我是Android新手。我想开发一个从片段回到主屏幕到退出应用程序的功能，但它不起作用。目前，一旦我按下后退按钮，它将提示退出应用程序。如何返回主活动以从onKeyDown退出应用程序？
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener, FragmentFlickrGridImage.OnFragmentInteractionListener
{
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
if (savedInstanceState == null) {
Fragment fragment = null;
Class fragmentClass = null;
fragment = new MyFragment("0");
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
}
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.setItemIconTintList(null);
navigationView.setNavigationItemSelectedListener(this);
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} 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.share) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
Fragment fragment = null;
Class fragmentClass = null;
String title="";
if (id == R.id.one) {
fragment = new MyFragment("1");
title="One";
} else if (id == R.id.two) {
fragment = new MyFragment("2");
title ="two";
} else if (id == R.id.three) {
fragment = new MyFragment("3");
title="three";
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
getSupportActionBar().setTitle(title);
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
public void onFragmentInteraction(Uri uri) {
}
// confirm when user click back key
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode != KeyEvent.KEYCODE_BACK) return super.onKeyDown(keyCode, event);
DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
switch (which){
case DialogInterface.BUTTON_POSITIVE:
//Yes button clicked
finish();
break;
case DialogInterface.BUTTON_NEGATIVE:
//No button clicked
break;
}
}
};
AlertDialog.Builder builder = new AlertDialog.Builder(this)
.setMessage("Are you sure?")
.setPositiveButton("Yes", dialogClickListener)
.setNegativeButton("No", dialogClickListener)
.setCancelable(false)
.setTitle("Exit");
builder.show();
return super.onKeyDown(keyCode, event);
}
下面是我在实现addtobackstack
时得到的错误代码Application is not responding: AppWindowToken{bcc0a65 token=Token{1a7f775c ActivityRecord{3ccf31cf u0 com.google.android.googlequicksearchbox/com.google.android.launcher.GEL t310}}}. It has been 5010.0ms since event, 5010.0ms since wait started.
Reason: Waiting because no window has focus but there is a focused application that may eventually add a window when it finishes starting up.
答案 0 :(得分：0)
替换片段时添加fragmentManager.addToBackStack(null);
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment);
fragmentManager.addToBackStack(null);
fragmentManager.commit();
通过调用addToBackStack（），替换事务将保存到后台，因此用户可以通过按“返回”按钮来反转事务并恢复上一个片段。
在onBackPressed()
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
// Add your Dialogue or whatever to alert
moveTaskToBack(true);
}
return super.onKeyDown(keyCode, event);
}
@Override
public void onBackPressed() {
super.onBackPressed();
onKeyDown(KeyEvent.KEYCODE_BACK, null);
}
答案 1 :(得分：0)
以下是我的解决方案
定义名为backstact_count的变量以保存已替换fragments的数量。
当您用另一个替换时，您需要将当前fragment添加到backstack
public void replaceFragment(Fragment fragment, boolean isAddToBackStack) {
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.flContent, fragment);
if (isAddToBackStack) {
transaction.addToBackStack(fragment.getClass().toString());
backstack_count ++;
}
transaction.commit();
}
然后覆盖onBackPressed（）
@Override
public void onBackPressed() {
if (backstack_count > 0) {
FragmentManager manager = getSupportFragmentManager();
manager.popBackStack();
backstack_count--;
} else {
super.onBackPressed();
}
}
最后，在onCreate（）中编辑替换的片段，如下所示：
replaceFragment(fragment, true);
答案 2 :(得分：0)
尝试在onBackPressed方法中调用moveTaskToBack(true)。包装逻辑以检查片段是否是您要从此方法周围退出的片段。看看我在这个问题上的答案：