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Getting started with CSS in Xamarin Forms 3

There’s a couple of new things coming with this new version of Xamarin Forms, and one of them is Cascading StyleSheets (CSS). Yup, that’s right, CSS in XAML. Not sure yet how useful it will be and how adopted it will become – as quite a few features are still missing but I’m guessing it will be a welcome addition to anyone wanting to transition from web development.

So, to jump right into it – there’s two ways of adding CSS to your application:

  • first is by dropping your styles right into your elements resources and wrapping it into a CDATA tag
  • And the second one involved actual .css files added as embedded resources into your project

And once you have the CSS included, you use it by either specifying StyleClass or the shorthand class property on your XAML element.

To exemplify we’ll do some changes to a new Xamarin Forms project that uses the master detail template. So go ahead – File > New project and upgrade it to Xamarin Forms 3.

First off, the CDATA way. Let’s say we want to make the elements of our list orange. Go to the ItemsPage and inside the XAML, above the <ContentPage.ToolbarItems> tag, drop this:

<ContentPage.Resources>
    <StyleSheet>
        <![CDATA[
        
            .my-list-item {
                padding: 20;             
                background-color: orange;
                color: white;    
            }
        
        ]]>
    </StyleSheet>
</ContentPage.Resources>



Extending your Xamarin Forms AdMob renderer to display Microsoft Ads on UWP

So far we’ve been displaying ads only on Android and iOS through AdMob and our AdMob renderer. Google dropped support for Windows Phone altogether and never bothered with UWP so AdMob is not a choice in this particular situation.

Fortunately, Microsoft is also in the advertising business and  they’ve now nicely integrated everything in the developer dashboard and Visual Studio making it rather easy to display ads in your application. We’re going to be building on our existing AdMob code from the article linked above, and we’re going to extend it to use the Microsoft Advertising SDK to display ads on UWP.

To start of, go to your Windows developer dashboard, select your app – Monetize – In-app ads and create a new banner unit.

https://lh3.googleusercontent.com/hugSsTj_AAb5XlshTd4UmVx4kutKzbOy-YAfjPcqutbHUk7lDjKc93jydZ8fmZWOIQyZ5O3i5Hn91Sc89n9NuKJJN-n2fgs13BW5g4EBTpId7oRaiR5NR4GASWEexGz-nFJkPoQy




Upgrading to Xamarin Forms 3

Upgrading between major versions of Xamarin tends to break stuff and lead to projects no longer building due to weird errors. In their innocence, most devs will take these errors for real, try to understand them, to fix them and when they fail, they will Google them; when most of the times, the fix is to close Visual Studio, open it back up, and then do a clean build of your solution. Now let’s have a look at Xamarin Forms 3 (bear in mind this is a pre-release version, so these might be solved by the time of  the actual release).

Open up your existing project, or create a new Master Detail project using .NET Standard.

https://lh5.googleusercontent.com/jR42Oj0Cy0aiBtQBVAUkYe30rHALlzAc3sQAMl9mdVQTWM-x5f-05fbSv-_g077SfSVW21HMUpl20v6oNo2NhYNqYSKhxIAD-_x_MnkdJDSNbHxfJWqu8Igs4BoElnEMPihkLdcf




Day 1 – Building an app – “Best Before” – with Ionic 3.x – Learning Angular 5.X / TypeScript

This video marks the beginning of a series in which I take on Ionic 3.X and Angular 5.X in developing an app called “Best Before”. In these first 2 hours we go through the project setup as I will be working with Visual Studio 2017 & there isn’t any official support for Ionic 3 projects in VS yet.

We’re also playing with some sample Angular fiddles and we look at some of the differences between Angular 1 and 2+.

You can follow this series live by subscribing to my Twitch channel: Twitch.tv/startdebugging. This way you will get notified when I go live. Or you can view the video recording on our YouTube channel page: YouTube.com




UWP – Using an Acrylic Brush in your Xamarin Forms MasterDetail menu

Right, so you are one of those guys targeting UWP with their Xamarin Forms app… aand you want to use the new Acrylic brush to make you application stand out. Say no more.

Gazeta Acrylic menu on UWP

We won’t be using any 3rd party library/package to do this and we’ll be working in the platform specific project; so open up your MainPage.xaml.cs found inside your UWP project. First thing we need to do is grab a reference to your MasterDetail Master page. In my case, the MasterDetail represents my MainPage so things are pretty straight forward.

var masterPage = (app.MainPage as Xamarin.Forms.MasterDetailPage).Master;



AdMob Smart Banner sizing in Xamarin Forms

In one of my previous posts on How To: Add AdMob to your Xamarin Forms app I was giving a piece of code that looked like this:

<controls:AdMobView WidthRequest="320" HeightRequest="50" />

That will work perfectly with Banner ads but it’s going to fail if you choose to use a Smart Banner. For a Smart Banner, the sizing is a bit more complicated – it depends on the height of your device. You can find out more on AdMob ad unit sizing here.

