title: flutter ListView KeepAlive after some scroll
body: <p>I want to <code>keepAlive</code> my widgets which are already rendered in <code>ListView</code>. I was tried with <code>addAutomaticKeepAlives:true</code> properties which provide by <code>ListView</code> class.</p>
  
  <p>Here is my sample code which I was used. Same issue in <code>SliverChildBuilderDelegate</code> delegate which provide by <code>SliverList</code>.</p>
  
  <pre><code>ListView.builder(
      itemBuilder: (context,index){
        return Card(
          child: Container(
            child: Image.asset("images/${index+1}.jpg",fit: BoxFit.cover,),
            height: 250.0,
          ),
        );
      },
      addAutomaticKeepAlives: true,
      itemCount:40 ,
  );
  </code></pre>