controller.js

const 
crypto = require('crypto'),
config = require('./config'),
NodeCache = require( "node-cache" ),
rp = require('request-promise');

var mycache = new NodeCache();


//steps 1,2,3
module.exports.home = async (req,res,next)=>{    
  let token = mycache.get("aTempTokenKey");
  if(token){
    try{
      let paths = await getLinksAsync(token); 
      res.render('gallery', { imgs: paths, layout:false});
    }catch(error){
      return next(new Error("Error getting images from Dropbox"));
    }
  }else{
  	res.redirect('/login');
  }
}  

//steps 4,5,6
module.exports.login = (req,res,next)=>{

  //create a random state value
  let state = crypto.randomBytes(16).toString('hex');

  //Save state and temporarysession for 10 mins
  mycache.set(state, "aTempSessionValue", 600);

  let dbxRedirect= config.DBX_OAUTH_DOMAIN 
  + config.DBX_OAUTH_PATH 
  + "?response_type=code&client_id="+config.DBX_APP_KEY
  + "&redirect_uri="+config.OAUTH_REDIRECT_URL 
  + "&state="+state;
  
  res.redirect(dbxRedirect);
}


//steps 8-12
module.exports.oauthredirect = async (req,res,next)=>{

	if(req.query.error_description){
		return next( new Error(req.query.error_description));
	} 

	let state= req.query.state;
	if(!mycache.get(state)){
		return next(new Error("session expired or invalid state"));
	} 

  //Exchange code for token
  if(req.query.code ){

  	let options={
  		url: config.DBX_API_DOMAIN + config.DBX_TOKEN_PATH, 
      //build query string
      qs: {'code': req.query.code, 
      'grant_type': 'authorization_code', 
      'client_id': config.DBX_APP_KEY, 
      'client_secret':config.DBX_APP_SECRET,
      'redirect_uri':config.OAUTH_REDIRECT_URL}, 
      method: 'POST',
      json: true 
    }

    try{

    	let response = await rp(options);

      //we will replace later cache with a proper storage
      mycache.set("aTempTokenKey", response.access_token, 3600);
      res.redirect("/");

    }catch(error){
    	return next(new Error('error getting token. '+error.message));
    }        
  }
}



/*Gets temporary links for a set of files in the root folder of the app
It is a two step process:
1.  Get a list of all the paths of files in the folder
2.  Fetch a temporary link for each file in the folder */
async function getLinksAsync(token){

  //List images from the root of the app folder
  let result= await listImagePathsAsync(token,'');

  //Get a temporary link for each of those paths returned
  let temporaryLinkResults= await getTemporaryLinksForPathsAsync(token,result.paths);

  //Construct a new array only with the link field
  var temporaryLinks = temporaryLinkResults.map(function (entry) {
    return entry.link;
  });

  return temporaryLinks;
}


/*
Returns an object containing an array with the path_lower of each 
image file and if more files a cursor to continue */
async function listImagePathsAsync(token,path){

  let options={
    url: config.DBX_API_DOMAIN + config.DBX_LIST_FOLDER_PATH, 
    headers:{"Authorization":"Bearer "+token},
    method: 'POST',
    json: true ,
    body: {"path":path}
  }

  try{
    //Make request to Dropbox to get list of files
    let result = await rp(options);

    //Filter response to images only
    let entriesFiltered= result.entries.filter(function(entry){
      return entry.path_lower.search(/\.(gif|jpg|jpeg|tiff|png)$/i) > -1;
    });        

    //Get an array from the entries with only the path_lower fields
    var paths = entriesFiltered.map(function (entry) {
      return entry.path_lower;
    });

    //return a cursor only if there are more files in the current folder
    let response= {};
    response.paths= paths;
    if(result.hasmore) response.cursor= result.cursor;        
    return response;

  }catch(error){
    return next(new Error('error listing folder. '+error.message));
  }        
} 


//Returns an array with temporary links from an array with file paths
function getTemporaryLinksForPathsAsync(token,paths){

  var promises = [];
  let options={
    url: config.DBX_API_DOMAIN + config.DBX_GET_TEMPORARY_LINK_PATH, 
    headers:{"Authorization":"Bearer "+token},
    method: 'POST',
    json: true
  }

  //Create a promise for each path and push it to an array of promises
  paths.forEach((path_lower)=>{
    options.body = {"path":path_lower};
    promises.push(rp(options));
  });

  //returns a promise that fullfills once all the promises in the array complete or one fails
  return Promise.all(promises);
}