Develop Reference

Back vs. Up - Intended behavior

The navigation design guide explains:
When the previously viewed screen is also the hierarchical parent of the current screen, pressing the Back button has the same result as pressing an Up button—this is a common occurrence.
up vs back - navigation guide
I have a MainActivity A which opens another activity B when touching a navigation entry in the NavigationDrawer. Activity A is set to be the parent of activity B in the AndroidManifest: android:parentActivityName=".MainActivity"
I followed this android documentaion to add up navigation to activity B. It shows how to implement onOptionsItemSelected in activity B:
override fun onOptionsItemSelected(item: MenuItem): Boolean {
when (item.itemId) {
android.R.id.home -> {
// Respond to the action bar's Up/Home button
NavUtils.navigateUpFromSameTask(this)
return true
}
}
return super.onOptionsItemSelected(item)
}
When I press back from Activity B the state of Activity A was saved and the NavigationDrawer is opened. If I use the up navigation though, onCreate()of activity A is called and it lost its state (the drawer is closed etc.).
This is not the quoted "same result".
When I replace the NavUtils.navigateUpFromSameTask(this) with a simple finish() it has the same behavior as pressing back - the state of activity A is kept.
Naturally I would prefer the way using finish. So what is the intended behavior? Do the guides contradict each other or am missing something?

It is an unfortunate reality that Google leaves documentation up for longer than it is relevant, and will even post two different pieces of documentation that directly contradict each other.
In the case of the Up button, your link says
The Up button appears in the app bar and is used to navigate within an app based on the hierarchical relationships between screens. [...]
The Back button appears in the system navigation bar and is used to navigate, in reverse chronological order, through the history of screens the user has recently worked with. It is generally based on the temporal relationships between screens, rather than the app's hierarchy.
However, there is also this article, which says
When the system Back button would not exit your app, such as when you are on your own task and not on the start destination, the Up button should function identically to the system Back button.
So... which one should you trust?
I assert that you should trust the second one. The first one was posted years ago; I don't know its exact age, but you can tell that it's old because the screenshots all use the Holo theme. The second one, on the other hand, is part of Android's Architecture Components, so is significantly newer. In general, I'd go with the newest piece of documentation.
Additionally, I think that Google was wrong to say for all these years that the Up button should work differently from the Back button. As someone who spent a lot of time thinking about navigation in my app, I see where they were coming from, but real-world users always get confused when Up did something "different".
So I'd go ahead and just finish() your activity when the user presses the Up button, and not worry about those two articles you found.

I think the change of policy to suggest consistency between the up icon and the system back button is a good idea. However the advice that you should:
"navigate in reverse-chronological order through the history of screens"
Is too crude, or at least should be clarified what they mean by "screen".
eg. When you have a bottom nav bar, the back button / up icon should take you back up the hierarchy within your tab section before jumping to the section previously visited. And you should preserve the previous state when revisiting a tab section (which may be drilled down into a lower level screen).

Android ActoinBar homeAsUp only works once

I'm working on an app that only uses a single Activity and switches out fragments as they are needed with the navigation drawer.
Now we want to navigate back from one of these fragments by using the homeAsUp button in the ActionBar.
I have followed all the steps to set the button up. From disabling the navigation drawer setDrawerIndicatorEnabled(false) and calling setDisplayHomeAsUpEnabled(true) in the fragment's onCreateView().
I also set setHomeButtonEnabled(true) in the MainActivity's onCreate() however because the app is already in the MainActivity, we cannot specify a Parent Activity.
Whenever I run a fresh install of the app, the homeAsUp button works and is registered in the onBackPressed(), not onOptionsItemSelected() method. However, when I close the app and run it again the button does not even register clicks.
In onBackPressed() I check a few conditions, but it does not block the button press.
In onOptionsItemSelected() I check for android.R.id.home.
Unfortunately, I cannot post the code.
This post describes what I'm trying to achieve: Switching between Android Navigation Drawer image and Up caret when using fragments

I managed to fix the issue I was experiencing. It was a very simple mistake.
Because I'm not the original author of this code, I went through the MainActivity thoroughly.
As it turns out the original author called setDisplayHomeAsUpEnabled in the onCreate function (which is extremely long), but near the end he also called setSupportActionBar, which made the first call of setDisplayHomeAsUpEnabled useless. Moving setDisplayHomeAsUpEnabled below setSupportActionBar fixed my problem.
If you did everything correctly, make sure that your code is written in the correct order.
Furthermore, if you use custom toolbars in other fragments, remember to set your original Support Action Bar by calling setSupportActionBar(toolbar.find(this)) in onResume of your MainActivity.

