我的代码有问题。通常活动生命周期是-在启动应用程序时:onCreate -

但由于某种原因，当我按下home键或任务切换键时，我的应用程序也会调用onDestroy，当我回到已经打开的应用程序时，它会调用onCreate。

我的应用程序有2个Activites：-LoginActivity，如果没有人登录，或者如果按下注销按钮，应该首先出现-MainActivity来处理主要功能。

请帮我确定是什么导致了这种不正常的生命周期行为。

希望如此，谢谢你！

主要活动:

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

public Helper helper = new Helper(this);

public static final String SP_DATA = "SP_DATA";
public static final String FB_LOGIN_STATUS = "FB_LOGIN_STATUS";
public static final String FLRT = "FLRT";

public SharedPreferences sharedPreferences;

public FragmentManager fragmentManager = getFragmentManager();

@Override
protected void onCreate(Bundle savedInstanceState) {
    Log.d(FLRT, "onCreate");
    helper.makeToast("onCreate");

    sharedPreferences = getSharedPreferences(SP_DATA, MODE_PRIVATE);
    boolean bAlreadyLoggedIn = sharedPreferences.getBoolean(FB_LOGIN_STATUS, false);

    if (bAlreadyLoggedIn) {
        Log.d(FLRT, "Already logged in");
    }
    else {
        Log.d(FLRT, "Not logged in");

        Intent intent = new Intent(MainActivity.this, LoginActivity.class);
        startActivity(intent);
    }

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);


    showFragment(new FirstFragment(),"FirstFragment");

    Button btnLogout = (Button) findViewById(R.id.btnLogout);

    btnLogout.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            logout();

            showActivity(MainActivity.this, LoginActivity.class);
        }
    });

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    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);
}

private void showActivity(Context context_fromActivity, Class<?> class_toActivty){
    Intent intent = new Intent(context_fromActivity, class_toActivty);
    //intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

    startActivity(intent);

    MainActivity.this.finish();
}

private void logout() {
    Log.d(FLRT, "Logging out...");

    sharedPreferences = getSharedPreferences(SP_DATA, MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();
    editor.putBoolean(FB_LOGIN_STATUS, false);
    editor.commit();
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else if (getFragmentManager().getBackStackEntryCount() > 1 ){
        getFragmentManager().popBackStack();
    } 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.main, 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) {
    switch (item.getItemId()) {
        case R.id.nav_first_layout:
            showFragment(new FirstFragment(), "FirstFragment");
            break;
        case R.id.nav_second_layout:
            showFragment(new SecondFragment(), "SecondFragment");
            break;
        case R.id.nav_third_layout:
            showFragment(new ThirdFragment(), "ThirdFragment");
            break;
        case R.id.nav_share:

            break;
        case R.id.nav_send:

            break;
    }

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

private void showFragment(Fragment fragment, String sFragmentTAG) {
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

    if (fragmentManager.findFragmentByTag(sFragmentTAG) != null) {
        Log.d(FLRT, "Fragment found, using existing one: " + sFragmentTAG);
        fragment = fragmentManager.findFragmentByTag(sFragmentTAG);
    }

    fragmentTransaction.replace(R.id.fragmentContainer, fragment, sFragmentTAG);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();
}

@Override
protected void onStart() {
    super.onStart();
    Log.d(FLRT, "onStart");
    helper.makeToast("onStart");
}

@Override
protected void onResume() {
    super.onResume();
    Log.d(FLRT, "onResume");
    helper.makeToast("onResume");
}

@Override
protected void onPause() {
    super.onPause();
    Log.d(FLRT, "onPause");
    helper.makeToast("onPause");
}

@Override
protected void onStop() {
    super.onStop();
    Log.d(FLRT, "onStop");
    helper.makeToast("onStop");
}

@Override
protected void onDestroy() {
    super.onDestroy();
    Log.d(FLRT, "onDestroy");
    helper.makeToast("onDestroy");
}
}

登录活动：

public class LoginActivity extends AppCompatActivity {

public static final String SP_DATA = "SP_DATA";
public static final String FB_LOGIN_STATUS = "FB_LOGIN_STATUS";
public static final String FLRT = "FLRT";

public SharedPreferences sharedPreferences;

private Button btn_start;

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

    btn_start = (Button) findViewById(R.id.btn_start);

    btn_start.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            login();

            showActivity(LoginActivity.this, MainActivity.class);;
        }
    });
}

private void showActivity(Context context_fromActivity, Class<?> class_toActivty){
    Intent intent = new Intent(context_fromActivity, class_toActivty);
    //intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

    startActivity(intent);

    LoginActivity.this.finish();
}

private void login() {
    Log.d(FLRT, "Logging in...");

    sharedPreferences = getSharedPreferences(SP_DATA, MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();
    editor.putBoolean(FB_LOGIN_STATUS, true);
    editor.commit();
}
}