当我尝试在片段中显示或隐藏浮动操作按钮时出错

Question

我试图从一个片段移动到另一个隐藏浮动操作按钮的片段，然后 go 从第二个片段返回到第一个片段并再次显示按钮。 浮动操作按钮位于主要活动内。

Mainactivity.java：

package cl.itn.cam;

import android.os.Bundle;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;
import androidx.navigation.fragment.NavHostFragment;

import android.view.View;

public class NCR extends AppCompatActivity {

    FragmentTransaction transaction;
    Fragment fragmentFirst, fragmentSecond;
    FloatingActionButton fab = findViewById(R.id.fab);

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

        fragmentFirst = new FirstFragment();
        fragmentSecond = new SecondFragment();


    }

    public FloatingActionButton getFloatingActionButton() {
        return fab;

    }

    public void showFloatingActionButton() {
        fab.show();
    }

    public void hideFloatingActionButton() {
        fab.hide();
    }

}

FirstFragment.java：

public class FirstFragment extends Fragment {

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {


        final FloatingActionButton fab = ((NCR) getActivity()).getFloatingActionButton();

        if (fab != null) {
            ((NCR) getActivity()).hideFloatingActionButton();
        }

        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_first, container, false);

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        view.findViewById(R.id.button_first).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                NavHostFragment.findNavController(FirstFragment.this)
                        .navigate(R.id.action_FirstFragment_to_SecondFragment);
            }
        });
    }
}

SecondFragment.java：

public class SecondFragment extends Fragment {

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {


        final FloatingActionButton fab = ((NCR) getActivity()).getFloatingActionButton();

        if (fab != null) {
            ((NCR) getActivity()).showFloatingActionButton();
        }



        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_second, container, false);
    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        view.findViewById(R.id.button_second).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(SecondFragment.this)
                        .navigate(R.id.action_SecondFragment_to_FirstFragment);
            }
        });
    }

应用程序在尝试使用以下堆栈跟踪显示主要活动时崩溃：

** 2021-03-19 11:04:56.979 32471-32471/cl.itn.cam E/AndroidRuntime：致命异常：主进程：cl.itn.cam，PID：32471 java.lang.RuntimeException：无法实例化活动ComponentInfo{cl.itn.cam/cl.itn.cam.NCR}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3401) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3620) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) at android .app.se rvertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2183) at android.os .Handler.dispatchMessage(Handler.java:107) at android.os.Looper.loop(Looper.java:241) at android.app.ActivityThread.main(ActivityThread.java:7604) at java.lang.reflect.Method. invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at Z4D236D9A2D102C5FE6AD1C5 0DA4BEC50Z.android.internal.os.ZygoteInit.main(ZygoteInit.java:941) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()'在 null object 参考 **

Answer 1

尝试更换

final FloatingActionButton fab = ((NCR) getActivity()).getFloatingActionButton();

和

FloatingActionButton fab = findViewById(R.id.fab);

在你的片段中。 这应该消除错误

Answer 2

解决了！

在公共 class NCR，我替换了“FloatingActionButton fab = findViewById(R.id.fab);” 通过“FloatingActionButton fab;”，然后在 onCreate 事件中我添加了“fab = findViewById(R.id.fab);”

谢谢！