# Weather application (JavaScript)

Susanna Huhtanen
ihmis.suski@gmail.com 2012
Marta Maria Casetti
mmcasetti@gmail.com 2013
How to plan an application that uses asynchronous calls. Asynchronous calls will be presented through a weather application.

# Weather application

        In this guide well construct a weather application using asynchronous calls. Weather information in this example is fetched from geonames.org and the application is using the ICAO codes  to place your weather request. To write and run all the code examples yourself, you need an editor to write code in, Terminal and GNOME 3 or higher installed into your computer. In this guide we we'll go through the following parts:

           Planning the graphical user interface
           Asynchronous calls
           The main program file
           Local library GeoNames
           Autotools and Icons

After reading this tutorial, you should see this in your screen:

# Planning the graphical user interface

Structuring an application for GNOME 3 means you will be using GTK+. The most important thing is to remember that the main window will only accept one widget. You must plan your structure accordingly (this example is using Gtk.Grid). A useful method is to draw out the main window and place every widget needed inside that box. By looking at an image of your future application it is easier to tell what are the relations between widgets. For example Gtk.Grid places your widgets in relation to other widgets, so after the first widget is in place, placing widgets can be done in relation to any widget on the grid.

# Asynchronous calls

With many programming languages, all operations are run synchronously - you tell the program to do something, and it will wait until that action completes before proceeding. This is however bad for graphical user interfaces, as then the whole application will be frozen while the program waits for the operation. Going asynchronous (async) helps here. With async calls, your UI won't be blocked with any requests. Async calls make your application more flexible and better equipped to handle situations when calls take more time than expected or for some reason get jammed. Async calls can be used for example file system I/O and for slower calculations in the background.

In this example we have to get data from geonames.org. While we do that we want the rest of our program to continue. If we wouldn't get any information from geonames.org for the lack of internet connection and this would be a synchronous application we would never get to the point where our main_quit() is processed correctly and the application would have to killed from Terminal.

# The different parts of the program