RxScreenshotDetector.java

private static final String TAG = "RxScreenshotDetector";
private static final String EXTERNAL_CONTENT_URI_MATCHER =
        MediaStore.Images.Media.EXTERNAL_CONTENT_URI.toString();
private static final String[] PROJECTION = new String[] {
        MediaStore.Images.Media.DISPLAY_NAME, MediaStore.Images.Media.DATA,
        MediaStore.Images.Media.DATE_ADDED
};
private static final String SORT_ORDER = MediaStore.Images.Media.DATE_ADDED + " DESC";
private static final long DEFAULT_DETECT_WINDOW_SECONDS = 10;

final ContentResolver contentResolver = context.getContentResolver();
final ContentObserver contentObserver = new ContentObserver(null) {
    @Override
    public void onChange(boolean selfChange, Uri uri) {
        Log.d(TAG, "onChange: " + selfChange + ", " + uri.toString());
        if (uri.toString().matches(EXTERNAL_CONTENT_URI_MATCHER)) {
            Cursor cursor = null;
            try {
                cursor = contentResolver.query(uri, PROJECTION, null, null,
                        SORT_ORDER);
                if (cursor != null && cursor.moveToFirst()) {
                    String path = cursor.getString(
                            cursor.getColumnIndex(MediaStore.Images.Media.DATA));
                    long dateAdded = cursor.getLong(cursor.getColumnIndex(
                            MediaStore.Images.Media.DATE_ADDED));
                    long currentTime = System.currentTimeMillis() / 1000;
                    Log.d(TAG, "path: " + path + ", dateAdded: " + dateAdded +
                            ", currentTime: " + currentTime);
                    if (path.toLowerCase().contains("screenshot") &&
                            Math.abs(currentTime - dateAdded) <=
                                    DEFAULT_DETECT_WINDOW_SECONDS) {
                        // screenshot added!
                    }
                }
            } catch (Exception e) {
                Log.d(TAG, "open cursor fail");
            } finally {
                if (cursor != null) {
                    cursor.close();
                }
            }
        }
        super.onChange(selfChange, uri);
    }
};
contentResolver.registerContentObserver(
        MediaStore.Images.Media.EXTERNAL_CONTENT_URI, true, contentObserver);

RxScreenshotDetectorDemo.java

RxScreenshotDetector.start(getApplicationContext())
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .compose(this.<String>bindUntilEvent(ActivityEvent.PAUSE))
        .subscribe(new Subscriber<String>() {
            @Override
            public void onCompleted() {

            }

            @Override
            public void onError(Throwable e) {
                e.printStackTrace();
            }

            @Override
            public void onNext(String path) {
                mTextView.setText(mTextView.getText() + "\nScreenshot: " + path);
            }
        });