因为detachFromFlutterEngine是FlutterBoostFragment在onCreateView做的。

但是super.onCreateView(inflater, container, savedInstanceState) 里面最终会把当前engine的platformview attach上去的操作（也就导致了第二个tab 错误的把第一个tab的platformview再次attach，所以出问题了）
flutterEngine.getPlatformViewsController().attachToView(this)

FlutterBoostFragment

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            FlutterBoost.instance().getPlugin().onContainerCreated(this);
            View view = super.onCreateView(inflater, container, savedInstanceState);
            flutterView = FlutterBoostUtils.findFlutterView(view);
            // Detach FlutterView from engine before |onResume|.
            flutterView.detachFromFlutterEngine();
            if (DEBUG) Log.d(TAG, "#onCreateView: " + flutterView + ", " + this);
            if (view == flutterView) {
                // fix https://github.com/alibaba/flutter_boost/issues/1732
                FrameLayout frameLayout = new FrameLayout(view.getContext());
                frameLayout.addView(view);
                return frameLayout;
            }
            return view;
        }

目前BoostFragment中的时序如下：

onCreateView(attachEngine -> detachEngine)  -> onResume或者onHiddenChanged (detachEngineIfNeed -> attachEngine)

这个时许中，会出现crash的其实有两个点：

1. Frgament1 attachEngine后，切换Fragment2，onCreateView中又attachEngine
2. onCreateView中detachEngine后，onResume中执行detachEngineIfNeed

上述问题的crash点出在1上，解决方法其实就是两次 attachEngine 中间添加一次 detachEngine 。但时机不好控制，所以基于目前没有 PlatformView 的情况下，能够正常运行，那么一个折中的方案就是，在Fragment2 attachEngine 之前，把 PlatformView 从Fragment1的 flutterView 中移除掉。

但一般无法直接获取到Fragment1中的 flutterView ，所以这里可以通过继承 PlatformViewsController 的方法来实现：

public class FlutterBoostPlatformViewsController extends PlatformViewsController {

    public FlutterView mCurrentFlutterView;

    @Override
    public void attachToView(@NonNull FlutterView newFlutterView) {
        super.attachToView(newFlutterView);
        mCurrentFlutterView = newFlutterView;
    }

    @Override
    public void detachFromView() {
        if (mCurrentFlutterView == null) {
            return;
        }
        super.detachFromView();
        mCurrentFlutterView = null;
    }

    public void removePlatformWrapperOrParents() {
        if (mCurrentFlutterView != null) {
            List<View> needRemoveViews = new ArrayList<>();
            int childCount = mCurrentFlutterView.getChildCount();
            for (int i = 0; i < childCount; i++) {
                View view = mCurrentFlutterView.getChildAt(i);
                if (view.getClass().getName().contains("PlatformViewWrapper") || view instanceof FlutterMutatorView) {
                    needRemoveViews.add(view);
                }
            }
            if (!needRemoveViews.isEmpty()) {
                for (View needRemoveView : needRemoveViews) {
                    mCurrentFlutterView.removeView(needRemoveView);
                }
            }
        }
    }
}

然后在构建FlutterEngine的地方使用 FlutterBoostPlatformViewsController 代替 PlatformViewsController 。

public FlutterEngine provideFlutterEngine(@NonNull Context context) {
    return new FlutterEngine(
            context,
            null,
            null,
            new FlutterBoostPlatformViewsController(),
            null,
            true,
            false);
}

最后在 FlutterBoostFragment 的 onCreateView 方法开始调用 removePlatformWrapperOrParents 把 PlatformView 给移掉。

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        FlutterEngine flutterEngine = getFlutterEngine();
        if (flutterEngine != null) {
            PlatformViewsController platformViewsController =
                    flutterEngine.getPlatformViewsController();
            if (platformViewsController instanceof FlutterBoostPlatformViewsController) {
                ((FlutterBoostPlatformViewsController) platformViewsController).removePlatformWrapperOrParents();
            }
        }
        ......
    }

这种方法只能解决attachEngine后又attachEngine的crash，运行后就会出现detachEngine后detachEngineIfNeed的crash点，就是个NPE，解决方法也简单，上面的 FlutterBoostPlatformViewsController 中已经做了保护， detachFromView 中加了空判定。

为什么不能在FlutterBoostFragment里面的 didFragmentHide把 performDetach加回来？

protected void didFragmentHide() {
    FlutterBoost.instance().getPlugin().onContainerDisappeared(this);
    // We defer |performDetach| call to new Flutter container's |onResume|;
    // performDetach();
    if (DEBUG) Log.d(TAG, "#didFragmentHide: " + this + ", isOpaque=" + isOpaque());
}

大概是，无法保证多个Fragment的情况下，可见Fragment的onResume方法和需要hide的fragment的didFragmentHide的方法的调用顺序把。

确实有这方面的问题。不过如果是在一个FragmentTransaction 顺序 操作多个fragment 就没问题了