[PWA] 16. Clean IDB

When we save items to IndexDB, we need to think about clean the items or keep those in a short list.

IndexController.prototype._onSocketMessage = function(data) {
  var messages = JSON.parse(data);

  this._dbPromise.then(function(db) {
    if (!db) return;

    var tx = db.transaction('wittrs', 'readwrite');
    var store = tx.objectStore('wittrs');
    messages.forEach(function(message) {
      store.put(message);
    });

    // TODO: keep the newest 30 entries in 'wittrs',
    // but delete the rest.
    //
    // Hint: you can use .openCursor(null, 'prev') to
    // open a cursor that goes through an index/store
    // backwards.
    store.index('by-date').openCursor(null, 'prev')
        .then((cursor)=>{
          return cursor.advance(); // only 30 itmes are kept
        }).then(function deleteCursor(cursor){
            if(!cursor){
              return;
            }else{
              cursor.delete();
              return cursor.continue().then(deleteCursor);
            }
        })
  });

  this._postsView.addPosts(messages);
};