android - 旋转后 fragment 字段为空

Question

我有 fragment ，我在其中调用自定义 View 类(Dragger)

public class CanvasFragment extends Fragment {

    private Dragger dragger;

    public CanvasFragment() {
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_canvas, container, false);

        dragger = (Dragger) rootView.findViewById(R.id.dragger);

        int[] symbolIndex = {};

        dragger.setElementsArray(symbolIndex);
        return rootView;
    }

    public void addIconToCanvas(int id){
        dragger.addIcon(id);
    }
}

我还有另一个带有 RecyclerView 和适配器的 fragment ，在其中我通过 的主机 Activity 调用 CanvasFragment 的 方法 addIconToCanvas() >CanvasFragment.

public class ItemAdapter extends RecyclerView.Adapter<ItemAdapter.ViewHolder> {

    private int[] data;
    private Context context;

    public static class ViewHolder extends RecyclerView.ViewHolder {
        public ImageView itemView;
        public ViewHolder(View v) {
            super(v);
            itemView = (ImageView) v.findViewById(R.id.im_item);
        }
    }

    public ItemAdapter(Context context, int[] myDataset) {
        data = myDataset;
        this.context = context;
    }

    @Override
    public ItemAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_view, parent, false);
        ViewHolder vh = new ViewHolder(v);
        return vh;
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, final int position) {
        holder.itemView.setImageResource(data[position]);
        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
            ((MainActivity)context).addItemToCanvas(data[position]);
            }
        });
    }

    @Override
    public int getItemCount() {
        return data.length;
    }
}

我发现 CanvasFragment 中的 dragger 旋转后得到新的引用，但是当我从适配器调用 addIconToCanvas() 时 - dragger 为空。

我真的无法理解为什么字段获得引用然后丢失它)))

有谁知道，这有什么问题吗？

提前致谢!

附:宿主 Activity

public class MainActivity extends AppCompatActivity {

    private CanvasFragment canvasFragment = new CanvasFragment();
    private ItemsFragment itemsFragment = new ItemsFragment();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                .add(R.id.container_canvas, canvasFragment)
                .commit();
            getSupportFragmentManager().beginTransaction()
                .add(R.id.container_items, itemsFragment)
                .commit();
        }

    }

    public void addItemToCanvas(int id){
        canvasFragment.addIconToCanvas(id);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();

        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

用线

private CanvasFragment canvasFragment = new CanvasFragment();

每次创建 Activity 时，您都将 Fragment 的新实例分配给变量 canvasFragment。但前提是 savedInstanceState == null 您将这个 CanvasFragment 实例添加到 UI。

Fragment 方法 onCreateView() 只有在需要显示 Fragment 时才会被调用，所以 dragger 配置更改后新创建的实例保持为空。

你能做什么？

如果您想将它添加到 UI，则只创建一个 CanvasFragment 实例。在 savedInstanceState != null 的情况下，找到已经存在的 Fragment，如下所示:

canvasFragment = (CanvasFragment)getSupportFragmentManager().findFragmentById(R.id.container_canvas);