How do I exit a fragment and return to my drawer activity using the back button?

I'm trying to make an app which has the DrawerActivity as the main activity. I have implemented 8 fragments within it, which correspond to each item in the drawer. Now, the problem I have is, whenever I try to press the back button to go back to the DrawerActivity from the fragment, I end up exiting the app instead. I've been searching on forums for three days now, and have not found any solution to this. Quite frustrated, I stupidly deleted my code, so I can't really show it right now. Can anyone simply explain to me how I should exit a fragment using the back button, and return to my main activity?
Current scenario:
DrawerActivity contains eight fragments, all independent of each other. Let's call them fragments A through H. When I go to fragment A, I should be able to open the navigationDrawer and head to any other fragments from B to H. However, on pressing the back button, I should also be able to go back to the drawer activity's main page.
Things I have tried which didn't work are
1. Using onBackPressed and popBackStack.
2. Creating custom listener.
3. Using fragmentTransaction.

I can't believe how stupid I am. I was using my Gmail app on my phone when I suddenly realized that the first fragment in the drawer activity is always the default fragment, and the app should always revert to it on the back button press. What I was thinking was that the drawer activity would go back to the original state when back is pressed, but I didn't account for the fact that fragments are part of the activity itself.
Thank you all for trying to understand my problem, which wasn't really a problem at all... It seems I learned something in spite of myself.

Have you tried overriding onBackPressed() without calling the superclass method? (So that you prevent finish() from being called.)
#Override
public void onBackPressed() {
//show Fragment A
}
Call it without the "super.onBackPressed()" and it should work.

You must add your fragment to the android back Stack like this:
getSupportFragmentManager()
.beginTransaction()
.replace(R.id.your_container,fragment,fragment.TAG)
.addToBackStack(null)
.commit();

Display Navigation Drawer in each Activity after connecting Fragments with Activities (Android - Github tutorial)

I am trying to implement a navigation drawer (left slide menu) in my Android project. My application has a main activity till now, in which you could navigate to others with Intent method through buttons.
So, I searched for a navigation drawer and I found/followed the exact steps of THIS tutorial. My code related to the menu is exactly the same, so for that reason, I am not posting it below.
Related to my code:
The three fragments that I created, were just extended the class Fragment and where completely empty.
The ONLY change from the code in the tutorial, is inside the res/layout/activity_main.xml, because I changed the com.codepath.examples.navdrawerdemo.FragmentNavigationDrawer to 'my_project's_name'.FragmentNavigationDrawer.
What I managed to do:
I managed to create the menu that I want and display it ONLY if I make the MainActivity.java as the launcher inside the AndroidManifest. When I see the menu, I can go to each fragment through it, but I can't go to any of my Activities because I didn't link the fragments with the activities that I want. My menu, also does this transition that it should, so I am completely fine with the style of it.
What I want to do:
I just want to have the same menu inside every activity that I have already created, and when I click in an item in the menu, I want to navigate to an activity that I want BUT I want the activity to still display the menu.
I searched for solutions, but when I put back my first Activity as launcher in the Android Manifest, and set a layout for example inside the Fragment for it, it's just a useless layout. And when I tried to do an Intent to call my Activity inside the Fragment, the menu disappeared.
P.S I didn't changed any of my Activitie's code or of their layouts. I added only the code from the tutorial!

Android OnPrepareOptionsMenu does not work as expected on API 16

In my app I have one fragment that I add by calling replace() on fragmentTransaction.
This fragment, let´s call it Fragment1, has some menu items that are added to action bar by calling onCreateOptionsMenu and setHasOptionsMenu(true) in onCreate().
From this fragment I then add another fragment that is added by calling add() on FragmentTransaction.
This fragment2 also has options menu but removes options for the first fragment. Then, on API 19 when I click on Back Button, it takes me back to Fragment1 and the options for this menu are added back, since the activity that hosts both of these fragments calls onCreateOptionsMenu and then onPrepare options menu. Also the same methods are called in Fragment1.
When I run my app on API 16, all the mentioned methods are called as well, but either the Menu Items for Fragment1 are not added back at all, or the three-dot-menu-group is added to a wrong place. I assume there is something different in the lower API, maybe even a bug. How should I fix this issue? Anyone with some experience?
This one is what it looks like when going back from the second fragment ot the first one and what it should look like on API 16:
This is how it is supposed to work and what it looks like on API 19 after going back from the second fragment to the first one:
I don´t attach any code because I think it is not relevant for this question. There is just some different behavior in different platforms and I just don´t see what I should do about it. Thanks for any help.