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前言

Item这块，前面看了一下貌似没什么太大作用的 KeyedSubtree ，接下来看下这两个貌似跟KeepAlive有关系的东西，他俩又有什么区别;

注释部分

总结一下，AutoMaticKeepAlive是通过监听KeepAliveNotification，来构建child，处理KeepAlive的一个widget；

而 KeepAlive 是一个通过给定的数值，来处理KeepAlive的Widget，他并不会自动处理事件；一般用于SliverList和SliverGrid；

运行原理

KeepAlive

首先从简单的KeepAlive开始：

其所做的事也不复杂：

当被调用ApplyParentData的时候，检查KeepAlive设置，如果不需要KeepAlive，那么触发重绘。否则就不用处理；

当然会更新ParentData中的KeepAlive属性，而那个是在SlverList中，用来判断是否需要放到KeepAliveBucket持久化的判断依据，比如说之前提到的 _destroyOrCacheChild 方法：

再次拿的时候就会从这个bucket中拿：

而KeepAlive被触发调用的地方就在AutomaticKeepAlive这块；

AutomaticKeepAlive

首先是AutomaticKeepAlive的build方法：

只是简单的构造了一个K呃呃篇Alive，并将_child 作为其child；

而_child 是这么构造的：

结合注释所述，那么核心逻辑就是这个_addClient方法了；

在这段代码中，所做的事就三件：

• 通过_createCallback(handler)方法，来创建KeppAliveNotification 的回调；
• 更新缓存的handle及其回调；
• 判断更新parentData的时机；

创建callback

这段代码中，注释反而是比代码长，去掉注释部分，代码是这样的：

VoidCallback _createCallback(Listenable handle) {
  return () {
    assert(() {
      if (!mounted) {
        throw FlutterError(
          'AutomaticKeepAlive handle triggered after AutomaticKeepAlive was disposed.\n'
          'Widgets should always trigger their KeepAliveNotification handle when they are '
          'deactivated, so that they (or their handle) do not send spurious events later '
          'when they are no longer in the tree.',
        );
      }
      return true;
    }());
    _handles!.remove(handle);
    if (_handles!.isEmpty) {
      if (SchedulerBinding.instance!.schedulerPhase.index < SchedulerPhase.persistentCallbacks.index) {
        setState(() { _keepingAlive = false; });
      } else {
        _keepingAlive = false;
        scheduleMicrotask(() {
          if (mounted && _handles!.isEmpty) {
            setState(() {
              assert(!_keepingAlive);
            });
          }
        });
      }
    }
  };
}

抛开前面的assert部分，其所做的事其实简单的来说就是：

• 移除缓存的回调；
• 如果没有缓存的回调了：
  • 如果当前构建过程处于build、layout、paint等方法之前，那么打脏，下一帧再更新；
  • 如果已经开始构建了，那么将打脏操作加入到一个微队列中（也就是说放到下下帧中？）；

关于为什么这么做；注释中是这么解释的：

// We were probably notified by a descendant when they were yanked out
// of our subtree somehow. We're probably in the middle of build or
// layout, so there's really nothing we can do to clean up this mess
// short of just scheduling another build to do the cleanup. This is
// very unfortunate, and means (for instance) that garbage collection
// of these resources won't happen for another 16ms.
//
// The problem is there's really no way for us to distinguish these
// cases:
//
//  * We haven't built yet (or missed out chance to build), but
//    someone above us notified our descendant and our descendant is
//    disconnecting from us. If we could mark ourselves dirty we would
//    be able to clean everything this frame. (This is a pretty
//    unlikely scenario in practice. Usually things change before
//    build/layout, not during build/layout.)
//
//  * Our child changed, and as our old child went away, it notified
//    us. We can't setState, since we _just_ built. We can't apply the
//    parent data information to our child because we don't _have_ a
//    child at this instant. We really want to be able to change our
//    mind about how we built, so we can give the KeepAlive widget a
//    new value, but it's too late.
//
//  * A deep descendant in another build scope just got yanked, and in
//    the process notified us. We could apply new parent data
//    information, but it may or may not get applied this frame,
//    depending on whether said child is in the same layout scope.
//
//  * A descendant is being moved from one position under us to
//    another position under us. They just notified us of the removal,
//    at some point in the future they will notify us of the addition.
//    We don't want to do anything. (This is why we check that
//    _handles is still empty below.)
//
//  * We're being notified in the paint phase, or even in a post-frame
//    callback. Either way it is far too late for us to make our
//    parent lay out again this frame, so the garbage won't get
//    collected this frame.
//
//  * We are being torn out of the tree ourselves, as is our
//    descendant, and it notified us while it was being deactivated.
//    We don't need to do anything, but we don't know yet because we
//    haven't been deactivated yet. (This is why we check mounted
//    below before calling setState.)
//
// Long story short, we have to schedule a new frame and request a
// frame there, but this is generally a bad practice, and you should
// avoid it if possible.

简单的来说就是，整个构建过程，应该是一个原子性的操作；所以如果在构建过程中被子View或者其他方式通知了重构，那么只能再安排一次构建来做这个操作；

判断更新parentData的时机

在这一步所做的事：

• 如果child不为空，那么直接调用 _updateParentDataOfChild 方法；
• 如果为空，那么就按注释中说的那样，放到结尾再更新ParentData；

而这个 _updateParentDataOfChild 方法所做的事，就是字面意思：

最终会调用到 KeepAlive 自己的applyParentData方法：

总结：

这两个Widget所做的事，无非就是通过监听KeepAliveNotification，来更新SliverList中存放的ParentData，进而实现SliverList在销毁的时候不调用Item本身的销毁方法，而是放到一个Map中缓存起来；