Ad height Screen height
32 dp ≤ 400 dp
50 dp > 400 dp and ≤ 720 dp
90 dp > 720 dp

Now, dps are density-independent pixels. Do not mistake them for your device’s screen resolution. This unit of measure is what both Android and Xamarin Forms use when it comes to sizes. In order to calculate you screen height in dps we’ll get the screen resolution and divide it by the density; Again this density value isn’t your screen dpi, it’s the scale at which pixels are grouped into dps. For example, for a screen width of 1080 pixels and a density of 3, you will have an actual width of 360dp.

var dpHeight = Resources.DisplayMetrics.HeightPixels / Resources.DisplayMetrics.Density;

So that’s how you get your screen height in dps. Now we need to implement the logic behind that size table and wrap it all into a function.

private int GetSmartBannerDpHeight()
{
    var dpHeight = Resources.DisplayMetrics.HeightPixels / Resources.DisplayMetrics.Density;

    if (dpHeight <= 400) return 32; 
    if (dpHeight > 400 && dpHeight <= 720) return 50;
    return 90;
}

 

The AdMob renderer

And that’s it, now in your AdMobRenderer you go ahead and specify the new AdBanner size and use the newly implemented method to request the proper height based on the device’s screen. The complete renderer for Android would look like this:

[assembly: ExportRenderer(typeof(AdMobView), typeof(AdMobRenderer))]

namespace GazetaSporturilor.Droid.Renderers
{
    public class AdMobRenderer : ViewRenderer
    {
        public AdMobRenderer(Context context) : base(context)
        {

        }

        private int GetSmartBannerDpHeight()
        {
            var dpHeight = Resources.DisplayMetrics.HeightPixels / Resources.DisplayMetrics.Density;

            if (dpHeight <= 400) return 32;
            if (dpHeight > 400 && dpHeight <= 720) return 50;
            return 90;
        }

        protected override void OnElementChanged(ElementChangedEventArgs<View> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                var ad = new AdView(Context)
                {
                    AdSize = AdSize.SmartBanner,
                    AdUnitId = "_your_admob_ad_unit_id_goes_here_"
                };

                var requestbuilder = new AdRequest.Builder();

#if !DEBUG
                ad.LoadAd(requestbuilder.Build());
                e.NewElement.HeightRequest = GetSmartBannerDpHeight();
#endif

                SetNativeControl(ad);
            }
        }
    }
}

That’s about it.




Xamarin ListView performance & replacing it with Syncfusion SfListView

While Xamarin keeps adding features and improves the performance of Xamarin Forms with each and every update, what they offer in terms of cross-platform user controls is not always enough. In my case, I’ve got an RSS reader app which aggregates news articles from different sources and displays them in a ListView like this:

Gazeta Xamarin ListView

While I like how the app looks, it has a big issue – performance. Even on high-end devices the scrolling is sluggish and on low-end devices it keeps on throwing OutOfMemory exceptions due to the images that are being loaded. So, a change was needed. In this article I’ll only cover the first one – the scrolling performance; we’ll have a look at the OutOfMemory exceptions another time.

The Item template

First thing you need to look at when troubleshooting performance is the ListView ItemTemplate. Any kind of optimization that you can do at this level will have a big impact on the overall performance of your ListView. Look at things like:

  • reducing the number of XAML elements. The fewer elements to render, the better
  • same goes for nesting. Avoid nesting elements and creating deep hierarchies. It will take way too long to render them
  • make sure your ItemSource is an IList and not an IEnumerable collection. IEnumerable doesn’t support random access
  • don’t change the layout based on your BindingContext. Use a DataTemplateSelector instead

You should already see some improvements in scrolling after making these changes. Next on the list is your caching strategy.

Caching strategy

By default Xamarin uses the RetainElement caching strategy for Android and iOS which means that it will create one instance of your ItemTemplate for each item in your list. Change your ListView’s caching strategy to RecycleElement to reuse containers that are no longer in view instead of creating new elements every time. This will increase performance by removing initialization costs.

<ListView CachingStrategy="RecycleElement">
    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell>
              ...
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

If by any chance you’re using a DataTemplateSelector, then you should use the RecycleElementAndDataTemplate caching strategy. For more details on caching strategies you can check Xamarin’s documentation on ListView performance.

Syncfusion ListView

If you got this far and your performance issues aren’t solved, then it’s time looking at other options. In my case, I gave Syncfusion SfListView a try because they’re known for their control suites and they offer their Xamarin controls for free under the same conditions as Visual Studio Community (more or less). To get started head over to Syncfusion’s website an claim your free comunity license if you haven’t already.

Next, you need to add the SfListView package to your project. Syncfusion packages are available through their own NuGet repository. To be able to access it you’ll need to add it to you NuGet sources. A complete guide on how to do that can be found here. One you’ve done that, a simple search for SfListView in NuGet will result in the desired package. Install the package in your core/cross-platform project and in all your platform projects as well; it will automatically pick up the correct DLLs based on your project’s target.

