1.
One Activity to rule them all
How to architect a mobile app on the fast
&
What to avoid to keep it clean

2.
The problem
● Android framework does not impose
structure/architecture
● Any sensible application needs some form of a
priori architectural thinking/setup
● Creating a heavily outlined and well-thought-out
architecture is often overkill for simple apps

3.
Outline
● One screen = one fragment + one layout file
● Activity is used to orchestrate flow between
fragments/screens
● Activity also supplies all data needed to the
screens, and handles all API to underlying data,
network etc. layers

4.
SAMF diagram

5.
Pros & Cons
+ Quick setup
+ Modular
+ Easy to implement views across multiple screens
- Can be unwieldy with too many screens
- Complex UI flows hard to handle
- Dealing with state loss can be messy
Use for simple, contained screens without too many animations

6.
// Get the code: https://github.com/olrandir/samf_3aadm
// com.olrandir.samf.fragments.HomeFragment
public class HomeFragment extends BaseFragment {
...
public interface HomeActions {
public SamfPerson getStoredPerson();
public void goToProfile();
}
}
// ==============================
// com.olrandir.samf.MainActivity
public class MainActivity
extends AppCompatActivity
implements HomeFragment.HomeActions
{
....
@Override
public SamfPerson getStoredPerson() {
return getUser();
}
@Override
public void goToProfile() {
transitionTo(new ProfileFragment());
}
}
The fragment defnes an
interface with thefunctionality it requires
The activity implements it

7.
// com.olrandir.samf.fragments.HomeFragment
import com.olrandir.samf.models.SamfPerson;
public class HomeFragment extends BaseFragment {
....
@Override
public void onStart(){
super.onStart();
// Initialize our data
SamfPerson user = ((HomeActions)getActivity()).getStoredPerson();
String name = getString(R.string.home_text_greeting_nameUnknown);
// Fill in fields with initial data, if available
if( user != null && !TextUtils.isEmpty(user.getName()) ){
name = user.getName();
}
mGreeting.setText(
getString(R.string.home_text_greeting).replace("$1", name)
);
}
}

8.
// com.olrandir.samf.fragments.ProfileFragment
public class ProfileFragment extends BaseFragment {
....
public interface ProfileActions {
public SamfPerson getStoredPerson();
public void setStoredPerson(SamfPerson person);
public void goToHome();
}
}
// ==============================
// com.olrandir.samf.MainActivity
public class MainActivity extends AppCompatActivity
implements ProfileFragment.ProfileActions,
HomeFragment.HomeActions
{
....
@Override
public void setStoredPerson(SamfPerson person) {
storeUser(person);
}
@Override
public void goToHome() {
transitionTo(new HomeFragment());
}
}
Note the
accumulation
of “implements”
clauses!

9.
Activity views
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout ...>
<FrameLayout
android:id="@+id/mainactivity_content"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
<FloatingActionButton
android:id="@+id/activity_button_edit"
... />
</RelativeLayout>

10.
// com.olrandir.samf.MainActivity
public class MainActivity extends AppCompatActivity
implements ProfileFragment.ProfileActions, HomeFragment.HomeActions {
BaseFragment currentScreen;
@Override
protected void onCreate(Bundle savedInstanceState) {
...
// Create first screen
if( currentScreen==null ){
currentScreen = new HomeFragment();
}
getSupportFragmentManager().beginTransaction()
.replace(R.id.mainactivity_content, currentScreen).commit();
// Add a backstack listener
getSupportFragmentManager().addOnBackStackChangedListener(
new OnBackStackChangedListener() {
public void onBackStackChanged() {
currentScreen = (BaseFragment) getSupportFragmentManager()
.findFragmentById(R.id.mainactivity_content);
}
}
);
}
}
Get a handle for thecurrent screen fragment

11.
// com.olrandir.samf.MainActivity
public class MainActivity
extends AppCompatActivity
implements ProfileFragment.ProfileActions, HomeFragment.HomeActions,
View.OnClickListener {
BaseFragment currentScreen;
FloatingActionButton editButton;
@Override
protected void onCreate(Bundle savedInstanceState) {
...
// Initialize views belonging to the main activity
editButton = (FloatingActionButton)
findViewById(R.id.activity_button_edit);
editButton.setOnClickListener(this);
}
@Override
public void onClick(View v) {
if( currentScreen != null ){
currentScreen.onFABClick();
}
}
}
The activity creates the
view, then delegates its
interactions to the current
fragment

12.
// com.olrandir.samf.fragments.HomeFragment
public class HomeFragment extends BaseFragment {
...
@Override
public void onFABClick() {
((HomeActions)getActivity()).goToProfile();
}
}
// ===========================================
// com.olrandir.samf.fragments.ProfileFragment
public class ProfileFragment extends BaseFragment {
...
@Override
public void onFABClick() {
showMessage(“You’re already in the profile!”);
}
}
Different fragments
can have different
actions for the button

13.
// com.olrandir.samf.fragments.BaseFragment
public abstract class BaseFragment extends Fragment {
...
public abstract void onFABClick();
}
// ========================================
// com.olrandir.samf.fragments.HomeFragment
public class HomeFragment extends BaseFragment {
...
@Override
public void onFABClick() {
((HomeActions)getActivity()).goToProfile();
}
}
Fragments have interfaces;
the activity can defne its
API in a parent fragment all
screens inherit from

14.
Mitigating
If you have many screens, the activity can become unwieldy (many
interfaces & functions to implement).
Solutions:
1. Use a single interface for all fragments; this allows you to be more
specific and careful with your Fragment-Activity API.
2. Break the pattern: use >1 activities, and group similar fragments
and 1 activity into flows
e.g. LoginActivity with LoginFragment and ForgotPasswordFragment
Luckily, it’s relatively easy to achieve this: have all activities extend a
parent Activity, implement lower layer APIs there.

15.
Thank you!
dimitris@forky.gr | github: olrandir
code available: https://github.com/olrandir/samf_3aadm
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