Skip to main content

Promises and Asynchron calls in Metro Application and JavaScript

After I wrote this post I realize that I should write a post about promises in general. Because of this I wrote the following post.
The new Windows brought a new type of applications that are called Metro Applications. These applications are design to use fewer resources as possible and to be used very easily on tablets. Metro Applications can be written not only in XAML and C# but also in HTML5 and JavaScript. This is something new for Microsoft world; we can develop native application in JavaScript. Now JavaScript is a first-class citizen in Metro world.
Of course we cannot talk about Metro Applications without talking about asynchrony methods. We need this type of methods as we need water. When we make a call to a service or we want to write something to the disk we need these calls to be asynchronous.
The most used patterns for asynchronous calls in JavaScript is the callback pattern, where we specify as parameter the callbacks functions that will be called when the given actions ends. This pattern is very used in jQuery worlds.
Microsoft decides to use another well know standard for asynchron calls. The guys from Common/JS propose another standard that was adopted by Microsoft. The name of this standards is Promises/A and is very similar with how asynchronous task work on .NET 4.5. Also this standard is based on fluent pattern, because of this we can read and understand any code written more easily. If you already worked with Node.js, Ruby or jQuery 1.5 maybe you already used this standard without knowing.
Basically a promise represents a value that will be fulfilled in the future. The exact moment cannot be defined. In a Metro Application written in JavaScript any promise return an object that have two methods that are very important and similar – then and done.
promise.then(onComplete, onError, onProgress)
The then method have three parameters where we can specify the functions (handlers) that will be called when:
  • onComplete - the call end with success
  • onError – is called when during the execution of a code an error appeared
  • onProgress - a function that is called when the promise reports progress.
Of course we can omit any of the 2th a 3th parameter. The only one that is mandatory is the first one. The done function has the same signature as the then, the difference is the error handling. This function will throw any error that appeared in the call execution, in comparison with then function that will only return an error state.
Using these promises we don’t need to rely on anonymous functions that are defined inline. We can define functions in our code that can be specified in the then or done. When an error is thrown is more easily to detect the original location of the exceptions.
In Metro Applications all the Windows Runtime and Windows Library for JavaScript is defined using this standard. Because of this this knowing this standard is a have to and is not a nice to have (know).
fooWebService.get("http://www.microsoft.com")
.then(function(result) {
// process result
return fooWebService.get("http://windows.azure.com/")
})
.then(function(result) {
// process result
}
We can have as many then appended one after another. The condition is to have in each onComplete function to return another promise. If we don’t do this, the onComplete of the second then will be immediately called, without waiting the response of the first call.
The WinJS.Promise object defined by the framework came with some helper methods. Besides then and done functions the most important functions are:
  • join – create a promise that permits us to specify a list of promises and only when all this promises are fulfilled the onComplete is called.
  • any – create a promise that is fulfilled when one of the promises is fullfilled
  • wrap – wrap a non-promise function into a promise. It is very useful when we have a chain of promises.
For example we can use any to wait any specified promises from the list to be fullfiled. When one of them is fullfiled the callback is automatically caled.
WinJS.Promise.any([someMethod1Async(), someMethod2Async("someParam")])
.then(function() { console.log("done");});
Promises are not only defined by the system, we can also defined promises very easily. When we create a promise we need to create and return a WinJS.Promise object. The constructor accepts a function that has three parameters (the onComplete, onError, onProgress parameters). In the next example we defined a promise that wait for 10 seconds and call the complete function (this a one of the classic examples):
   function wait10SecondsAsync() {
return new WinJS.Promise(function (onComplete, onError, onProgress) {
var seconds = 10;
var intervalId = window.setInterval(function () {
seconds--;
onProgress(seconds);
if (seconds < 1) {
window.clearInterval(intervalId);
onComplete();
}
}, 1000);
});
}
In the end some small hints when working with promises:
  • always name suffix an asynchrone method with “Async”
  • when more than one promise are bound don’t forget to return the promises on each onComplete function
  • for the last call of an asynchrone chain call is recommended to use done and not then (done throws any exceptions to the upper code)
  • use WinJS.Promise.any and WinJs.Promise.wait when you need to wait for one or more promises
In conclusion promises are very powerful when we are talking about asynchrone calls in JavaScript language. When are used clevered, promises can help us not only to write beautiful cod but the code it almost writes by himself.

Comments

Popular posts from this blog

How to check in AngularJS if a service was register or not

There are cases when you need to check in a service or a controller was register in AngularJS.
For example a valid use case is when you have the same implementation running on multiple application. In this case, you may want to intercept the HTTP provider and add a custom step there. This step don’t needs to run on all the application, only in the one where the service exist and register.
A solution for this case would be to have a flag in the configuration that specify this. In the core you would have an IF that would check the value of this flag.
Another solution is to check if a specific service was register in AngularJS or not. If the service was register that you would execute your own logic.
To check if a service was register or not in AngularJS container you need to call the ‘has’ method of ‘inhector’. It will return TRUE if the service was register.
if ($injector.has('httpInterceptorService')) { $httpProvider.interceptors.push('httpInterceptorService&#…

ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded

Today blog post will be started with the following error when running DB tests on the CI machine:
threw exception: System.InvalidOperationException: The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer' registered in the application config file for the ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application. See http://go.microsoft.com/fwlink/?LinkId=260882 for more information. at System.Data.Entity.Infrastructure.DependencyResolution.ProviderServicesFactory.GetInstance(String providerTypeName, String providerInvariantName) This error happened only on the Continuous Integration machine. On the devs machines, everything has fine. The classic problem – on my machine it’s working. The CI has the following configuration:

TeamCity.NET 4.51EF 6.0.2VS2013
It seems that there …

Run native .NET application in Docker (.NET Framework 4.6.2)

Scope
The main scope of this post is to see how we can run a legacy application written in .NET Framework in Docker.

Context
First of all, let’s define what is a legacy application in our context. By a legacy application we understand an application that runs .NET Framework 3.5 or higher in a production environment where we don’t have any more the people or documentation that would help us to understand what is happening behind the scene.
In this scenarios, you might want to migrate the current solution from a standard environment to Docker. There are many advantages for such a migration, like:

Continuous DeploymentTestingIsolationSecurity at container levelVersioning ControlEnvironment Standardization
Until now, we didn’t had the possibility to run a .NET application in Docker. With .NET Core, there was support for .NET Core in Docker, but migration from a full .NET framework to .NET Core can be costly and even impossible. Not only because of lack of features, but also because once you…