Android FileObserver onEvent 未被调用

Question

Can someone possibly help me with this?

I want to observe a file to see if it gets modified so that I can update the activity. After several tests, I've determined it's just plain not working. Am I doing something wrong?

I'm creating a FileObserver with an onEvent method to display a Toast and log data just to see if it's working, however the onEvent is never getting called. I have tried it both with an existing and a new file, but it doesn't seem to work in either case.

    Context context = this;
    File fileFolder = context.getFilesDir();

    String fileName = "quest";
    FileObserver questObserver = new FileObserver(fileFolder.getPath()) { // also tried fileFolder.getName()
        @Override
        public void onEvent(int event, String path) {
            Toast.makeText(getApplicationContext(), "onEvent fired", Toast.LENGTH_LONG).show();
            Log.d(TAG, "FileObserver().onEvent");
        }
    };
    questObserver.startWatching();

    /* create file */
    ObjectOutputStream objectOut = null;
    try {
        FileOutputStream fileOut = context.openFileOutput(fileName, Context.MODE_PRIVATE);
        objectOut = new ObjectOutputStream(fileOut);
        objectOut.writeObject(new Quest());
        fileOut.getFD().sync();
    } catch (IOException e) {
        Log.d(TAG, e.getMessage());
    } finally {
        if (objectOut != null) {
            try {
                objectOut.close();
            } catch (IOException e) {
                Log.d(TAG, e.getMessage());
            }
        }
    }

    /* read file */
    ObjectInputStream objectIn = null;
    Quest quest = null;

     try {
         FileInputStream fileIn = context.openFileInput(fileName);
         objectIn = new ObjectInputStream(fileIn);
         quest = (Quest) objectIn.readObject();
     } catch (FileNotFoundException e) {
         // Do nothing
     } catch (IOException e) {
         e.printStackTrace();
     } catch (ClassNotFoundException e) {
         e.printStackTrace();
     } finally {
         if (objectIn != null) {
             try {
                 objectIn.close();
             } catch (IOException e) {
                 Log.d(TAG, e.getMessage());
             }
         }
     }
     Toast.makeText(context, quest.getTitle(), Toast.LENGTH_LONG).show();

    questObserver.stopWatching();

Any help would be greatly appreciated.

Answer 1

' public abstract void onEvent (int event, String path) " -

This method is invoked on a special FileObserver thread. It runs independently of any threads, so take care to use appropriate synchronization! Consider using post(Runnable) to shift event handling work to the main thread to avoid concurrency problems.

http://developer.android.com/reference/android/os/FileObserver.html

If you put the toast through a handler.post(new Runnable(){...}), that should work.

Answer 2

Assuming your file doesn't (always) exist you should probably put your observer on the files folder, obtained like so:

Context ctx = ...;
File filesFolder = ctx.getFilesDir();

Note that this will also ensure that the filesFolder directory will be created.

Your observer will now be notified whenever a file is written, deleted or updated using for instance Context#.openFileOutput(..) - and you can filter in your FileObserver for the file name, in your example "quest".