How do you implement a FileObserver from an Android Service

Please see this post. I think you are missing the observer.startWatching() call after you setup your observer.

 observer = new FileObserver(pathToWatch) { // set up a file observer to watch this directory on sd card

     @Override
     public void onEvent(int event, String file) {
         //if(event == FileObserver.CREATE && !file.equals(".probe")){ // check if its a "create" and not equal to .probe because thats created every time camera is launched
         Log.d(TAG, "File created [" + pathToWatch + file + "]");

         Toast.makeText(getBaseContext(), file + " was saved!", Toast.LENGTH_LONG).show();
         //}
     }
 };
 observer.startWatching(); //START OBSERVING 

Add .show() after toast, i.e.

Toast.makeText(getBaseContext(), file + " was saved!", toast.LENGTH_LONG).show();                  

Here is the full code to create a service that listen for new file in a directory.

Firstly, you need to create the service that listen for new file entry in the directory. (E.g Camera)

MediaListenerService.java


import android.app.Service;
import android.content.Intent;
import android.os.FileObserver;
import android.os.Handler;
import android.os.IBinder;
import android.os.Looper;
import android.util.Log;
import android.widget.Toast;
import java.io.File;

    public class MediaListenerService extends Service {

        public static FileObserver observer;

        public MediaListenerService() {
        }

        @Override
        public IBinder onBind(Intent intent) {
            return null;
        }

        @Override
        public void onCreate() {
            super.onCreate();
            startWatching();
        }

        private void startWatching() {

        //The desired path to watch or monitor
        //E.g Camera folder
            final String pathToWatch = android.os.Environment.getExternalStorageDirectory().toString() + "/DCIM/Camera/";
            Toast.makeText(this, "My Service Started and trying to watch " + pathToWatch, Toast.LENGTH_LONG).show();

            observer = new FileObserver(pathToWatch, FileObserver.ALL_EVENTS) { // set up a file observer to watch this directory
                @Override
                public void onEvent(int event, final String file) {
                    if (event == FileObserver.CREATE || event == FileObserver.CLOSE_WRITE || event == FileObserver.MODIFY || event == FileObserver.MOVED_TO && !file.equals(".probe")) { // check that it's not equal to .probe because thats created every time camera is launched
                        Log.d("MediaListenerService", "File created [" + pathToWatch + file + "]");

                        new Handler(Looper.getMainLooper()).post(new Runnable() {
                            @Override
                            public void run() {
                                Toast.makeText(getBaseContext(), file + " was saved!", Toast.LENGTH_LONG).show();


                            }
                        });
                    }
                }
            };
            observer.startWatching();
        }
    }

Next step, you need to Declare the service in AndroidManifest.xml inside tag

<service
    android:name=".service.MediaListenerService"
    android:enabled="true"
    android:exported="false" >
</service>

And also don't forget to add a permission:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

If you're writing for Android 6 or above, you'll need to request the permission dynamically too, as per these instructions: https://developer.android.com/training/permissions/requesting

Now start the service from your Activity.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    startService(new Intent(getBaseContext(), MediaListenerService.class));
}

If you want to make your service started on boot, just simply create a receiver that listen to android.intent.action.BOOT_COMPLETED and then launch the service from that.

Hope this helps.

One more thing FileObserver doesn't observe sub directory. If you want to observe sub-directories too Check out this post.

An open-source RecursiveFileObserver acts as advanced FileObserver that is recursive for all directories beneath the directory you chose

package com.owncloud.android.utils;

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

import android.os.FileObserver;

public class RecursiveFileObserver extends FileObserver {

    public static int CHANGES_ONLY = CLOSE_WRITE | MOVE_SELF | MOVED_FROM;

    List<SingleFileObserver> mObservers;
    String mPath;
    int mMask;

    public RecursiveFileObserver(String path) {
        this(path, ALL_EVENTS);
    }

    public RecursiveFileObserver(String path, int mask) {
        super(path, mask);
        mPath = path;
        mMask = mask;
    }

    @Override
    public void startWatching() {
        if (mObservers != null) return;
        mObservers = new ArrayList<SingleFileObserver>();
        Stack<String> stack = new Stack<String>();
        stack.push(mPath);

        while (!stack.empty()) {
            String parent = stack.pop();
            mObservers.add(new SingleFileObserver(parent, mMask));
            File path = new File(parent);
            File[] files = path.listFiles();
            if (files == null) continue;
            for (int i = 0; i < files.length; ++i) {
                if (files[i].isDirectory() && !files[i].getName().equals(".")
                    && !files[i].getName().equals("..")) {
                    stack.push(files[i].getPath());
                }
            }
        }
        for (int i = 0; i < mObservers.size(); i++)
            mObservers.get(i).startWatching();
    }

    @Override
    public void stopWatching() {
        if (mObservers == null) return;

        for (int i = 0; i < mObservers.size(); ++i)
            mObservers.get(i).stopWatching();

        mObservers.clear();
        mObservers = null;
    }

    @Override
    public void onEvent(int event, String path) {

    }

    private class SingleFileObserver extends FileObserver {
        private String mPath;

        public SingleFileObserver(String path, int mask) {
            super(path, mask);
            mPath = path;
        }

        @Override
        public void onEvent(int event, String path) {
            String newPath = mPath + "/" + path;
            RecursiveFileObserver.this.onEvent(event, newPath);
        } 

    }
}

Source on GitHub