0 Comments

Image with red dots - intended to look fancy

Some apps require quite a bit of content which is fairly static but changes over time and then the app should adjust and provide the user with the new content. Let's assume we want an app that provides us with quotes and their authors. We could just add the quotes to our app but whenever we wanted to update the app we would have to redeploy our app to the store(s). This can range from an inconvenience to requiring technical expertise for updating the app for simply correcting such a simple thing as a comma. So it becomes evident that in these cases we would like to separate the content from the app itself.

Hosting content does not require running any logic on the server. We do not need any other service than that of a simple file share. With perhaps one or two additional requirements regarding security etc. but more on that later. And that is exactly what Azure Blob Storage can provide us with.

Setting up the blob storage

You are required to have an Azure Account to create a blob storage, the steps, therefore, you can find here. On Azure create a blob storage, under containers, create a Container if you haven't done so already and then upload your data to it. In this sample, we will upload a single JSON file.

Showing Blobstorage Container with one JSON File

In a real application, we could also provide multiple other files including videos and other static files. But for this simple demo, we will stick to a lonely JSON file. We can access the content by calling the URL:

Sample get request with Postman

Having something on a public server always raises questions about security and the sorts. So let's have a look at them.

Security

So let's start with what you get out of the box. The easiest security is if your data is public. Like on a website but for your app. The default you can limit the anonymous access to your blob storage as follows:

  • read-only container: which will allow everyone to read at the container level i.e. "look at the directory" and list all the blobs within.
  • read-only blob: here the caller will have to know which blob he wants to open and is limited to reading the blob storage itself.
  • no read access for anonymous: this will restrict the access to authenticated parties only.

In our sample, we will stick with anonymous blob access. But if you are interested in adding some extra layers of security be sure to check out the Azure Storage security guide which explains the different methods of authentication from shared keys all the way to using Azure Active Directory (Azure AD) for securing the access of your data. Which in summary means Blob Storage is not only quick and convenient in the beginning but can also be modified to add some serious layers of protection.

The client

AppInAction

On the client, we will want to consume the hosted resource and use it in our app. You can do this rather simply within a .Net Standard library using JSON.Net as follows:

string quotesJson;
using (var httpClient = new HttpClient())
{
    var response = await httpClient.GetAsync("https://gnabberonlinestorage.blob.core.windows.net/alpha/quotes.json");
    quotesJson = await response.Content.ReadAsStringAsync();
}
_quotes = JsonConvert.DeserializeObject<List<QuoteInfo>>(quotesJson);

While the above sample works and will get us our data it is not really smart, it will always pull the entire file even if nothing has changed. Fortunately, Azure Blob Storage supports ETags which allows us to be smarter when creating the call, by adding the If-None-Match header to our request as follows:

public async Task Init()
{
    if (_quote != null) return;

    IsBusy = true;
    string quotesJson;
    using (var httpClient = new HttpClient())
    {
        if(!string.IsNullOrEmpty(CurrentEtagVersion)) httpClient.DefaultRequestHeaders.Add("If-None-Match", CurrentEtagVersion);
        var response = await httpClient.GetAsync("https://gnabberonlinestorage.blob.core.windows.net/alpha/quotes.json");

        quotesJson = response.StatusCode == HttpStatusCode.NotModified
            ? ReadQuotesFromCache()
            : await response.Content.ReadAsStringAsync();

        UpdateLocalCache(response.Headers.ETag, quotesJson);
    }
    _quotes = JsonConvert.DeserializeObject<List<QuoteInfo>>(quotesJson);

