}) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    final InheritedElement ancestor = _inheritedWidgets == null ? null : _inheritedWidgets[T];
    if (ancestor != null) {
      assert(ancestor is InheritedElement);
      return dependOnInheritedElement(ancestor, aspect: aspect);
    }
    _hadUnsatisfiedDependencies = true;
    return null;
  }

查找指定类型widget的Element

@Deprecated(
    'Use getElementForInheritedWidgetOfExactType instead. '
    'This feature was deprecated after v1.12.1.'
  )
  @override
  InheritedElement ancestorInheritedElementForWidgetOfExactType(Type targetType) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    final InheritedElement ancestor = _inheritedWidgets == null ? null : _inheritedWidgets[targetType];
    return ancestor;
  }

  @override
  InheritedElement getElementForInheritedWidgetOfExactType<T extends InheritedWidget>() {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    final InheritedElement ancestor = _inheritedWidgets == null ? null : _inheritedWidgets[T];
    return ancestor;
  }

查找指定类型的widget

@Deprecated(
    'Use findAncestorWidgetOfExactType instead. '
    'This feature was deprecated after v1.12.1.'
  )
  @override
  Widget ancestorWidgetOfExactType(Type targetType) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    while (ancestor != null && ancestor.widget.runtimeType != targetType)
      ancestor = ancestor._parent;
    return ancestor?.widget;
  }

  @override
  T findAncestorWidgetOfExactType<T extends Widget>() {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    while (ancestor != null && ancestor.widget.runtimeType != T)
      ancestor = ancestor._parent;
    return ancestor?.widget;
  }

InheritedModel

由以下代码可知，InheritedModel与InheritedWidget的不同在于

• InheritedModel将_ dependents的value设置为Set<T>，
• 并且在其他element依赖于InheritedModelElement的时候传递一个T类型的值存储在Set中
• 当InheritedModelElement notifyDependent的时候会将Set值传递给InheritedModel的updateShouldNotifyDependent方法，在方法中可以判断该依赖于InheritedModelElement的element是否需要刷新。

abstract class InheritedModel<T> extends InheritedWidget {
...
@protected
  bool updateShouldNotifyDependent(covariant InheritedModel<T> oldWidget, Set<T> dependencies);
...
}

class InheritedModelElement<T> extends InheritedElement {
  /// Creates an element that uses the given widget as its configuration.
  InheritedModelElement(InheritedModel<T> widget) : super(widget);

  @override
  InheritedModel<T> get widget => super.widget as InheritedModel<T>;

  @override
  void updateDependencies(Element dependent, Object aspect) {
    final Set<T> dependencies = getDependencies(dependent) as Set<T>;
    if (dependencies != null && dependencies.isEmpty)
      return;

    if (aspect == null) {
      setDependencies(dependent, HashSet<T>());
    } else {
      assert(aspect is T);
      setDependencies(dependent, (dependencies ?? HashSet<T>())..add(aspect as T));
    }
  }

  @override
  void notifyDependent(InheritedModel<T> oldWidget, Element dependent) {
    final Set<T> dependencies = getDependencies(dependent) as Set<T>;
    if (dependencies == null)
      return;
    if (dependencies.isEmpty || widget.updateShouldNotifyDependent(oldWidget, dependencies))
      dependent.didChangeDependencies();
  }
}