How can I display list of recently opened files?

Question

Can anyone provide me with the guidelines on how to retrieve a list of recently opened files in a folder?

In my application, I have a folder which contains .epub files. I would like to display a sorted list of recently read/opened books by the user.

Answer 1

The problem is actually not so trivial. There are two reasons - history's persistence and IO blocking operations.

In fact to ensure persistence of history we can use some solutions:

• database (seems most reasonable but requires most effort)
• internal file (should be the easiest way)
• shared preferences

So, I used second method. In memory I keep just ArrayList<Uri>, and to save it to file I convert it to List<String>, as android.net.Uri doesn't support serialisation. In internal memory I save serialised object using ObjectIOStream.

So, see the code:

public class FileHistory {
    private static final String FILE_NAME = "file-history-v1";
    private static final int HISTORY_SIZE = 20;

    @NonNull
    private final Context mAppContext;
    // This is a executor where I can post any runnable
    // and all of them will be executed in one pipeline
    // keeping posting order.
    @NonNull
    private final OneThreadExecutor mExecutor;

    @Nullable
    private ArrayList<Uri> mInternalFilesHistory;
    @NonNull
    private MutableLiveData<List<Uri>> mFilesHistory = new MutableLiveData<>();

    public FileHistory(@NonNull final Context appContext,
                       @NonNull final OneThreadExecutor executor) {
        this.mAppContext = appContext;
        this.mExecutor = executor;
        loadHistory();
    }

    public void addEntry(@NonNull final Uri entry) {
        if (mInternalFilesHistory == null) {
            // The fileHistory is not ready yet.
            // Schedule adding entry as next task of serial executor.
            mExecutor.execute(() -> addEntry(entry));
            return;
        }

        // Remove entry if exists and add it as first element.
        CollectionUtils.removeFirst(mInternalFilesHistory, uri -> uri.equals(entry));
        mInternalFilesHistory.add(0, entry);
        if (mInternalFilesHistory.size() > HISTORY_SIZE) {
            ArrayList<Uri> trimmed = new ArrayList<>(HISTORY_SIZE + 1);
            trimmed.addAll(mInternalFilesHistory.subList(0, HISTORY_SIZE));
            mInternalFilesHistory = trimmed;
        }

        mExecutor.execute(this::rePostHistory);
        mExecutor.execute(this::saveToInternalStorage);
    }

    @NonNull
    public MutableLiveData<List<Uri>> getFilesHistory() {
        return mFilesHistory;
    }

    private void loadHistory() {
        mExecutor.execute(this::loadFromInternalStorage);
        mExecutor.execute(this::rePostHistory);
    }

    private void rePostHistory() {
        if (mInternalFilesHistory != null) {
            mFilesHistory.postValue(Collections.unmodifiableList(mInternalFilesHistory));
        }
    }

    @SuppressWarnings("unchecked")
    @WorkerThread
    private void loadFromInternalStorage() {
        try {
            FileInputStream fis = mAppContext.openFileInput(FILE_NAME);
            ObjectInputStream ois = new ObjectInputStream(fis);

            ArrayList<String> entries = (ArrayList<String>) ois.readObject();
            List<Uri> mapped = CollectionUtils.map(entries, Uri::parse);

            if (mInternalFilesHistory == null) {
                mInternalFilesHistory = new ArrayList<>(HISTORY_SIZE + 1);
            } else {
                mInternalFilesHistory.clear();
            }
            mInternalFilesHistory.addAll(mapped);

            fis.close();
            ois.close();
        } catch (Exception ex) {
            mInternalFilesHistory = new ArrayList<>(HISTORY_SIZE + 1);
        }
    }

    @WorkerThread
    private void saveToInternalStorage() {
        try {
            FileOutputStream fis = mAppContext.openFileOutput(FILE_NAME, Context.MODE_PRIVATE);
            ObjectOutputStream oos = new ObjectOutputStream(fis);

            if (mInternalFilesHistory == null) {
                mInternalFilesHistory = new ArrayList<>();
            }

            List<String> converted = CollectionUtils.map(mInternalFilesHistory, Uri::toString);
            oos.writeObject(converted);

            fis.close();
            oos.close();
        } catch (IOException ignored) {
        }
    }

}

As you can see, internal storage is use to keep that file. So, there is no need to add any additional permissions. Synchronisation is ensured by using executor which will execute all request, one by one, so even if IO will be slow order or requests will be saved.

We do not block thread with IO operations, because all operations using IO are on WorkerThread. About the result we will be notified via LiveData from android.arch.

In my opinion this is kind of the simplest solution. If we need to keep stats, dates etc. we can save List<MyHistoryEntry>, as long MyHistoryEntry will be serialisable.

As a better approach I would suggest to use database (easier migration etc.).