    PickAndSetQuote();
    IsBusy = false;
}

If the local and remote ETag match, we will not receive any data with the call leaving us with a very small data footprint for this call. The code handling the caching is shown below. Note that for accessing the preferences Xamarin.Essentials were used:

public string CurrentEtagVersion => Preferences.Get(EtagKey, string.Empty);

private void UpdateLocalCache(EntityTagHeaderValue eTag, string quotesJson)
{
    // Only update the cache if we need to
    if (eTag == null || CurrentEtagVersion == eTag.Tag) return;
    Preferences.Set(EtagKey, eTag.Tag);
    File.WriteAllText(_quotesFilename, quotesJson);
}

private string ReadQuotesFromCache()
{
    if (!File.Exists(_quotesFilename)) return string.Empty;
    return File.ReadAllText(_quotesFilename);
}

I will leave it there with this sample but since we are already storing the data in a local cache we could also consider making this app fully Offline capable. With Xamarin Essentials, which we are already using, we can check if we have a network connection and what kind of connection. This information allows us to decide if we want/can access the remote storage or rather load the data from the initial cache.

You can find the entire client sample code on GitHub.

Conclusion

In this post, we saw how you can use Azure Blob storage as a backend service to host the content of your app without having to implement any web server. You can add security layers to the storage. Tracking changes on the backend are provided out of the box via HTTP ETags.

But how much will this cost me? Probably less than you would think but check out the Blob Storage pricing to get your exact number.

0 Comments

pexels-photo-316465


When developing an app your design might require to use a font that is not available from iOS out of the box. So let’s see how a custom font can be added to ones app and how to use it in a Storyboard or Code.

Adding the font

Assuming you already have the font, note that iOS supports fonts that are stored in TTF or OTF formats. Custom fonts are copied into the Resource folder of your iOS project.

image of ios project, showing the custom font file in a subfolder named fonts in the resoucres folder

Ensure that the properties of the font file are set to BundleResource.

Creating the Fonts subfolder is optional. But since the Resources projects tends to collect a couple of items in larger projects. Let’s tidy things up from the start Smile

To use the font we will have to update the Info.plist file with the following lines:

image

Note: At the time of writing in Visual Studio 15.5.4 you will have to open the Info.plist file in an XML Editor. For this right click the file and select “Open with…” then choose the XML (Text) Editor. In Visual Studio for Mac the GUI Info.plist editor supports editing the source directly.

Using custom fonts in Storyboards

After adding for example a UILabel to the storyboard, select it. In the options click on the font and choose the custom font.

Storyboard

Using custom fonts in Code

In code behind using a custom font is pretty straight forward. For example in a label we can set the Font attribute as follows:

image

The result when running app with the combined storyboard and new label is:

Custom Font Screenshot

Conclusion

In this blogpost we went through the steps that have to be taken to add a custom font. Note that fonts are loaded during the start up of the app and if you go bonkers with them you might notice some performance impact.

You can find a small sample on GitHub. For setting the constraints in the code defined UI PureLayout.Net was used.

0 Comments

pexels-photo-64774

Update: Hey there thank you for reading my blog, since I wrote this post I have learnt a lot and have found a better way to migrate your Xamarin Apps so please check out my latest blogpost on this matter.

If you haven’t heard or dived into .Net Standard you are in for a treat. In short it provides a way to share code across platforms but in contrast to the PCL it gives you so many more platform specific features. For an in depth overview check out the official docs.

Note: If you are starting a new Project today with Visual Studio (VS) 2015.4, you will not be able to select .Net Standard by default. But the new templates for .Net Standard will be included in Visual Studio 15.5.

Creating the .Net Standard Class Library

Migrating an existing app to .Net Standard is pretty straight forward. Step one add a .Net Standard Library to replace your PCL project.

The VS Add New Project dialog, under Visual C# select Class Library (.Net Standard)

Migrating your source code

Then drag and drop all of your existing files from the PCL project to your .Net Standard library. Note that you don’t want to copy the packages.config or any of the files under properties.

CopyFiles

Now you can delete your PCL project that you have migrated. If you do so from Visual Studio note that the Project is still available in the file system. Which means you still have it if you forgot something, but also means that you will have to delete it later on if you want to remove it from the source control workspace.

