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Building Offline First Apps Saket Narayan @saketme uncommon.is

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Never make the user wait, • when consuming content • when producing content

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Parts 1. Architecture 2. User Experience 3. Libraries

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Part 1, Architecture

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Simplifying state management is the key to offline support

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But state management is difficult

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Source: Jake Wharton

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Source: Jake Wharton

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Solution, Persistence as the Single Source of Truth

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Some graphs to prove my point

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Time Productivity

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Time Productivity

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Time Productivity

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No content

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No content

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No content

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Usecases

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Usecase 1, Login

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LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; api.login(username, password, listener); The usual way

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LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; api.login(username, password, listener); The usual way

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LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; api.login(username, password, listener); The usual way

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LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; api.login(username, password, listener); The usual way

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LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; api.login(username, password, listener); The usual way

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Call call = api.login(…); void onDestroy() { call.cancel(); } What if the Activity gets destroyed?

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Memory leaks Lifecycle issues when using Fragments No support for orientation changes Response may come while Activity is minimized Problems

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void onSubmitClick() { LoginListener listener = (authToken) -> { database.saveAuthToken(authToken); startActivity(ContentFeedActivity.startIntent(this)); }; authRepository.login(username, password, listener); } The PSST way

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void onSubmitClick() { authRepository.login(username, password); } The PSST way

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void onSubmitClick() { authRepository.login(username, password); } @Singleton public class AuthRepository { api.login(username, password) .doOnSuccess(authToken -> database.saveAuthToken(authToken)) .subscribe(); } The PSST way

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void onSubmitClick() { authRepository.login(username, password); } @Singleton public class AuthRepository { api.login(username, password) .doOnSuccess(authToken -> database.saveAuthToken(authToken)) .subscribe(); } The PSST way

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void onStart() { authRepository.streamUserSession() .takeUntil(lifecycle.onStop()) .subscribe(userSession -> { if (userSession.isLoggedIn()) { // Start next Activity. } else { // Show login form. } }); } The PSST way

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void onStart() { authRepository.streamUserSession() .takeUntil(lifecycle.onStop()) .subscribe(userSession -> { if (userSession.isLoggedIn()) { // Start next Activity. } else { // Show login form. } }); } The PSST way Observable

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void onStart() { authRepository.streamUserSession() .takeUntil(lifecycle.onStop()) .subscribe(userSession -> { if (userSession.isLoggedIn()) { // Start next Activity. } else { // Show login form. } }); } The PSST way

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void onStart() { authRepository.streamUserSession() .takeUntil(lifecycle.onStop()) .subscribe(userSession -> { if (userSession.isLoggedIn()) { // Start next Activity. } else { // Show login form. } }); } The PSST way

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Flow of data is in a single place and not scattered. Every problem can be traced to an event, action, result, or model. Advantages

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Usecase 2, Trello

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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The user is being blocked by a non-dismissible progress indicator
 Chances of losing user input on process death/phone restart. Problems

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 NetworkListener listener = () -> {
 hideProgressDialog();
 
 database.saveCard(TrelloCard.create(title));
 reloadBoardFromDatabase();
 };
 
 showProgressDialog();
 api.saveNewCard(title, listener);
 } The usual way

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void onAddNewCard(String title) {
 repository.createNewCard(title);
 } PSST

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Repository.java void createNewCard(String name) {
 database.saveCard(TrelloCard.create(name))
 .andThen(scheduleSyncWithServer())
 .subscribe();
 } PSST

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Repository.java void createNewCard(String name) {
 database.saveCard(TrelloCard.create(name))
 .andThen(scheduleSyncWithServer())
 .subscribe();
 } PSST

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Repository.java void createNewCard(String name) {
 database.saveCard(TrelloCard.create(name))
 .andThen(scheduleSyncWithServer())
 .subscribe();
 } PSST

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Usecase 3, Content Details

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void onCreate() { UserContent contentToShow = getIntent().getParcelable(KEY_CONTENT); votesCountView.setText(contentToShow.votes() + " points”); } The usual way

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void onCreate() { UserContent contentToShow = getIntent().getParcelable(KEY_CONTENT); votesCountView.setText(contentToShow.votes() + " points”); } The usual way

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void onCreate() { UserContent contentToShow = getIntent().getParcelable(KEY_CONTENT); votesCountView.setText(contentToShow.votes() + " points”); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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void onUpvoteClick() { int newVoteCount = contentToShow.votes() + 1; NetworkListener listener = () -> { hideProgressDialog(); contentToShow = contentToShow.withVotes(newVoteCount); votesCountView.setText(newVoteCount + " points"); database.updateUserContent(contentToShow); }; showProgressDialog(); api.voteOnUserContent(contentToShow.id(), newVoteCount, listener); } The usual way

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Problem: UI is being driven from multiple sources void onCreate() { ... votesCountView.setText(contentToShow.votes() + " points”); } void onUpvoteClick() { ... votesCountView.setText(newVoteCount + " points"); ... }

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String id = getIntent().getStringExtra(KEY_CONTENT_ID); contentRepository.streamUserContent(id) .takeUntil(lifecycle.onDestroy()) .subscribe(userContent -> { loadImage(userContent.imageUrl()); votesCountView.setText( userContent.votes() + " points” ); upvoteButton.setOnClickListener(o -> contentRepository.upvote(userContent) ); }); The PSST way

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String id = getIntent().getStringExtra(KEY_CONTENT_ID); contentRepository.streamUserContent(id) .takeUntil(lifecycle.onDestroy()) .subscribe(userContent -> { loadImage(userContent.imageUrl()); votesCountView.setText( userContent.votes() + " points” ); upvoteButton.setOnClickListener(o -> contentRepository.upvote(userContent) ); }); The PSST way

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String id = getIntent().getStringExtra(KEY_CONTENT_ID); contentRepository.streamUserContent(id) .takeUntil(lifecycle.onDestroy()) .subscribe(userContent -> { loadImage(userContent.imageUrl()); votesCountView.setText( userContent.votes() + " points” ); upvoteButton.setOnClickListener(o -> contentRepository.upvote(userContent) ); }); The PSST way

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String id = getIntent().getStringExtra(KEY_CONTENT_ID); contentRepository.streamUserContent(id) .takeUntil(lifecycle.onDestroy()) .subscribe(userContent -> { loadImage(userContent.imageUrl()); votesCountView.setText( userContent.votes() + " points” ); upvoteButton.setOnClickListener(o -> contentRepository.upvote(userContent) ); }); The PSST way

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String id = getIntent().getStringExtra(KEY_CONTENT_ID); contentRepository.streamUserContent(id) .takeUntil(lifecycle.onDestroy()) .subscribe(userContent -> { loadImage(userContent.imageUrl()); votesCountView.setText( userContent.votes() + " points” ); upvoteButton.setOnClickListener(o -> contentRepository.upvote(userContent) ); }); The PSST way

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Part 2, User Experience

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If a network call does not involve any input validation, assume that it’ll succeed. Never make the user wait

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ContentRepository.java void onUpvoteClick(UserContent content) { // Assume success. database.upvote(content.id(), content.votes() + 1); performApiCallInBackground(); } Never make the user wait

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ContentRepository.java void onUpvoteClick(UserContent content) { // Assume success. database.upvote(content.id(), content.votes() + 1); performApiCallInBackground(); } Never make the user wait

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If a network call’s progress has to be communicated, consider displaying the progress state inline in UI.
 
 Totally avoid covering the screen with a progress bar. Never make the user wait

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void postComment(UserContent parent, String comment) { api.postComment(parent.id(), comment) .map(response -> ProgressState.POSTED) .onErrorReturnItem(ProgressState.FAILED) .startWith(ProgressState.IN_FLIGHT) .subscribe(progressState -> { database.createOrUpdateComment( comment, progressState ); }); } Never make the user wait

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void postComment(UserContent parent, String comment) { api.postComment(parent.id(), comment) .map(response -> ProgressState.POSTED) .onErrorReturnItem(ProgressState.FAILED) .startWith(ProgressState.IN_FLIGHT) .subscribe(progressState -> { database.createOrUpdateComment( comment, progressState ); }); } Never make the user wait

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void postComment(UserContent parent, String comment) { api.postComment(parent.id(), comment) .map(response -> ProgressState.POSTED) .onErrorReturnItem(ProgressState.FAILED) .startWith(ProgressState.IN_FLIGHT) .subscribe(progressState -> { database.createOrUpdateComment( comment, progressState ); }); } Never make the user wait

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void postComment(UserContent parent, String comment) { api.postComment(parent.id(), comment) .map(response -> ProgressState.POSTED) .onErrorReturnItem(ProgressState.FAILED) .startWith(ProgressState.IN_FLIGHT) .subscribe(progressState -> { database.createOrUpdateComment( comment, progressState ); }); } Never make the user wait

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void postComment(UserContent parent, String comment) { api.postComment(parent.id(), comment) .map(response -> ProgressState.POSTED) .onErrorReturnItem(ProgressState.FAILED) .startWith(ProgressState.IN_FLIGHT) .subscribe(progressState -> { database.createOrUpdateComment( comment, progressState ); }); } Never make the user wait

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contentRepository.postCommentAndStreamProgress(…) .subscribe(progressEvent -> { switch (progressEvent.state()) { case IN_FLIGHT: statusView.setText("Posting…"); break; case POSTED: statusView.setText("Posted at " + progressEvent.timestamp()); break; case FAILED: statusView.setText("Failed. Tap to retry."); break; } }); Never make the user wait

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Instant gratification Increased perceived speed of the app Advantages

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Notifications can also be used for communicating progress.
 Never make the user wait

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Notifications can also be used for communicating progress.
 Never make the user wait

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Notifications can also be used for communicating progress.
 Never make the user wait

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Part 3, Libraries

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Store github.com/NYTimes/Store

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android-job github.com/evernote/android-job

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onComplete()