0. TL;DR
For the impatient coder, a working version of the following implementation can be found on GitHub. This is the same answer written on another Stack Overflow post.
After rewriting the login activity code several times in many different apps, the easy (and not so elegant) solution was create the Google API client as a Application class object. But, since the connection state affect the UX flow, I never was happy about with this approach.
Reducing our problem only to the connection concept, we may consider that:
- It hides the Google API client.
- It has finite states.
- It is a (rather) unique.
- The current state affect the behavior of the app.
1. Proxy Pattern
Since the Connection encapsulates the GoogleApiClient, it will implement the ConnectionCallbacks and OnConnectionFailedListener:
@Override
public void onConnected(Bundle hint) {
changeState(State.OPENED);
}
@Override
public void onConnectionSuspended(int cause) {
changeState(State.CLOSED);
connect();
}
@Override
public void onConnectionFailed(ConnectionResult result) {
if (currentState.equals(State.CLOSED) && result.hasResolution()) {
changeState(State.CREATED);
connectionResult = result;
} else {
connect();
}
}
Activities can communicate to the Connection class through the methods connect, disconnect, and revoke, but their behaviors are decided by the current state. The following methods are required by the state machine:
protected void onSignIn() {
if (!googleApiClient.isConnected() && !googleApiClient.isConnecting()) {
googleApiClient.connect();
}
}
protected void onSignOut() {
if (googleApiClient.isConnected()) {
Plus.AccountApi.clearDefaultAccount(googleApiClient);
googleApiClient.disconnect();
googleApiClient.connect();
changeState(State.CLOSED);
}
}
protected void onSignUp() {
Activity activity = activityWeakReference.get();
try {
changeState(State.OPENING);
connectionResult.startResolutionForResult(activity, REQUEST_CODE);
} catch (IntentSender.SendIntentException e) {
changeState(State.CREATED);
googleApiClient.connect();
}
}
protected void onRevoke() {
Plus.AccountApi.clearDefaultAccount(googleApiClient);
Plus.AccountApi.revokeAccessAndDisconnect(googleApiClient);
googleApiClient = googleApiClientBuilder.build();
googleApiClient.connect();
changeState(State.CLOSED);
}
2. State Pattern
This is a behavioral pattern the allow an object to alter its behavior when its internal state changes. The GoF Design Patterns book describes how a TCP connection can be represent by this pattern (which is also our case).
A state from a state machine should be a singleton, and the easiest away of doing it in Java was to create Enum named State as follows:
public enum State {
CREATED {
@Override
void connect(Connection connection) {
connection.onSignUp();
}
@Override
void disconnect(Connection connection) {
connection.onSignOut();
}
},
OPENING {},
OPENED {
@Override
void disconnect(Connection connection) {
connection.onSignOut();
}
@Override
void revoke(Connection connection) {
connection.onRevoke();
}
},
CLOSED {
@Override
void connect(Connection connection) {
connection.onSignIn();
}
};
void connect(Connection connection) {}
void disconnect(Connection connection) {}
void revoke(Connection connection) {}
The Connection class holds the context, i.e. the current state, which defines how the Connection methods connect, disconnect, and revoke will behave:
public void connect() {
currentState.connect(this);
}
public void disconnect() {
currentState.disconnect(this);
}
public void revoke() {
currentState.revoke(this);
}
private void changeState(State state) {
currentState = state;
setChanged();
notifyObservers(state);
}
3. Singleton Pattern
Since there is not need to recreate this class repeatedly, we provide it as a singleton:
public static Connection getInstance(Activity activity) {
if (null == sConnection) {
sConnection = new Connection(activity);
}
return sConnection;
}
public void onActivityResult(int result) {
if (result == Activity.RESULT_OK) {
changeState(State.CREATED);
} else {
changeState(State.CLOSED);
}
onSignIn();
}
private Connection(Activity activity) {
activityWeakReference = new WeakReference<>(activity);
googleApiClientBuilder = new GoogleApiClient
.Builder(activity)
.addConnectionCallbacks(this)
.addOnConnectionFailedListener(this)
.addApi(Plus.API, Plus.PlusOptions.builder().build())
.addScope(new Scope("email"));
googleApiClient = googleApiClientBuilder.build();
currentState = State.CLOSED;
}
4. Observable Pattern
The Connection class extends Java Observable, so 1 or more activities can observe the state changes:
@Override
protected void onCreate(Bundle bundle) {
connection = Connection.getInstance(this);
connection.addObserver(this);
}
@Override
protected void onStart() {
connection.connect();
}
@Override
protected void onDestroy() {
connection.deleteObserver(this);
connection.disconnect();
}
@Override
protected void onActivityResult(int request, int result, Intent data) {
if (Connection.REQUEST_CODE == request) {
connection.onActivityResult(result);
}
}
@Override
public void update(Observable observable, Object data) {
if (observable != connection) {
return;
}
// Your presentation logic goes here...
}