Next step is to add all the NuGet packages you have used in the PCL project. If you are having trouble adding some of the packages look out for NuGet packages that are no longer needed due to .Net Standard support such as file system access. In other cases it could be because the NuGet package has not (hopefully yet) migrated to .Net Standard. In that case check out this post and don’t forget to ask the maintainers of the project when the project will be available for .Net Standard Winking smile

Hooking up the projects

You can now add the reference to the .Net Standard in your Android and iOS project. If you created a new namespace I strongly recommend you refactor them after adding them to your projects. Or else your refactoring tool of choice will only do half the magic and you will still have some work left to do.

If you are using a UWP project please read the section bellow as you will need to make some additional steps to make it work.

Fixing the csproj for Xamarin Forms

Unfortunately when moving a Xamarin Forms app over to .Net Standard you will get weird compilation errors. The cause of this is that the XAML files are referenced in the csproj file:

02_1_RemoveEmbeddedResources

Simply remove them as they are not needed and the compile errors should be history.

When using UWP

If you are using UWP as a target (I.e. using the default Project provided up to VS 2015.4). You will have to remove and add the project anew:

If you are unsure if you really have to update your UWP project. Check if you have a project.json file in your UWP project. If the answer is yes, I’m afraid you will have to follow the following steps.

  1. Remove the UWP project from the solution in Visual Studio
  2. Rename the UWP project folder in the file explorer
  3. Add a new UWP project in Visual Studio (with the same name as the one just removed)
  4. Set the minimal supported Windows 10 version to the Fall creators update

    VS Dialog Window with Target and Minimum Version set to Fall Creators Update
  5. Add any NuGet references the just removed project had (you can peek into the project.json file in the renamed location if you are unsure which packages to add)
  6. Copy and paste all your UWP files (except the project.json)
  7. Add a reference to the Standard Library project

If you know an easier way to upgrade a UWP project, please let me know in the comments bellow Smile

Conclusion

In this post we went over the steps required to migrate an existing PCL project to .Net Standard. All the steps were done with Visual Studio 15.4.

3 Comments

pexels-photo-277803

When writing an app that only allows access to certain or all parts of the app when a user is logged in requires a login screen which can be presented to the user at every screen in the app I.e. as soon as he is required to login or re-login.

In this post you will see how to create a view that can be used to enter the username and password. Further we will look at how we can use this screen regardless of which screen is currently displayed to the user.

Writing a login view

Let’s consider a view as follows.

LoginScreen

It requires a username and password, the user can confirm his entry by hitting a button which will validate his entry.

After entering a correct login the user will be presented with the apps content. In our case a simple screen containing a logout button.

So far so good, but how will we ensure that the user only sees the content after she has logged in? How do we prevent the user form simply dismissing the page? Well let’s dive into this topic as next.

Login sites and Navigation

So implementing the view and even the business logic of a login site are quite straight forward but how do can we pop up the login view whenever the user is required to authenticate himself I.e. has to re-authenticate? And how do we prevent him from leaving the screen. Luckily all this can be solved by using the modal navigation backed in to Xamarin Forms. Utilizing a simple navigation service from a previous post, we can ensure invoke the navigation to the login page from any page or even when resuming the app or on start up:

Using modal navigation with a simple  navigation service allows us to implement a login dialog that can be pop over any view currently displayed and return the user to the sensitive content once he is properly authenticated.

