Async Vala Example - asynchronous

Question

In the Book "Introduction to Vala" by Dr Michael Lauer, he has mentioned that the lib Soup async api is broken. I'm struggling to write a simple example using session.queue_message that query radio stations using the service from radio-browser. Here is my code. I would appreciate any help form experienced Programmers like "Al Thomas". Thank you.
public class Station : Object {
// A globally unique identifier for the change of the station information
public string changeuuid { get; set; default = ""; }
// A globally unique identifier for the station
public string stationuuid { get; set; default = ""; }
// The name of the station
public string name { get; set; default = ""; }
// The stream URL provided by the user
public string url { get; set; default = ""; }
// and so on ... many properties
public string to_string () {
var builder = new StringBuilder ();
builder.append_printf ("\nchangeuuid = %s\n", changeuuid);
builder.append_printf ("stationuuid = %s\n", stationuuid);
builder.append_printf ("name = %s\n", name);
builder.append_printf ("url = %s\n", url);
return (owned) builder.str;
}
}
public class RadioBrowser : Object {
private static Soup.Session session;
// private static MainLoop main_loop;
public const string API_URL = "https://de1.api.radio-browser.info/json/stations";
public const string USER_AGENT = "github.aeldemery.radiolibrary";
public RadioBrowser (string user_agent = USER_AGENT, uint timeout = 50)
requires (timeout > 0)
{
Intl.setlocale ();
session = new Soup.Session ();
session.timeout = timeout;
session.user_agent = user_agent;
session.use_thread_context = true;
// main_loop = new MainLoop ();
}
private void check_response_status (Soup.Message msg) {
if (msg.status_code != 200) {
var str = "Error: Status message error %s.".printf (msg.reason_phrase);
error (str);
}
}
public Gee.ArrayList<Station> listStations () {
var stations = new Gee.ArrayList<Station> ();
var data_list = Datalist<string> ();
data_list.set_data ("limit", "100");
var parser = new Json.Parser ();
parser.array_element.connect ((pars, array, index) => {
var station = Json.gobject_deserialize (typeof (Station), array.get_element (index)) as Station;
assert_nonnull (station);
stations.add (station);
});
var msg = Soup.Form.request_new_from_datalist (
"POST",
API_URL,
data_list
);
// send_message works but not queue_message
// session.send_message (msg);
session.queue_message (msg, (sess, mess) => {
check_response_status (msg);
try {
parser.load_from_data ((string) msg.response_body.flatten ().data);
} catch (Error e) {
error ("Failed to parse data, error:" + e.message);
}
});
return stations;
}
}
int main (string[] args) {
var radio_browser = new RadioBrowser ();
var stations = radio_browser.listStations ();
assert_nonnull (stations);
foreach (var station in stations) {
print (station.to_string ());
}
return 0;
}

Answer 1

While I'm not Al Thomas, I still might be able to help. ;)
For async calls to work in there needs to be a main loop running, and typically from the main program thread. Thus you want to create and execute the main loop from your main() function, rather than in your application code:
int main (string[] args) {
var loop = new GLib.MainLoop ();
var radio_browser = new RadioBrowser ();
// set up async calls here
// then set the main loop running as the last thing
loop.run();
}
Also, if you want to wait for an async call to complete, you typically need to make the call using the yield keyword from another async function E.g:
public async Gee.ArrayList<Station> listStations () {
…
// When this call is made, execution of listStations() will be
// suspended until the soup response is received
yield session.send_async(msg);
// Execution then resumes normally
check_response_status (msg);
parser.load_from_data ((string) msg.response_body.flatten ().data);
…
return stations;
}
You can then call this from the (non-async) main function using the listStations.begin(…) notation:
int main (string[] args) {
var loop = new GLib.MainLoop ();
var radio_browser = new RadioBrowser ();
radio_browser.listStations.begin((obj, res) => {
var stations = radio_browser.listStations.end(res);
…
loop.quit();
});
loop.run();
}
As further reading, I would recommend the async section of the Vala Tutorial, and the asyc examples on the wiki as well.