先说需求吧：应用当前在FirstFragment，然后回到桌面，然后回到应用，回到应用需要显示SecondFragment

然后说下实现：Activity启动模式是singleTask，这样再次启动会走onNewIntent ，然后再onNewIntent里处理intent，根据传过来的type来显示Fragment。

说下问题： 每次回来的时候会看到FirstFragment切换到SecondFragment的痕迹，就是会短暂的显示一下FirstFragment再切换到SecondFragment。

下面是实现代码：

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";
    private FirstFragment mFirstFragment;
    private SecondFragment mSecondFragment;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mFirstFragment = new FirstFragment();
        mSecondFragment = new SecondFragment();
    }

    @Override
    protected void onStart() {
        super.onStart();
        Log.d(TAG, "onStart: ");
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        Log.d(TAG, "onRestart: ");
    }

    @Override
    protected void onResume() {
        super.onResume();
        Log.d(TAG, "onResume: ");
    }



    public void toFirstFragment(View view){
        switchFragment(mFirstFragment);
    }

    public void toSecondFragment(View view){
        switchFragment(mSecondFragment);
    }

    private Fragment lastOneFragment = null;
    private void switchFragment(Fragment currentFragment) {
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        if(!currentFragment.isAdded()){
            transaction.add(R.id.layout_container,currentFragment);
        }
        if (lastOneFragment!=null){
            transaction.hide(lastOneFragment);
        }
        lastOneFragment=currentFragment;
        transaction.show(currentFragment);
        transaction.commit();
    }

    @Override
    protected void onNewIntent(Intent intent) {
        super.onNewIntent(intent);
        Log.d(TAG, "onNewIntent: ");

        processIntent(intent);

    }

    private void processIntent(Intent intent) {
        int type = intent.getIntExtra("type", 0);
        Log.d(TAG, "processIntent: type = "+type);
        switch (type){
            case 1:
                switchFragment(mFirstFragment);
                break;
            case 2:
                switchFragment(mSecondFragment);
                break;
        }
    }
}

下面是输出的log：

2019-01-01 09:50:37.044 11798-11798/com.example.fragmenttest D/MainActivity: onNewIntent: 

2019-01-01 09:50:37.044 11798-11798/com.example.fragmenttest D/MainActivity: processIntent: type = 2

2019-01-01 09:50:37.062 11798-11798/com.example.fragmenttest D/SecondFragment: onStart: 

2019-01-01 09:50:37.066 11798-11798/com.example.fragmenttest D/MainActivity: onResume: 

2019-01-01 09:50:37.068 11798-11798/com.example.fragmenttest D/FirstFragment: onResume: 

2019-01-01 09:50:37.069 11798-11798/com.example.fragmenttest D/SecondFragment: onResume: