Fix an issue where the retry button wasn't doing anything. Preload the first pages of the next chapter (if available). Show a toast if no next/previous chapter is available. Other minor changes.

app/src/main/java/eu/kanade/mangafeed/data/source/base/BaseSource.java

@@ -80,8 +80,8 @@ public abstract class BaseSource {
         return defaultPageUrl;
     }
 
-    // Get the URL of the remaining pages that contains source images
-    protected String overrideRemainingPagesUrl(String defaultPageUrl) {
+    // Get the URL of the pages that contains source images
+    protected String overridePageUrl(String defaultPageUrl) {
         return defaultPageUrl;
     }
 

app/src/main/java/eu/kanade/mangafeed/data/source/base/Source.java

@@ -103,7 +103,7 @@ public abstract class Source extends BaseSource {
     public Observable<Page> getImageUrlFromPage(final Page page) {
         page.setStatus(Page.LOAD_PAGE);
         return mNetworkService
-                .getStringResponse(overrideRemainingPagesUrl(page.getUrl()), mRequestHeaders, null)
+                .getStringResponse(overridePageUrl(page.getUrl()), mRequestHeaders, null)
                 .flatMap(unparsedHtml -> Observable.just(parseHtmlToImageUrl(unparsedHtml)))
                 .onErrorResumeNext(e -> {
                     page.setStatus(Page.ERROR);

app/src/main/java/eu/kanade/mangafeed/data/source/online/english/Batoto.java

@@ -135,7 +135,7 @@ public class Batoto extends Source {
     }
 
     @Override
-    protected String overrideRemainingPagesUrl(String defaultPageUrl) {
+    protected String overridePageUrl(String defaultPageUrl) {
         int start = defaultPageUrl.indexOf("#") + 1;
         int end = defaultPageUrl.indexOf("_", start);
         String id = defaultPageUrl.substring(start, end);

app/src/main/java/eu/kanade/mangafeed/ui/base/presenter/BasePresenter.java

@@ -6,7 +6,6 @@ import android.support.annotation.NonNull;
 
 import de.greenrobot.event.EventBus;
 import icepick.Icepick;
-import nucleus.presenter.RxPresenter;
 import nucleus.view.ViewWithPresenter;
 
 public class BasePresenter<V extends ViewWithPresenter> extends RxPresenter<V> {

app/src/main/java/eu/kanade/mangafeed/ui/base/presenter/RxPresenter.java

@@ -0,0 +1,332 @@
+package eu.kanade.mangafeed.ui.base.presenter;
+
+import android.os.Bundle;
+import android.support.annotation.CallSuper;
+import android.support.annotation.Nullable;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map;
+
+import nucleus.presenter.Presenter;
+import nucleus.presenter.delivery.DeliverFirst;
+import nucleus.presenter.delivery.DeliverLatestCache;
+import nucleus.presenter.delivery.DeliverReplay;
+import nucleus.presenter.delivery.Delivery;
+import rx.Observable;
+import rx.Subscription;
+import rx.functions.Action1;
+import rx.functions.Action2;
+import rx.functions.Func0;
+import rx.internal.util.SubscriptionList;
+import rx.subjects.BehaviorSubject;
+
+/**
+ * This is an extension of {@link Presenter} which provides RxJava functionality.
+ *
+ * @param <View> a type of view.
+ */
+public class RxPresenter<View> extends Presenter<View> {
+
+    private static final String REQUESTED_KEY = RxPresenter.class.getName() + "#requested";
+
+    private final BehaviorSubject<View> views = BehaviorSubject.create();
+    private final SubscriptionList subscriptions = new SubscriptionList();
+
+    private final HashMap<Integer, Func0<Subscription>> restartables = new HashMap<>();
+    private final HashMap<Integer, Subscription> restartableSubscriptions = new HashMap<>();
+    private final ArrayList<Integer> requested = new ArrayList<>();
+
+    /**
+     * Returns an {@link rx.Observable} that emits the current attached view or null.
+     * See {@link BehaviorSubject} for more information.
+     *
+     * @return an observable that emits the current attached view or null.
+     */
+    public Observable<View> view() {
+        return views;
+    }
+
+    /**
+     * Registers a subscription to automatically unsubscribe it during onDestroy.
+     * See {@link SubscriptionList#add(Subscription) for details.}
+     *
+     * @param subscription a subscription to add.
+     */
+    public void add(Subscription subscription) {
+        subscriptions.add(subscription);
+    }
+
+    /**
+     * Removes and unsubscribes a subscription that has been registered with {@link #add} previously.
+     * See {@link SubscriptionList#remove(Subscription)} for details.
+     *
+     * @param subscription a subscription to remove.
+     */
+    public void remove(Subscription subscription) {
+        subscriptions.remove(subscription);
+    }
+
+    /**
+     * A restartable is any RxJava observable that can be started (subscribed) and
+     * should be automatically restarted (re-subscribed) after a process restart if
+     * it was still subscribed at the moment of saving presenter's state.
+     *
+     * Registers a factory. Re-subscribes the restartable after the process restart.
+     *
+     * @param restartableId id of the restartable
+     * @param factory       factory of the restartable
+     */
+    public void restartable(int restartableId, Func0<Subscription> factory) {
+        restartables.put(restartableId, factory);
+        if (requested.contains(restartableId))
+            start(restartableId);
+    }
+
+    /**
+     * Starts the given restartable.
+     *
+     * @param restartableId id of the restartable
+     */
+    public void start(int restartableId) {
+        stop(restartableId);
+        requested.add(restartableId);
+        restartableSubscriptions.put(restartableId, restartables.get(restartableId).call());
+    }
+
+    /**
+     * Unsubscribes a restartable
+     *
+     * @param restartableId id of a restartable.
+     */
+    public void stop(int restartableId) {
+        requested.remove((Integer)restartableId);
+        Subscription subscription = restartableSubscriptions.get(restartableId);
+        if (subscription != null)
+            subscription.unsubscribe();
+    }
+
+    /**
+     * Checks if a restartable is started.
+     *
+     * @param restartableId id of a restartable.
+     * @return True if the restartable is started, false otherwise.
+     */
+    public boolean isStarted(int restartableId) {
+        return requested.contains(restartableId);
+    }
+
+    /**
+     * This is a shortcut that can be used instead of combining together
+     * {@link #restartable(int, Func0)},
+     * {@link #deliverFirst()},
+     * {@link #split(Action2, Action2)}.
+     *
+     * @param restartableId     an id of the restartable.
+     * @param observableFactory a factory that should return an Observable when the restartable should run.
+     * @param onNext            a callback that will be called when received data should be delivered to view.
+     * @param onError           a callback that will be called if the source observable emits onError.
+     * @param <T>               the type of the observable.
+     */
+    public <T> void restartableFirst(int restartableId, final Func0<Observable<T>> observableFactory,
+                                     final Action2<View, T> onNext, @Nullable final Action2<View, Throwable> onError) {
+
+        restartable(restartableId, new Func0<Subscription>() {
+            @Override
+            public Subscription call() {
+                return observableFactory.call()
+                        .compose(RxPresenter.this.<T>deliverFirst())
+                        .subscribe(split(onNext, onError));
+            }
+        });
+    }
+
+    /**
+     * This is a shortcut for calling {@link #restartableFirst(int, Func0, Action2, Action2)} with the last parameter = null.
+     */
+    public <T> void restartableFirst(int restartableId, final Func0<Observable<T>> observableFactory, final Action2<View, T> onNext) {
+        restartableFirst(restartableId, observableFactory, onNext, null);
+    }
+
+    /**
+     * This is a shortcut that can be used instead of combining together
+     * {@link #restartable(int, Func0)},
+     * {@link #deliverLatestCache()},
+     * {@link #split(Action2, Action2)}.
+     *
+     * @param restartableId     an id of the restartable.
+     * @param observableFactory a factory that should return an Observable when the restartable should run.
+     * @param onNext            a callback that will be called when received data should be delivered to view.
+     * @param onError           a callback that will be called if the source observable emits onError.
+     * @param <T>               the type of the observable.
+     */
+    public <T> void restartableLatestCache(int restartableId, final Func0<Observable<T>> observableFactory,
+                                           final Action2<View, T> onNext, @Nullable final Action2<View, Throwable> onError) {
+
+        restartable(restartableId, new Func0<Subscription>() {
+            @Override
+            public Subscription call() {
+                return observableFactory.call()
+                        .compose(RxPresenter.this.<T>deliverLatestCache())
+                        .subscribe(split(onNext, onError));
+            }
+        });
+    }
+
+    /**
+     * This is a shortcut for calling {@link #restartableLatestCache(int, Func0, Action2, Action2)} with the last parameter = null.
+     */
+    public <T> void restartableLatestCache(int restartableId, final Func0<Observable<T>> observableFactory, final Action2<View, T> onNext) {
+        restartableLatestCache(restartableId, observableFactory, onNext, null);
+    }
+
+    /**
+     * This is a shortcut that can be used instead of combining together
+     * {@link #restartable(int, Func0)},
+     * {@link #deliverReplay()},
+     * {@link #split(Action2, Action2)}.
+     *
+     * @param restartableId     an id of the restartable.
+     * @param observableFactory a factory that should return an Observable when the restartable should run.
+     * @param onNext            a callback that will be called when received data should be delivered to view.
+     * @param onError           a callback that will be called if the source observable emits onError.
+     * @param <T>               the type of the observable.
+     */
+    public <T> void restartableReplay(int restartableId, final Func0<Observable<T>> observableFactory,
+                                      final Action2<View, T> onNext, @Nullable final Action2<View, Throwable> onError) {
+
+        restartable(restartableId, new Func0<Subscription>() {
+            @Override
+            public Subscription call() {
+                return observableFactory.call()
+                        .compose(RxPresenter.this.<T>deliverReplay())
+                        .subscribe(split(onNext, onError));
+            }
+        });
+    }
+
+    /**
+     * This is a shortcut for calling {@link #restartableReplay(int, Func0, Action2, Action2)} with the last parameter = null.
+     */
+    public <T> void restartableReplay(int restartableId, final Func0<Observable<T>> observableFactory, final Action2<View, T> onNext) {
+        restartableReplay(restartableId, observableFactory, onNext, null);
+    }
+
+    /**
+     * Returns an {@link rx.Observable.Transformer} that couples views with data that has been emitted by
+     * the source {@link rx.Observable}.
+     *
+     * {@link #deliverLatestCache} keeps the latest onNext value and emits it each time a new view gets attached.
+     * If a new onNext value appears while a view is attached, it will be delivered immediately.
+     *
+     * @param <T> the type of source observable emissions
+     */
+    public <T> DeliverLatestCache<View, T> deliverLatestCache() {
+        return new DeliverLatestCache<>(views);
+    }
+
+    /**
+     * Returns an {@link rx.Observable.Transformer} that couples views with data that has been emitted by
+     * the source {@link rx.Observable}.
+     *
+     * {@link #deliverFirst} delivers only the first onNext value that has been emitted by the source observable.
+     *
+     * @param <T> the type of source observable emissions
+     */
+    public <T> DeliverFirst<View, T> deliverFirst() {
+        return new DeliverFirst<>(views);
+    }
+
+    /**
+     * Returns an {@link rx.Observable.Transformer} that couples views with data that has been emitted by
+     * the source {@link rx.Observable}.
+     *
+     * {@link #deliverReplay} keeps all onNext values and emits them each time a new view gets attached.
+     * If a new onNext value appears while a view is attached, it will be delivered immediately.
+     *
+     * @param <T> the type of source observable emissions
+     */
+    public <T> DeliverReplay<View, T> deliverReplay() {
+        return new DeliverReplay<>(views);
+    }
+
+    /**
+     * Returns a method that can be used for manual restartable chain build. It returns an Action1 that splits
+     * a received {@link Delivery} into two {@link Action2} onNext and onError calls.
+     *
+     * @param onNext  a method that will be called if the delivery contains an emitted onNext value.
+     * @param onError a method that will be called if the delivery contains an onError throwable.
+     * @param <T>     a type on onNext value.
+     * @return an Action1 that splits a received {@link Delivery} into two {@link Action2} onNext and onError calls.
+     */
+    public <T> Action1<Delivery<View, T>> split(final Action2<View, T> onNext, @Nullable final Action2<View, Throwable> onError) {
+        return new Action1<Delivery<View, T>>() {
+            @Override
+            public void call(Delivery<View, T> delivery) {
+                delivery.split(onNext, onError);
+            }
+        };
+    }
+
+    /**
+     * This is a shortcut for calling {@link #split(Action2, Action2)} when the second parameter is null.
+     */
+    public <T> Action1<Delivery<View, T>> split(Action2<View, T> onNext) {
+        return split(onNext, null);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @CallSuper
+    @Override
+    protected void onCreate(Bundle savedState) {
+        if (savedState != null)
+            requested.addAll(savedState.getIntegerArrayList(REQUESTED_KEY));
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @CallSuper
+    @Override
+    protected void onDestroy() {
+        views.onCompleted();
+        subscriptions.unsubscribe();
+        for (Map.Entry<Integer, Subscription> entry : restartableSubscriptions.entrySet())
+            entry.getValue().unsubscribe();
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @CallSuper
+    @Override
+    protected void onSave(Bundle state) {
+        for (int i = requested.size() - 1; i >= 0; i--) {
+            int restartableId = requested.get(i);
+            Subscription subscription = restartableSubscriptions.get(restartableId);
+            if (subscription != null && subscription.isUnsubscribed())
+                requested.remove(i);
+        }
+        state.putIntegerArrayList(REQUESTED_KEY, requested);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @CallSuper
+    @Override
+    protected void onTakeView(View view) {
+        views.onNext(view);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @CallSuper
+    @Override
+    protected void onDropView() {
+        views.onNext(null);
+    }
+}

app/src/main/java/eu/kanade/mangafeed/ui/manga/myanimelist/MyAnimeListPresenter.java

@@ -84,6 +84,7 @@ public class MyAnimeListPresenter extends BasePresenter<MyAnimeListFragment> {
         chapterSync.last_chapter_read = chapterNumber;
 
         add(updateSubscription = myAnimeList.update(chapterSync)
+                .flatMap(response -> db.insertChapterSync(chapterSync).createObservable())
                 .subscribeOn(Schedulers.io())
                 .observeOn(AndroidSchedulers.mainThread())
                 .subscribe(response -> {},

app/src/main/java/eu/kanade/mangafeed/ui/reader/ReaderPresenter.java

@@ -45,6 +45,7 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
     private Chapter nextChapter;
     private Chapter previousChapter;
     private List<Page> pageList;
+    private List<Page> nextChapterPageList;
     private boolean isDownloaded;
     @State int currentPage;
 
@@ -56,6 +57,7 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
     private static final int GET_PAGE_LIST = 1;
     private static final int GET_PAGE_IMAGES = 2;
     private static final int RETRY_IMAGES = 3;
+    private static final int PRELOAD_NEXT_CHAPTER = 4;
 
     @Override
     protected void onCreate(Bundle savedState) {
@@ -81,7 +83,8 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
                 });
 
         restartableReplay(GET_PAGE_IMAGES,
-                this::getPageImagesObservable,
+                () -> getPageImagesObservable()
+                        .doOnCompleted(this::preloadNextChapter),
                 (view, page) -> {},
                 (view, error) -> Timber.e("An error occurred while downloading an image"));
 
@@ -89,6 +92,11 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
                 this::getRetryPageObservable,
                 (view, page) -> {},
                 (view, error) -> Timber.e("An error occurred while downloading an image"));
+
+        restartableLatestCache(PRELOAD_NEXT_CHAPTER,
+                this::getPreloadNextChapterObservable,
+                (view, pages) -> {},
+                (view, error) -> Timber.e("An error occurred while preloading a chapter"));
     }
 
     @Override
@@ -105,7 +113,7 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
 
     @Override
     protected void onDestroy() {
-        onChapterChange();
+        onChapterLeft();
         super.onDestroy();
     }
 
@@ -125,10 +133,73 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
         retryPageSubject.onNext(page);
     }
 
+    // Returns the page list of a chapter
+    private Observable<List<Page>> getPageListObservable() {
+        return isDownloaded ?
+                // Fetch the page list from disk
+                Observable.just(downloadManager.getSavedPageList(source, manga, chapter)) :
+                // Fetch the page list from cache or fallback to network
+                source.getCachedPageListOrPullFromNetwork(chapter.url)
+                        .subscribeOn(Schedulers.io())
+                        .observeOn(AndroidSchedulers.mainThread());
+    }
+
+    // Get the chapter images from network or disk
+    private Observable<Page> getPageImagesObservable() {
+        Observable<Page> pageObservable;
+
+        if (!isDownloaded) {
+            pageObservable = Observable.from(pageList)
+                    .filter(page -> page.getImageUrl() != null)
+                    .mergeWith(source.getRemainingImageUrlsFromPageList(pageList))
+                    .flatMap(source::getCachedImage, 3);
+        } else {
+            File chapterDir = downloadManager.getAbsoluteChapterDirectory(source, manga, chapter);
+            pageObservable = Observable.from(pageList)
+                    .flatMap(page -> downloadManager.getDownloadedImage(page, source, chapterDir));
+        }
+        return pageObservable
+                .subscribeOn(Schedulers.io())
+                .observeOn(AndroidSchedulers.mainThread());
+    }
+
+    // Listen for retry page events
+    private Observable<Page> getRetryPageObservable() {
+        return retryPageSubject
+                .observeOn(Schedulers.io())
+                .flatMap(page -> page.getImageUrl() == null ?
+                        source.getImageUrlFromPage(page) :
+                        Observable.just(page))
+                .flatMap(source::getCachedImage)
+                .observeOn(AndroidSchedulers.mainThread());
+    }
+
+    // Preload the first pages of the next chapter
+    private Observable<Page> getPreloadNextChapterObservable() {
+        return source.getCachedPageListOrPullFromNetwork(nextChapter.url)
+                .flatMap(pages -> {
+                    nextChapterPageList = pages;
+                    // Preload at most 5 pages
+                    int pagesToPreload = Math.min(pages.size(), 5);
+                    return Observable.from(pages)
+                            .take(pagesToPreload)
+                            .concatMap(source::getImageUrlFromPage)
+                            .doOnCompleted(this::stopPreloadingNextChapter);
+                })
+                .subscribeOn(Schedulers.io())
+                .observeOn(AndroidSchedulers.mainThread());
+    }
+
+    // Loads the given chapter
     private void loadChapter(Chapter chapter) {
+        // Before loading the chapter, stop preloading (if it's working) and save current progress
+        stopPreloadingNextChapter();
+
         this.chapter = chapter;
         isDownloaded = isChapterDownloaded(chapter);
-        if (chapter.last_page_read != 0 && !chapter.read)
+
+        // If the chapter is partially read, set the starting page to the last the user read
+        if (!chapter.read && chapter.last_page_read != 0)
             currentPage = chapter.last_page_read;
         else
             currentPage = 0;
@@ -136,11 +207,22 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
         // Reset next and previous chapter. They have to be fetched again
         nextChapter = null;
         previousChapter = null;
+        nextChapterPageList = null;
 
         start(GET_PAGE_LIST);
     }
 
-    private void onChapterChange() {
+    // Check whether the given chapter is downloaded
+    public boolean isChapterDownloaded(Chapter chapter) {
+        File dir = downloadManager.getAbsoluteChapterDirectory(source, manga, chapter);
+        List<Page> pageList = downloadManager.getSavedPageList(source, manga, chapter);
+
+        return pageList != null && pageList.size() + 1 == dir.listFiles().length;
+    }
+
+    // Called before loading another chapter or leaving the reader. It allows to do operations
+    // over the chapter read like saving progress
+    private void onChapterLeft() {
         if (pageList == null)
             return;
 
