android 抽屉导航每次都重新创建 fragment

Question

我正在加载一个简单的抽屉导航，里面有 3 个 fragment 。当我转到第一个 fragment 并单击该 fragment 中的一个按钮时，按钮的背景颜色会发生变化，然后我导航到第二个 fragment ，然后我又回到第一个 fragment ，我希望我的 fragment (带有颜色已更改的按钮)在后台.但它每次都会重新创建。我尝试了多种方法没有运气。由于我是 fragment 的新手，所以我坚持这一点。

public class SampleActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, Fragment1.OnFragmentInteractionListener,
        Fragment2.OnFragmentInteractionListener, Fragment3.OnFragmentInteractionListener {

    static Fragment fragment = null;

    private static final String FRAGMENT1_TAG = "FRAGMENT1_TAG";
    private static final String FRAGMENT2_TAG = "FRAGMENT2_TAG";
    private static final String FRAGMENT3_TAG = "FRAGMENT3_TAG";

    @Override
    public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
        super.onSaveInstanceState(outState, outPersistentState);
    }

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

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

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        // getMenuInflater().inflate(R.menu.dashboard, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

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

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        int id = item.getItemId();
        navigateToFragment(id);
        return true;
    }

    @Override
    public void onFragmentInteraction(Uri uri) {

    }

    public void navigateToFragment(int itemId) {
        // Handle navigation view item clicks here.
        String FRAGMENT_TAG="";

        switch (itemId) {

            case R.id.nav_fragment1:

                FRAGMENT_TAG = FRAGMENT1_TAG;
                fragment = Fragment1.newInstance();
                break;

            case R.id.nav_fragment2:

                FRAGMENT_TAG = FRAGMENT2_TAG;
                fragment = Fragment2.newInstance();
                break;

            case R.id.nav_fragment3:

                FRAGMENT_TAG = FRAGMENT3_TAG;
                fragment = Fragment3.newInstance();
                break;

            default:

                FRAGMENT_TAG = FRAGMENT1_TAG;
                fragment = Fragment1.newInstance();
                break;

        }

        FragmentManager supportFragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = supportFragmentManager.beginTransaction();
        Fragment inputFragment = supportFragmentManager.findFragmentByTag(FRAGMENT_TAG);

        if (inputFragment == null) {

            fragmentTransaction.replace(R.id.content_frame, fragment, FRAGMENT_TAG).addToBackStack(null).commit();

        } else {

            fragmentTransaction.replace(R.id.content_frame, inputFragment, FRAGMENT_TAG).addToBackStack(null).commit();

        }

        DrawerLayout drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawerLayout.closeDrawer(GravityCompat.START);
    }


}

Answer 1

您应该添加 Fragment2.newInstance(); 的代码以更好地了解行为。

这段代码可能会解决您的问题:

public class SampleActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, Fragment1.OnFragmentInteractionListener,
        Fragment2.OnFragmentInteractionListener, Fragment3.OnFragmentInteractionListener {

    static Fragment fragment1 = null;
    static Fragment fragment2 = null;
    static Fragment fragment3 = null;

    private static final String FRAGMENT1_TAG = "FRAGMENT1_TAG";
    private static final String FRAGMENT2_TAG = "FRAGMENT2_TAG";
    private static final String FRAGMENT3_TAG = "FRAGMENT3_TAG";

    @Override
    public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
        super.onSaveInstanceState(outState, outPersistentState);
    }

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

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

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        // getMenuInflater().inflate(R.menu.dashboard, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

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

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        int id = item.getItemId();
        navigateToFragment(id);
        return true;
    }

    @Override
    public void onFragmentInteraction(Uri uri) {

    }

    public void navigateToFragment(int itemId) {
        // Handle navigation view item clicks here.
        String FRAGMENT_TAG="";

        Fragment fragment = null;

        switch (itemId) {

            case R.id.nav_fragment2:

                FRAGMENT_TAG = FRAGMENT2_TAG;
                if(fragment2 == null) fragment2 = Fragment2.newInstance();
                fragment = fragment2;
                break;

            case R.id.nav_fragment3:

                FRAGMENT_TAG = FRAGMENT3_TAG;
                if(fragment3 == null) fragment3 = Fragment3.newInstance();
                fragment = fragment3;
                break;

            default:

                FRAGMENT_TAG = FRAGMENT1_TAG;
                if(fragment1 == null) fragment1 = Fragment1.newInstance();
                fragment = fragment1;
                break;

        }

        FragmentManager supportFragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = supportFragmentManager.beginTransaction();
        Fragment inputFragment = supportFragmentManager.findFragmentByTag(FRAGMENT_TAG);

        if (inputFragment == null) {

            fragmentTransaction.replace(R.id.content_frame, fragment, FRAGMENT_TAG).addToBackStack(null).commit();

        } else {

            fragmentTransaction.replace(R.id.content_frame, inputFragment, FRAGMENT_TAG).addToBackStack(null).commit();

        }

        DrawerLayout drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawerLayout.closeDrawer(GravityCompat.START);
    }
}

我建议您稍微重构一下代码以获得更优雅的解决方案。

请确保您是第一次使用 fragmentTransaction.add() 添加 fragment 。