Fragment Back-stack android

Question

我有三个Fragments。在每次单击按钮时，它将加载相应的Fragment。如果我再次尝试单击该按钮，它将重新创建一个新的Fragment并加载，但不是创建新的Fragment，而是需要显示现有的后筹码Fragment。

代码：

 public class MainActivity extends AppCompatActivity implements  View.OnClickListener {
    private Frag1 frag1;
    private  Frag2 frag2;
    private  Frag3 frag3;
    Button one,two,three;
    FragmentTransaction fragmentTransaction;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        registerViewComponent();
        registerClick();


    }

    private void registerClick() {
        one.setOnClickListener(this);
        two.setOnClickListener(this);
        three.setOnClickListener(this);
    }

    private void registerViewComponent() {
        one=(Button)findViewById(R.id.one);
        two=(Button)findViewById(R.id.two);
        three=(Button)findViewById(R.id.three);
    }

    @Override
    public void onClick(View view) {

        switch (view.getId())
        {
            case R.id.one:
                frag1=new Frag1();
                frag3=new Frag3();
                frag2=new Frag2();

                fragmentTransaction=getSupportFragmentManager().beginTransaction();


                Fragment fragment2 =        getSupportFragmentManager().findFragmentByTag("Frag1");
                Log.v("Tag","Tagg::"+fragment2);
                if (fragment2 instanceof Frag1){
                    Log.v("If","IF::"+fragment2);
                }
                else {


                    fragmentTransaction.add(R.id.stack_frame,frag1,"Frag1").addToBackStack("Frag1");
                    fragmentTransaction.commit();
                }

                break;
            case R.id.two:
      frag2=new Frag2();

                Fragment fragment = getSupportFragmentManager().findFragmentByTag("Frag2");
                Log.v("Tag","Tagg::"+fragment);
                if (fragment instanceof Frag2){
                    Log.v("If","IF::"+fragment);


                }
                else {

                    fragmentTransaction=getSupportFragmentManager().beginTransaction();
                    fragmentTransaction.add(R.id.stack_frame,frag2,"Frag2").addToBackStack("Frag2");
                    fragmentTransaction.commit();
                }
                break;
            case R.id.three:
                frag3=new Frag3();
                Fragment fragment1 = getSupportFragmentManager().findFragmentByTag("Frag3");
                Log.v("Tag","Tagg::"+fragment1);
                if (fragment1 instanceof Frag3){
                    Log.v("If","IF::"+fragment1);


                }
                else {

                    fragmentTransaction=getSupportFragmentManager().beginTransaction();
                    fragmentTransaction.add(R.id.stack_frame,frag3,"Frag3").addToBackStack("Frag3");

                    fragmentTransaction.commit();

                }

                break;
        }

    }

Answer 1

Frag3 frag3;     
Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.frame_main);
  if (fragment instanceof Frag3) {
     frag3 = (Frag3) fragment;
  } else {
     frag3 = new Frag3();
  }

-------------- OR ---------------------

 Frag3 frag3 = new Frag3();  
 List<Fragment> fragments = getSupportFragmentManager().getFragments();
 for(Fragment fragment : fragments){
   if(fragment instanceof Frag3){
    frag3 = (Frag3)fragment;
   }
 }

然后进行片段交易

fragmentTransaction=getSupportFragmentManager().beginTransaction(); 
fragmentTransaction.add(R.id.stack_frame,frag3,"Frag3").addToBackStack("Frag3");
fragmentTransaction.commit();