Android Fragment回退

MainActivity初始化和监听返回键
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mFragmentManager = getSupportFragmentManager();
        addFragment(new LoginFragment(), "login");
    }
    public void addFragment(Fragment fragment, String tag) {
        FragmentTransaction beginTransaction = mFragmentManager.beginTransaction();
        beginTransaction.add(R.id.login_content, fragment, tag);
        // 添加到回退栈,并定义标记
        beginTransaction.addToBackStack(tag);
        beginTransaction.commit();
    }
    /**
     * 监听返回键
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            // 获取当前回退栈中的Fragment个数
            int backStackEntryCount = mFragmentManager.getBackStackEntryCount();
            // 回退栈中至少有多个fragment,栈底部是首页
            if (backStackEntryCount > 1) {
                // 回退一步
                mFragmentManager.popBackStackImmediate();
                // 获取当前退到了哪一个Fragment上,重新获取当前的Fragment回退栈中的个数
                FragmentManager.BackStackEntry backStack = mFragmentManager.getBackStackEntryAt(mFragmentManager.getBackStackEntryCount() - 1);
                // 获取当前栈顶的Fragment的标记值
                String tag = backStack.getName();
                if (tag.equals("login")) {
                    if (isLogin) {
                        addFragment(new LoginFragment(), "login");
                        isLogin = false;
                    } else
                        finish();
                }
            } else {
                finish();
            }
        }
        return true;
    }

@OnClick({R.id.btn_register})
    public void onViewClicked(View view) {
        switch (view.getId()) {
            case R.id.btn_register:
                LoginActivity activity = (LoginActivity) getActivity();
                activity.addFragment(new RegisterFragment(), "register");
                FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
                fragmentTransaction.hide(getActivity().getSupportFragmentManager().findFragmentByTag("login"));
                fragmentTransaction.commit();
                break;
        }
    }

@OnClick({R.id.btn_back})
    public void onViewClicked(View view) {
        switch (view.getId()) {
            case R.id.btn_back:
                FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
                fragmentTransaction.show(getActivity().getSupportFragmentManager().findFragmentByTag("login"));
                fragmentTransaction.hide(getActivity().getSupportFragmentManager().findFragmentByTag("register"));
                fragmentTransaction.commit();
                break;
        }
    }