How to show the 1st fragment when back button is pressed? - xamarin.android

Question

I'm new to Mobile Development
I am currently developing a Telephone Directory app from Xamarin Android, in this app, I have 4 Fragment(HomeFragment, AboutFragment, DirectoryFragment, and SyncFragment).
HomeFragment is the first fragment that shows from the app. When I click the item About in the side bar then pressed the back button, it works well because i used AddToBackStack(null) before commit() in the main activity.
But the problem is, let's say I open the app then it shows the HomeFragment fist by default, when i navigate to the AboutFragment, it shows the AboutPage, then if I navigate to the DirectoryFragment next and click the back button, i keep on going back to the AboutPage/AboutFragment, which is what i want to achieve is, it should go back to the HomePage/HomeFragment
In short, what i want to achieve is like the Navigation behavior of the Gmail App.
Anyway this is my code in Main Activity
switch (e.MenuItem.ItemId)
{
case (Resource.Id.nav_home):
FragmentTransaction ft = FragmentManager.BeginTransaction();
HomeFragment home = new HomeFragment();
ft.Replace(Resource.Id.HomeFrameLayout, home);
HideSoftKeyboard();
mDrawerLayout.AddDrawerListener(mDrawerToggle);
ft.AddToBackStack(null);
ft.Commit();
break;
case (Resource.Id.nav_about):
FragmentTransaction ft1 = FragmentManager.BeginTransaction();
AboutFragment about = new AboutFragment();
ft1.Replace(Resource.Id.HomeFrameLayout, about);
HideSoftKeyboard();
ft1.AddToBackStack(null);
ft1.Commit();
break;
case (Resource.Id.nav_etel):
FragmentTransaction ft2 = FragmentManager.BeginTransaction();
GHQFragment ghq = new DirectoryFragment();
ft2.Replace(Resource.Id.HomeFrameLayout, ghq);
HideSoftKeyboard();
ft2.AddToBackStack(null);
ft2.Commit();
break;
case (Resource.Id.nav_refresh):
if (CrossConnectivity.Current.IsConnected)
{
FragmentTransaction ft3 = FragmentManager.BeginTransaction();
SyncFragment sync = new SyncFragment();
ft3.Replace(Resource.Id.HomeFrameLayout, sync);
HideSoftKeyboard();
ft3.AddToBackStack(null);
ft3.Commit();
}
else
{
Toast.MakeText(this, "Please connect to the internet to sync records.", ToastLength.Long).Show();
}
break;

Answer 1

You can override the OnKeyDown method like following code.
public override bool OnKeyDown([GeneratedEnum] Keycode keyCode, KeyEvent e)
{
if (keyCode == Keycode.Back)
{
FragmentManager fragmentManager = this.FragmentManager;
int count = fragmentManager.BackStackEntryCount;
for (int i = 0; i < count; ++i)
{
fragmentManager.PopBackStack();
}
// your code
return false;
}
return base.OnKeyDown(keyCode, e);
}
I switch three fragment, when I click the back button. it switch to the first fragment.
Update
Do you want to achieve the result that you click back button then back to the desktop when you in the HomePage?
Here is code.
public override bool OnKeyDown([GeneratedEnum] Keycode keyCode, KeyEvent e)
{
if (keyCode == Keycode.Back)
{
FragmentManager fragmentManager = this.FragmentManager;
int count = fragmentManager.BackStackEntryCount;
if(count>0){
for (int i = 0; i < count; ++i)
{
fragmentManager.PopBackStack();
}
return false;
}
}
return base.OnKeyDown(keyCode, e);
}