从一个片段传递到另一个片段

Question

我在这里尝试创建一个菜单，在点击时从一个片段传递到另一个片段。当我按下菜单中的特定项目（活动）时，活动退出，故障是

    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();// this line has the problem

这是我完整的 logcat：

FATAL EXCEPTION: main                                                                               java.lang.NullPointerException                                        

在 android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:414) 在 android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:449) 在 android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:441) atbr.exemplozxingintegration.MainActivity.selectDrawerItem（MainActivity.java:84 atbr.exemplozxingintegration.MainActivity$1.onNavigationItemSelected(MainActivity.java:50) 在 android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:150) 在 android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811) 在 android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152) 在 android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958) 在 android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:308) 在 android.view.View.performClick(View.java:4439) 在 android.view.View$PerformClick.run(View.java:18395) 在 android.os.Handler.handleCallback(Handler.java:725) 在 android.os.Handler.dispatchMessage(Handler.java:92) 在 android.os.Looper.loop(Looper.java:176) 在 android.app.ActivityThread.main(ActivityThread.java:5319) 在 java.lang.reflect.Method.invokeNative(Native Method) 在 java.lang.reflect.Method.invoke(Method.java:511) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869) 在 dalvik.system.NativeStart.main(Native Method)

MainActivity.java

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        txResult = (TextView) findViewById(R.id.txResult);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        nvDrawer = (NavigationView) findViewById (R.id.nvView);
        setupDrawerContent(nvDrawer);}



    private void setupDrawerContent(NavigationView navigationView) {
        navigationView.setNavigationItemSelectedListener(
                new NavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(MenuItem menuItem) {
                        selectDrawerItem(menuItem);
                        return true;
                    }
                });
    }

    public void selectDrawerItem(MenuItem menuItem) {
        // Create a new fragment and specify the planet to show based on
        // position
        Fragment fragment = null;

        Class fragmentClass;
        switch(menuItem.getItemId()) {
            case R.id.nav_first_fragment:
                fragmentClass = MainActivity.class;
                break;
            case R.id.nav_second_fragment:
                fragmentClass = SecondActivity.class;
                break;
            case R.id.nav_third_fragment:
                fragmentClass = ThirdActivity.class;
                break;
            default:
                fragmentClass = MainActivity.class;
        }

        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Insert the fragment by replacing any existing fragment
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

        // Highlight the selected item, update the title, and close the drawer
        menuItem.setChecked(true);
        setTitle(menuItem.getTitle());
        mDrawer.closeDrawers();
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // The action bar home/up action should open or close the drawer.
        switch (item.getItemId()) {
            case android.R.id.home:
                mDrawer.openDrawer(GravityCompat.START);
                return true;
        }

        return super.onOptionsItemSelected(item);
    }

   // @Override
   // protected void onPostCreate(Bundle savedInstanceState) {
   //     super.onPostCreate(savedInstanceState);
   // 

SecondActivity.java

 private void setupDrawerContent(NavigationView navigationView) {
        navigationView.setNavigationItemSelectedListener(
                new NavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(MenuItem menuItem) {
                        selectDrawerItem(menuItem);
                        return true;
                    }
                });
    }

    public void selectDrawerItem(MenuItem menuItem) {
        // Create a new fragment and specify the planet to show based on
        // position
        Fragment fragment = null;

        Class fragmentClass;
        switch(menuItem.getItemId()) {
            case R.id.nav_first_fragment:
                fragmentClass = MainActivity.class;
                break;
            case R.id.nav_second_fragment:
                fragmentClass = SecondActivity.class;
                break;
            case R.id.nav_third_fragment:
                fragmentClass = ThirdActivity.class;
                break;
            default:
                fragmentClass = SecondActivity.class;
        }

        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Insert the fragment by replacing any existing fragment
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

        // Highlight the selected item, update the title, and close the drawer
        menuItem.setChecked(true);
        setTitle(menuItem.getTitle());
        mDrawer.closeDrawers();
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // The action bar home/up action should open or close the drawer.
        switch (item.getItemId()) {
            case android.R.id.home:
                mDrawer.openDrawer(GravityCompat.START);
                return true;
        }

        return super.onOptionsItemSelected(item);
    }

    // @Override
    // protected void onPostCreate(Bundle savedInstanceState) {
    //     super.onPostCreate(savedInstanceState);
    // }

Answer 1

您的问题不清楚，因为您没有发布完整的课程代码，以及您遇到了什么错误。

我所理解的是，在选择导航抽屉项时要更改片段。类似于 Gmail 应用，当用户从收件箱文件夹移动到已发送文件夹或任何其他文件夹时。

如果你想这样做，你需要做一个活动，插入一个片段。选择导航抽屉中的项目时，应根据导航抽屉中选择的选项将当前片段替换为新片段。

检查以下两个链接，这将为您提供完整的想法。

链接1：https://androidbelieve.com/material-navigation-drawer-with-header/

链接2：http://joerichard.net/android/android-how-to-change-fragments-when-navigation-drawer-item-selected/

Answer 2

FragmentTransaction.replace 调用remove(Fragment) 然后add(int, Fragment, String) 实际替换片段。因此，在您的情况下，容器视图中可能存在 no 片段，因此您正在调用 remove(null)。因此，请检查您是否必须添加或替换片段。

您可以检查容器视图中是否有任何片段并进行相应操作：

 FragmentManager fragmentManager = getSupportFragmentManager();
 android.support.v4.app.Fragment currentFragment = getSupportFragmentManager().findFragmentById(R.id.flContent);
 if(currentFragment==null){ 
    fragmentManager.beginTransaction().add(R.id.flContent, fragment).commit();
 }else{
   fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
 }

希望这能解决您的问题。