replace Container with SizedBox

[tjx666]

Now, when some child is not loaded, it would be replaced by an empty container, but it will cause It size fit parent.

For Example:

import 'package:flutter/material.dart';
import './lazy_indexed_stack.dart';

class TabView extends StatefulWidget {
  int index;
  TabView({Key key, this.index}) : super(key: key);

  @override
  _TabViewState createState() => _TabViewState();
}

class _TabViewState extends State<TabView> {
  @override
  void initState() {
    super.initState();
    print('init tabView ${widget.index}');
  }

  @override
  void dispose() {
    super.dispose();
    print('dispose tabView ${widget.index}');
  }

  @override
  Widget build(BuildContext context) {
    print('build tabView ${widget.index}');

    return Container(
      width: 200,
      height: 200,
      color: Colors.lightGreenAccent,
      child: Center(
        child: Text('view ${widget.index}'),
      ),
    );
  }
}

class TestLazyIndexedStack extends StatefulWidget {
  TestLazyIndexedStack({Key key}) : super(key: key);

  @override
  _TestLazyIndexedStackState createState() => _TestLazyIndexedStackState();
}

class _TestLazyIndexedStackState extends State<TestLazyIndexedStack> {
  int viewIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('测试 IndexedStack'),
      ),
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        children: [
          Container(
            color: Colors.lightBlueAccent,
            child: LazyIndexedStack(
              index: viewIndex,
              itemCount: 3,
              itemBuilder: (context, index) {
                return TabView(index: index);
              },
            ),
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: List<Widget>.generate(
              3,
              (index) => FlatButton(
                onPressed: () {
                  setState(() {
                    viewIndex = index;
                  });
                },
                child: Text('$index'),
              ),
            ),
          )
        ],
      ),
    );
  }
}

result in:

When replace Container with SizedBox:

  @override
  void initState() {
    _loaded = [];
    _children = [];
    for (int i = 0; i < widget.itemCount; ++i) {
      if (i == widget.index) {
        _children.add(widget.itemBuilder(context, i));
        _loaded.add(true);
      } else {
        _children.add(const SizedBox());
        _loaded.add(false);
      }
    }
    super.initState();
  }