The document discusses architectural practices for developing mobile applications on the Android platform, emphasizing the importance of having a structured architecture while avoiding over-complication for simple apps. It outlines a fragment-based approach where activities orchestrate data flow and manage UI interactions, along with code examples for implementing interfaces between fragments and activities. Additionally, it offers pros and cons of this architecture and suggests solutions for managing complexity in applications with many screens.

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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