@@ -158,6 +240,7 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
         db.insertChapter(chapter).executeAsBlocking();
     }
 
+    // Check whether the chapter has been read
     private boolean isChapterFinished() {
         return !chapter.read && currentPage == pageList.size() - 1;
     }
@@ -185,46 +268,6 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
         }
     }
 
-    private Observable<List<Page>> getPageListObservable() {
-        if (!isDownloaded)
-            return source.getCachedPageListOrPullFromNetwork(chapter.url)
-                    .subscribeOn(Schedulers.io())
-                    .observeOn(AndroidSchedulers.mainThread());
-        else
-            return Observable.just(downloadManager.getSavedPageList(source, manga, chapter));
-    }
-
-    private Observable<Page> getPageImagesObservable() {
-        Observable<Page> pages;
-
-        if (!isDownloaded) {
-            pages = Observable.from(pageList)
-                    .filter(page -> page.getImageUrl() != null)
-                    .mergeWith(source.getRemainingImageUrlsFromPageList(pageList))
-                    .flatMap(source::getCachedImage, 3);
-        } else {
-            File chapterDir = downloadManager.getAbsoluteChapterDirectory(source, manga, chapter);
-
-            pages = Observable.from(pageList)
-                    .flatMap(page -> downloadManager.getDownloadedImage(page, source, chapterDir));
-        }
-        return pages
-                .subscribeOn(Schedulers.io())
-                .observeOn(AndroidSchedulers.mainThread());
-    }
-
-    private Observable<Page> getRetryPageObservable() {
-        return retryPageSubject
-                .flatMap(page -> {
-                    if (page.getImageUrl() == null)
-                        return source.getImageUrlFromPage(page);
-                    return Observable.just(page);
-                })
-                .flatMap(source::getCachedImage)
-                .subscribeOn(Schedulers.io())
-                .observeOn(AndroidSchedulers.mainThread());
-    }
-
     public void setCurrentPage(int currentPage) {
         this.currentPage = currentPage;
     }
@@ -247,33 +290,49 @@ public class ReaderPresenter extends BasePresenter<ReaderActivity> {
                 .subscribe(result -> previousChapter = result));
     }
 
-    public boolean isChapterDownloaded(Chapter chapter) {
-        File dir = downloadManager.getAbsoluteChapterDirectory(source, manga, chapter);
-        List<Page> pageList = downloadManager.getSavedPageList(source, manga, chapter);
-
-        return pageList != null && pageList.size() + 1 == dir.listFiles().length;
-    }
-
     public void loadNextChapter() {
-        if (nextChapter != null) {
-            onChapterChange();
+        if (hasNextChapter()) {
+            onChapterLeft();
             loadChapter(nextChapter);
         }
     }
 
     public void loadPreviousChapter() {
-        if (previousChapter != null) {
-            onChapterChange();
+        if (hasPreviousChapter()) {
+            onChapterLeft();
             loadChapter(previousChapter);
         }
     }
 
-    public Manga getManga() {
-        return manga;
+    public boolean hasNextChapter() {
+        return nextChapter != null;
+    }
+
+    public boolean hasPreviousChapter() {
+        return previousChapter != null;
+    }
+
+    private void preloadNextChapter() {
+        if (hasNextChapter() && !isChapterDownloaded(nextChapter)) {
+            start(PRELOAD_NEXT_CHAPTER);
+        }
+    }
+
+    private void stopPreloadingNextChapter() {
+        if (isStarted(PRELOAD_NEXT_CHAPTER)) {
+            stop(PRELOAD_NEXT_CHAPTER);
+            if (nextChapterPageList != null)
+                source.savePageList(nextChapter.url, nextChapterPageList);
+        }
     }
 
     public void updateMangaViewer(int viewer) {
         manga.viewer = viewer;
         db.insertManga(manga).executeAsBlocking();
     }
+
+    public Manga getManga() {
+        return manga;
+    }
+
 }

app/src/main/java/eu/kanade/mangafeed/ui/reader/viewer/base/BaseReader.java

@@ -5,18 +5,23 @@ import android.view.ViewGroup;
 
 import java.util.List;
 
+import eu.kanade.mangafeed.R;
 import eu.kanade.mangafeed.data.source.model.Page;
 import eu.kanade.mangafeed.ui.reader.ReaderActivity;
+import eu.kanade.mangafeed.ui.reader.ReaderPresenter;
+import eu.kanade.mangafeed.util.ToastUtil;
 
 public abstract class BaseReader {
 
     protected ReaderActivity activity;
+    protected ReaderPresenter presenter;
     protected ViewGroup container;
     protected int currentPosition;
 
     public BaseReader(ReaderActivity activity) {
         this.activity = activity;
         this.container = activity.getContainer();
+        this.presenter = activity.getPresenter();
     }
 
     public void updatePageNumber() {
@@ -34,13 +39,22 @@ public abstract class BaseReader {
     }
 
     public void requestNextChapter() {
-        activity.getPresenter().setCurrentPage(getCurrentPosition());
-        activity.getPresenter().loadNextChapter();
+        if (presenter.hasNextChapter()) {
+            presenter.setCurrentPage(getCurrentPosition());
+            presenter.loadNextChapter();
+        } else {
+            ToastUtil.showShort(activity, R.string.no_next_chapter);
+        }
+
     }
 
     public void requestPreviousChapter() {
-        activity.getPresenter().setCurrentPage(getCurrentPosition());
-        activity.getPresenter().loadPreviousChapter();
+        if (presenter.hasPreviousChapter()) {
+            presenter.setCurrentPage(getCurrentPosition());
+            presenter.loadPreviousChapter();
+        } else {
+            ToastUtil.showShort(activity, R.string.no_previous_chapter);
+        }
     }
 
     public void destroy() {}

app/src/main/java/eu/kanade/mangafeed/ui/reader/viewer/horizontal/HorizontalReader.java

@@ -30,7 +30,7 @@ public abstract class HorizontalReader extends BaseReader {
         transitionsSubscription = activity.getPreferences().enableTransitions().asObservable()
                 .subscribe(value -> transitions = value);
 
-        viewPager.setOffscreenPageLimit(3);
+        viewPager.setOffscreenPageLimit(2);
         viewPager.addOnPageChangeListener(new HorizontalViewPager.SimpleOnPageChangeListener() {
             @Override
             public void onPageSelected(int position) {

app/src/main/java/eu/kanade/mangafeed/ui/reader/viewer/vertical/VerticalReader.java

@@ -31,7 +31,7 @@ public class VerticalReader extends BaseReader {
         transitionsSubscription = activity.getPreferences().enableTransitions().asObservable()
                 .subscribe(value -> transitions = value);
 
-        viewPager.setOffscreenPageLimit(3);
+        viewPager.setOffscreenPageLimit(2);
         viewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
             @Override
             public void onPageSelected(int position) {

app/src/main/res/values/strings.xml

@@ -92,6 +92,8 @@
     <string name="chapter_progress">Page: %1$d</string>
     <string name="page_list_error">Error fetching page list. Is network available?</string>
     <string name="chapter_subtitle">Chapter %1$s</string>
+    <string name="no_next_chapter">Next chapter not found</string>
+    <string name="no_previous_chapter">Previous chapter not found</string>
 
     <!-- Library update service notifications -->
     <string name="notification_progress">Update progress: %1$d/%2$d</string>