Monthly Archives: January 2017

Building a Xamarin app with Azure Functions and OneDrive

Preface

(you can skip this if you are not interested in the underlying idea, no hard feelings!)

A while ago my brother -who sells windows (no, not the OS) and doors- asked me if I could build a small app for his business.
The key feature he wanted to have in his app is a map that would show all the references of the company. Users should then be able to filter on the type of window and the colors they are interested in and see the addresses on the map where this type of window is installed. This would make it easy for them to see how these windows and colors look like in real life.
Of course this list of references should be dynamic and easy to update. My brother wouldn’t mind ‘updating a text file’ but he has an overview of all references in Excel.

I thought this was actually a nice idea, so I decided to make a small prototype of this use case.

The setup

The complete setup of this PoC will look something like this:
Xamarin Azure Funtcion Overview

For this blogpost, I’m going to focus on the bottom part.
Xamarin Azure Http Trigger Overview
I’ll dive deeper in the rest of the flow in upcoming blogposts.

Implementing the Azure Function

So what are these Azure Functions? Azure Functions is a way to create a serverless, event-driven ‘backend’ on Azure. We can do all sorts of stuff with it, but in this example we are going to use it as an http endpoint that returns some XML data.
Let’s open up the Azure Portal and create a new Function App:
Create new function app

Give at a name, set the settings and Create it!
Create new function app

Wait a few moments for Azure to create your Function App…

When it’s created, we’ll be presented with some sort of ‘Quick Start’ wizard. For this scenario, we’ll use Webhook + API as we want to create an endpoint to request data from. And of course, we select C# as programming language (because JS isn’t a real programming language… 😉 ). In fact, when we select C#, it is actually C# Script we are going to use.
Creating new Webhook API app
Create!
Take a look at the code that has been generated for you.

This code is executed whenever we make a request to our service.
Click the ‘Run’ button and watch the output in the Log Window bellow.
TIP: you can change the parameters that are used to test run you service by clicking on the ‘Test‘ tab on the right of your screen. There you can add query string parameters, define request headers or a request body. So, as a test, you could add a query string parameter named ‘name‘ and give it a value. Adjust your script by inserting this just above the return statement:  log.Info(name);  “Save and run” and you should see the value of the name parameter being printed in your logs.

Click the ‘Integrate‘ tab on the left of your screen. In this screen we can specify additional inputs. Inputs are ‘things’ we can use inside our function. Just to be clear, this has NOTHING to do with GET or POST parameters, these are objects like a database for example that we can use inside our function. In our case, we want to specify a file in which all of our addresses are defined. Click the ‘New Input’ button and select ‘External File (preview)’.

Azure Function External File input

In the next screen we can configure where the service should fetch the file from. In my PoC I wanted to get the file from OneDrive. You can do this by clicking the ‘new‘ button next to External File connection. A blade will open up and in there you can select which api to use in order to fetch your file.

Azure Function External File API

Once you select an API, the next step is configuring and authorizing your Function. As soon as everything is configured, the only thing you need to do is specify the path to the file you would like to access. I uploaded a small XML-file with some addresses to my OneDrive and entered the path in  External File (Preview) input. Please note the ‘File parameter name‘, you can change it if you want, but we will need it in just a second.

Azure Function External file input

Click save and we can finally start to write some code!!

Back in the ‘Develop tab‘, the first thing we need to do, is change the signature of our Run method. We just declared this External File as input, and now we want to access it in our method. We can simply do this by adding an additional input parameter. Mind you that the name should exactly match the name you filled in as ‘File parameter name’. If it doesn’t match, the Log window will tell you.

Now, let’s write some logic! In my sample XML, I listed addresses of Ordina and Microsoft in Belgium. Via a query parameter I would like to specify of which company I would like to see the addresses. We can access the request’s query string parameters via the req parameter. Getting the ‘company‘ request parameter can be done as follows:

Next we want to process our XML inputFile . The easiest way to do this is by using a XDocument and use Linq to get the things we are interested in. In order to be able to use XDocument , we need to reference the System.Xml.Linq dll. In C# Script -which we are using here- you can do this by adding  #r "System.Xml.Linq" at the top of our script file. Now we can define a using for the System.Xml.Linq  namespace and write all of our logic:

As you can see in the code above, I return the resulting list as an anonymous type, and not the XML list directly. This is because Azure determines the serialization of the response data itself, using the Accept header of the request ( Application/xml  or Application/json ). This way the caller of the endpoint can decide in which format he wants the result. And XML data serialized as JSON is just evil, bad, dirty, …

When this is done, we can go and create our client, in my case I wanted to create a Xamarin client.

Creating the Xamarin Forms Client

Nothing fancy here, I’m just going to create a very basic UI, just to show how easy it is to communicate with the Azure Function. In future posts I’ll pimp up the UI for the Xamarin app. In this example, there will be a ListView on the page which will allow the user to select a company. The addresses of the selected company will be shown in an other ListView .

So go ahead and create your Xamarin Forms (XAML) project. I’ve added a  Address model and a MainViewModel to the project so my project looks like this:

Xamarin Solution overview

The Address model represents the addresses I get back from my Azure Function and want to display in my app.  The model looks like this:

Tip: I could very rapidly create this model by taking the a JSON response from my Azure Function, open the Address class, go to ‘Edit‘, ‘Paste Special‘, ‘Paste JSON as Classes‘. This will generate classes that corresponds to the given JSON. It will also generate a ‘RootObject‘ class, but in this case I removed everything except the Address class. Productivity++! 🙂 I also added an extra property AddressDetails which just concats the individual properties.

As already stated, the UI of my application is really simple. So this is what the XAML of my MainPage looks like:

In the code-behind of the MainPage.xaml, I set the BindingContext to a new instance of the MainViewModel :

And now the final peace of the puzzle: implementing the MainViewModel .

First, I’m going to add the properties to the ViewModel which I want to expose to the view:

I think all properties speak for themselves. The only thing we need to do now, is making a request to our Azure Function whenever the user selects a company. Therefore, when we set a new value to the SelectedCompany property, we will call a method that will talk to our function.

The code above is triggered whenever the SelectedCompany changes (user selects it from the ListView ). We take the URL to the HTTP Trigger Azure Function, append the extra company query string parameter and do the call via the HttpClient . When we get a response, we read the string content; which is JSON as we specified the Accept-header to be application/json . We take this string and use JSON.NET to deserialize the json string to an IEnumerable of Address . Once it is deserialized, we set our property Addressess to these new addresses.

And that’s it! That’s all there is to it to get the data from our Azure function!!

Conclusion

Although this is quite a lengthy blogpost (but with a lot of code and images 😉 ), I think it illustrates how easy and simple it is to setup a backend for your mobile app using Azure Functions. Azure functions can do a lot more than what is in this example, so go ahead and play with it, it is real fun stuff!

In following blogposts I’m going to further expand this app, like for example showing the addresses on a map, allowing a ‘third party’ to update the Addresses XML list, …

If you have any further questions or remarks, you can always drop a comment below!

 

Enjoy!