如何在多个片段中处理 onBackPressed()

Question

我是Android初学者，我正在开发一个应用程序，其中我通过一个活动使用了多个片段。

private void displaySelectedScreen(int itemId) {
Fragment fragment = null;
switch (itemId) {
case R.id.nav_home:
fragment = new HomeFragment();
break;
case R.id.nav_attendance:
fragment = new AttendanceFragment();
break;
case R.id.nav_notification:
fragment = new NotificationFragment();
break;
case R.id.nav_event:
fragment = new EventFragment();
break;
case R.id.nav_fee:
fragment = new FeesFragment();
break;
case R.id.nav_news:
fragment = new NewsFragment();
break;
}
if (fragment != null) {
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.frame_container, fragment);
ft.addToBackStack(null);
ft.commit();
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
}

如果抽屉已经关闭，请使用onBackPressed()方法关闭应用程序。现在我想关闭一个应用程序，如果当前片段HomeFragment并且当前片段不是HomeFragment则替换为HomeFragment。我不知道，它是怎么回事？

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

Answer 1

您可以向片段添加标签。 这样，您只需检查标签名称。

Fragment fragment = new Fragment();
getFragmentManager().beginTransaction()
.replace(R.id.FrameLayoutId,fragment,"TAG_NAME")
.addToBackStack("SOURCE_TAG_NAME").commit(); 

这是检查片段标签的方法，将此检查放在活动的反压方法或closeApp()方法

function void closeApp(){
Fragment fragment = getFragmentManager().findFragmentByTag("HomeActivity");
if (fragment != null) {
if(fragment.isVisible()){
HomeActivity.this.finish(); 
}
}
}

Answer 2

在 NavigationActivity 中使用这段代码 onBackPressed(( 方法。我希望这能解决你。

@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
Fragment f = getVisibleFragment();// to get which fragment now 
if (f.getClass() != MainFragment.class) {
Fragment fragment = null;
try {
fragment = (Fragment) MainFragment.class.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.frame, fragment).commit(); // this is your framelayout
}else {
super.onBackPressed();
}
}
}

getVisibleFragment(( 函数

public Fragment getVisibleFragment() {
FragmentManager fragmentManager = NavigationActivity.this.getSupportFragmentManager();
List<Fragment> fragments = fragmentManager.getFragments();
if (fragments != null) {
for (Fragment fragment : fragments) {
if (fragment != null && fragment.isVisible())
return fragment;
}
}
return null;
}

Answer 3

简单起见，只需转到您的HomeFragment.java并编辑方法onBackPressed()即可使用finish()退出应用程序。
而对于其他Fragment.java，只需使用popBackStack()功能，可以在这里搜索： https://developer.android.com/reference/android/app/FragmentManager.html#popBackStack((

Answer 4

在您的 MainActivity 类中覆盖onBackPressed并显示对话框，如下所示：

@Override
public void onBackPressed() {
//show dialog box here 
}

在每个Fragment使用以下代码实现onResume()方法：

@Override
public void onResume() {
super.onResume();
getView().setFocusableInTouchMode(true);
getView().requestFocus();
getView().setOnKeyListener(new View.OnKeyListener() {
@Override
public boolean onKey(View v, int keyCode, KeyEvent event) {
if (event.getAction() == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK){
//homefragment class name     
Class fragmentClass = HomeFragment.class;
try {
Fragment  fragment = (Fragment) fragmentClass.newInstance();
FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
} catch (Exception e) {
e.printStackTrace();
}
return true;
}
return false;
}
});
}