ApplicationName.login = function (params) {
    function gotFS(fileSystem) {
        YourAppName.app.fileSystem = fileSystem;
        if (YourAppName.app.fileSystem == null) {
            return;
        };
        YourAppName.app.fileSystem.root.getFile("login.txt", { create: true }, GetFileEntryReader, FileSystemFail);
    };
 
 /*
 ...
 ...
 the other code
 ...
 ...
 */

        viewShowing: function () {
            //the problem is here, error when call requestFileSystem
   setTimeout(function () {
                window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, gotFS, fail);
            }, 1000)
        }
    };
    return viewModel;
};