I’ve been exploring NativeScript recently, and decided to go “back to the basics” to make sure I understood the foundation of NativeScript before using it on an upcoming production project. When going “back to the basics,” I mean reading the documentation provided by the Telerik team on the NativeScript website, and creating sample apps based upon the online documentation. When I was writing my demo app, I found that the NativeScript application launch event does not fire.

NativeScript Application Launch Event Does Not Fire

NativeScript apps have a core set of application-level events that fire throughout the app lifecycle (launch, suspend, resume, exit, low memory, and uncaught error). The launch event is supposed to be raised when the application launches; however, in my demo app, I could not get the lunch event to fire.

In my testing, I tried to simplify my code, and I had the following in my app.js file (which is the file acting as the application bootstrapper, launching the app):

var application = require("application");
application.mainModule = "views/main/main";
application.start();

// launch event raised when the application launches 
application.on(application.launchEvent, function (args) {
	console.log("App launching...");
});

When using the above code, the application launch event does not fire. When I originally wrote this code, it was probably too late at night, so I wasn’t really thinking what I wrote. Let’s break it down to see where my error was.

var application = require("application");

Line 1: load the “application” module, storing a reference to the module in the application variable.

application.mainModule = "views/main/main";

Line 2: set the main starting page to the view named “main”, located in “views/main”.

 application.start();

Line 3: start the application, loading the main view.

// launch event raised when the application launches
application.on(application.launchEvent, function (args) {
	console.log("App launching...");
});

Lines 5-8: tell the NativeScript runtime to print “App launching…” to the console when the application launches.

Launch Event Does Not Fire When Declared After the Application Starts

When I ran the above code, I couldn’t get my application to output to the console when it started. After several hours staring at my code in the middle of the night, I decided to sleep on it. When I woke up the next morning, I knew what I did wrong.

Did you see my mistake…it took me a restless night of sleep to realize the simple problem. One line 3, I started the application. After I started the application (lines 5-8), I told the NativeScript runtime to execute the anonymous function that outputs “App launching…” to the console. Let me say that again…After I start the application, I told the NativeScript runtime to execute a startup/launch command.

Launch Event Must Be Placed Before Application Start Command

To get my application to output the “App launching…” text to the console, all I had to do was tell my application to respond tot he launch event before the application started. Sounds simple, right? It is.

Here’s the updated code that I used in my app (after a good night’s sleep):

var application = require("application");
application.mainModule = "views/main/main";

// launch event raised when the application launches
// if this command is not placed before the application.start(), then it won't execute
application.on(application.launchEvent, function (args) {
	console.log("App launching...");
});

// start the application AFTER you tell it to respond to the launch event
application.start();

Programming 101: Tell Your Application to do Something Before it is Supposed to do “It”

So, the lesson learned is something I learned in my first computer science class: if you want your application to do something, you need to tell it do do it before it is supposed to happen. After thinking how I got caught up on something this simple, I thought back to a great lesson I learned with my 2 young boys – you need to set the expectation first; otherwise, they don’t/won’t do what you’re expecting.

If you ensure your application launch event registration code is executed before the application starts, your launch event code will executes when the application starts.