No way back

Though modal pages do not provide the user with a software button in the navigation bar to return to the previous page. The dedicated OS back button on Android and Windows 10 can still be used by the user. To ensure the user can not leave the page via the OS button the OnBackButtonPressed method has to be override as follows:

Instant Login View navigation

When requiring the user to log in on the initial start up of the app or resume. Often it is desired to simply overlay the login view over the page that should be displayed when the user is successfully authenticated. Want we do not want is the user to ever see the landing page before being logged in. To achieve this effect we can want to insert our check before the page is displayed to the user and then navigate without animation to the login page:

Improve user input experience

When creating the user login what would be nice is if the user could simply navigate from the username to the password field via the enter button on the keyboard. After completing the password it could directly verify the username and password when pressing enter. This can be done in the code behind of the login view as follows:

Conclusion

In this blog post we saw how we can create a view for a login page and also implement the logic behind it. Through modal navigation we saw how we can “capture” a user on a view and prevent him from leaving the view before he has entered some valid credentials.

We then improved the UX by showing the login page instantly when resuming or starting the app. Plus improving the entry of username and password.

You can find a sample of the login view on GitHub.

3 Comments

pexels-photo-297755

If you have ever written a Xamarin Forms app and wanted to navigate from within a View Model to another page. Then you have come over the issue that the navigation logic usually resides in the view. Since every best practice blog post, course and video tells us that the view and business/control logic should be separate. We are often left with the question how to integrate a View Model based navigation.

What we want to do

The goal of this post is to show you how we can initiate a navigation call from the View Model. Without adding dependencies to your view or writing call back methods.

Navigation 101

In Xamarin Forms there are two fundamental ways of navigating between pages:

  • Push
  • Modal

Push navigation is the most straight forward way for navigating in an app. The main page in the App.xaml.cs is a NavigationPage which contains the main page displayed to the user:

When navigating to the next page, the page gets pushed onto the navigation stack. The user can then navigate back via Screen (iOS & Windows) or Hardware button (Android & Windows). Navigating back will dispose the current page, pop a page from the navigation stack and display it to the user.

When navigating to a page using Modal. The target page is displayed in a separate navigation context. This results in the behaviour that the user is no longer displayed with a software back button from the OS. The back navigation call differs from Push. Thus it requires invoking a different method of the Navigation Page.

Modal navigation is a great option to capture the user on a view. This is not generally what we intend to do with a user. But if the user has to login or the users consent is required (are you really sure...) filling out a form. Then modal navigation allows keeping the user on a page and handling the navigation via custom defined logic.

Note: Modal navigation is an iOS concept in it’s origins. Under Android and Windows the user has system buttons to navigate the app. If the user should not be able to leave the page and return to the previous page I.e. user has to login first. The developer will have to override the OnBackButtonPressed with a return true in the code behind of the view.

Navigation TL;DR;

We have two navigation methods in Xamarin.Forms. These are Push and Modal. Each has different ways of invoking the navigation. Which also applies to the back navigation.

Using a navigation service

In general the navigation service will have to contain a Navigation Page to handle the navigation stack. Further it needs to offer a Method for navigating to a page via push or modal navigation and allow navigating back.

The initialisation of the navigation service can now be done in the App.xaml.cs by registering all the views with a corresponding key.

In a view model we can now navigate to a page by providing the service with the key of the target page. If required we can also pass in a parameter. Since the parameter is of type object there are no limitations there.

As mentioned before modal navigation leaves the current navigation stack and displays the page on it’s own. If we want to navigate from a modal root page via push to a child page. The modal root page has to be in a Navigation Page itself. This is what the navigation service does automatically. It further removes the navigation header of the modal root page.

Back navigation checks what back navigation is intended for the current page. A root page in a navigation stack is either the root page of the app or a root page in a modal navigation stack. In either case it will choose the appropriate back navigation method.
If the page is not the root page we simple navigate back to the previous page that was pushed onto the navigation stack.

Conclusion

Using the navigation service described in this blog post. It is possible to navigate from the view model to other pages. Doing so we do not add dependencies to the view stack e.g. Xamarin Forms. Nor do we have to write cumbersome call back methods for each navigation action over and over again.

I would like to thank Laurent Bugnion at this point who laid down the base of the described navigation service above in with his initial implementation of a Navigation Service for Xamarin Forms when using MVVM Light.

You can find a running sample on GitHub.