How to know when MediaRecorder has finished writing data to file

The FileObserver class suits your needs perfectly. Here is the documentation. It's easy to use. When a observed file is closed after writing, the onEvent callback is called with CLOSE_WRITE as the parameter.

MyFileObserver fb = new MyFileObserver(mediaFile_path, FileObserver.CLOSE_WRITE);
fb.startWatching();

class MyFileObserver extends FileObserver {

    public MyFileObserver (String path, int mask) {
        super(path, mask);
    }

    public void onEvent(int event, String path) {
        // start playing
    }
}

Don't forget to call stopWatching().

We solved similar problem with the following algo:

while (file not complete)
    sleep for 1 sec
    read the fourth byte of the file
    if it is not 0 (contains 'f' of the 'ftyp' header) then
        file is complete, break

The key point is that MediaRecorder writes the ftyp box at the very last moment. If it is in place, then the file is complete.