java - 如何使用 onCreate 方法在 MainActivity 中打开 fragment

Question

我创建了一个 Drawer Activity ，现在我有 MainActivity 和 RegisterFragment。 XML 和其他事情现在已经完成，我可以使用抽屉导航打开 RegisterFragment，但是如何在应用程序启动时显示 RegisterFragment？

主要 Activity

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();
        navigationView.setNavigationItemSelectedListener(this);
    }


//unnecessary code block removed

 @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        Fragment fragment = null;

        if (id == R.id.nav_home) {

        } else if (id == R.id.nav_gallery) {

            fragment = new RegisterFragment();

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_tools) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        if (fragment != null){
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = 
            fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.screen_area, fragment);
            fragmentTransaction.commit();
        }

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }

正如我所说，它基本上运行良好，我想在应用程序启动时打开 RegisterFragment。

Answer 1

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();
        navigationView.setNavigationItemSelectedListener(this);

       // change fragment at onCreate method
       changeFragment(new RegisterFragment());
    }


private void changeFragement(Fragment fragment){
      if (fragment != null){
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = 
        fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.screen_area, fragment);
        fragmentTransaction.commit();
    }
}


//unnecessary code block removed

 @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        Fragment fragment = null;

        if (id == R.id.nav_home) {

        } else if (id == R.id.nav_gallery) {

            fragment = new RegisterFragment();

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_tools) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        changeFragment(fragment);

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }