如何从Dropbox下载文件，我不知道我做错了什么

Question

请帮助...下载功能一直运行到Toast“Downloading ..”但之后没有，并且在目标文件夹中创建了一个空文件。 我认为Dropbox一定不能通过身份验证，请参阅。 我选择的源是在我的app文件夹里面，这是在Dropbox开发人员创建项目时生成的......请检查它是否正确

    final static private String APP_KEY = "xxxxxx";
    final static private String APP_SECRET = "xxxxxx";
    final static private String ACCESS_TOKEN = "xxxxxx";


    public static String DropboxDownloadPathFrom = "MyFolder/Get Started with Dropbox.pdf"; //This is my source inside the app folder
    public static String DropboxDownloadPathTo = "/storage/emulated/0/DropboxItems/hello";  //This is my destination
    public  static String dropboxdirpath="/storage/emulated/0/DropboxItems/";
    public static File dir=new File(dropboxdirpath);

    // In the class declaration section:
    static private DropboxAPI<AndroidAuthSession>  mDBApi;




    @Override

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

        AppKeyPair appKeys = new AppKeyPair(APP_KEY, APP_SECRET);
        AndroidAuthSession session = new AndroidAuthSession(appKeys);
        session.setOAuth2AccessToken(ACCESS_TOKEN); //I am using access token so no need for other authentication

        mDBApi = new DropboxAPI<AndroidAuthSession>(session);

        btn=(Button)findViewById(R.id.button);
        btn.setOnClickListener(click);
    }

private View.OnClickListener click=new View.OnClickListener() {

@Override
public void onClick(View view) {

DownloadFromDropboxFromPath(DropboxDownloadPathTo,DropboxDownloadPathFrom);

}};


//This is the download function 

private void DownloadFromDropboxFromPath (String downloadPathTo, String downloadPathFrom)

       DropboxDownloadPathTo = downloadPathTo;
        DropboxDownloadPathFrom = downloadPathFrom;
       private void DownloadFromDropboxFromPath(){
           {    dir.mkdirs();
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                Toast.makeText(getApplicationContext(), "Download file ...", Toast.LENGTH_SHORT).show();
                Thread th = new Thread(new Runnable() {
                    public void run() {

                        File file = new File(DropboxDownloadPathTo + DropboxDownloadPathFrom.substring(DropboxDownloadPathFrom.lastIndexOf('.')));
                        if (file.exists()) file.delete();
                        try {
                            FileOutputStream outputStream = new FileOutputStream(file);

                            MainActivity.mDBApi.getFile(DropboxDownloadPathFrom, null, outputStream, null);

                           getMain().runOnUiThread(new Runnable() {

                                @Override
                                public void run() {
                                    Toast.makeText(getApplicationContext(), "File successfully downloaded.", Toast.LENGTH_SHORT).show();
                                }
                            });
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                });
                th.start();
            }
        });
    }






}
    public MainActivity getMain()
    {
        return this;
    }

}