Now that you’ve got everything installed, it’s time to replace the standart ListView. To do so, add the following namespace in your page/view:

xmlns:sflv="clr-namespace:Syncfusion.ListView.XForms;assembly=Syncfusion.SfListView.XForms"

And then replace the ListView tag with sflv:ListView, the ListView.ItemTemplate with sflv:SfListView.ItemTemplate and remove the ViewCell from your hierarchy – it’s not needed. Additionaly, if you’ve been using the CachingStrategy property, remove that as well – SfListView recycles elements by default. You should end up with something like this:

<sflv:ListView>
    <sflv:SfListView.ItemTemplate>
        <DataTemplate>
           ...
        </DataTemplate>
    </sflv:SfListView.ItemTemplate>
</sflv:SfListView>

That’s it. If you have any questions let me know in the comments section below. Also, if you’ve got any other tips to share that would improve ListView performance, please do.




How To: Add AdMob to your Xamarin Forms app

One of the first things people think about when developing for a new platform / using a new technology is monetization; and in my case the question is: how easy is it to integrate AdMob? For Xamarin Forms the answer would be: “It depends” – it depends on luck & on the complexity of what you want to achieve; but I will detail this as we move along.

The first thing you need to do is add the required components to your projects. For this walktrough I will be using Visual Studio but it should be relatively the same when using Xamarin Studio. Here, things go separate ways for each of the platforms:

  • for Android – add Nuget package Xamarin.GooglePlayServices.Ads.Lite
  • for iOS – add Nuget package Xamarin.Google.iOS.MobileAds
  • for Windows Phone – download the SDK from here and add it as a reference platform no longer supported

By now, you Android project should no longer be building & you should be receiving a COMPILETODALVIK : UNEXPECTED TOP-LEVEL error. To fix that, go into your Droid project properties, select the Android Options tab and then under Advanced modify the value for the Java Max Heap Size to 1G. Your project should now build without any errors.

Next, inside your shared / PCL project add a new Content View and call it AdMobView. Remove the code generated inside it’s constructor & it should look like this:

public class AdMobView : ContentView
{
    public AdMobView() { }
}

Add this new view to your page. In XAML you can do it like this:

<controls:AdMobView />

Make sure NOTHING interferes with the control. By nothing I mean – overlapping controls, page padding, control margins / spacing, etc. If you have something overlapping the ad control, ads will not display & you won’t receive an error, so be careful.

Next, it’s time to add the custom view renderers; and again, we’ll handle each platform:

Android

Add a new class called AdMobRenderer with the code below. Make sure to keep the ExportRenderer attribute above the namespace, otherwise the magic won’t happen.

[assembly: ExportRenderer(typeof(AdMobView), typeof(AdMobRenderer))]

namespace AdExample.Droid.Renderers
{
    public class AdMobRenderer : ViewRenderer
    {
        public AdMobRenderer(Context context) : base(context)
        {

        }

        private int GetSmartBannerDpHeight()
        {
            var dpHeight = Resources.DisplayMetrics.HeightPixels / Resources.DisplayMetrics.Density;

            if (dpHeight <= 400) return 32;
            if (dpHeight > 400 && dpHeight <= 720) return 50;
            return 90;
        }

        protected override void OnElementChanged(ElementChangedEventArgs<View> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                var ad = new AdView(Context)
                {
                    AdSize = AdSize.SmartBanner,
                    AdUnitId = "ca-app-pub-xxxxxxxxxxxxxxxx/xxxxxxxxxx"
                };

                var requestbuilder = new AdRequest.Builder();

                ad.LoadAd(requestbuilder.Build());
                e.NewElement.HeightRequest = GetSmartBannerDpHeight();

                SetNativeControl(ad);
            }
        }
    }
}

Next, you need to modify your AndroidManifest.xml file to add the AdActivity & required permissions for displaying ads: ACCESS_NETWORK_STATE, INTERNET; just like in the example below.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">
	<uses-sdk android:minSdkVersion="15" />
	<application>
    <activity android:name="com.google.android.gms.ads.AdActivity" android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize" android:theme="@android:style/Theme.Translucent" />
  </application>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
  <uses-permission android:name="android.permission.INTERNET" />
</manifest>

That’s it. Your Android build should now display ads inside the AdMobView content view.

iOS

Start off by adding one line in your AppDelegate.cs to initialize the SDK with your application ID. Careful, do not mistake this with your ad unit ID! Add this just before the LoadApplication call.

MobileAds.Configure("ca-app-pub-xxxxxxxxxxxxxxxx~xxxxxxxxxx");

Then, same as before, just add a new class called AdMobRenderer and copy-paste the code below replacing the AdmobID with the ID of your banner unit.

[assembly: ExportRenderer(typeof(AdMobView), typeof(AdMobRenderer))]

namespace GazetaSporturilor.iOS.Renderers
{
    public class AdMobRenderer : ViewRenderer
    {
        BannerView adView;
        bool viewOnScreen;

        protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.View> e)
        {
            base.OnElementChanged(e);

            if (e.NewElement == null)
                return;

            if (e.OldElement == null)
            {
                adView = new BannerView(AdSizeCons.SmartBannerPortrait)
                {                    
                    AdUnitID = "ca-app-pub-xxxxxxxxxxxxxxxx/xxxxxxxxxx",
                    RootViewController = GetRootViewController()
                };

                adView.AdReceived += (sender, args) =>
                {
                    if (!viewOnScreen) this.AddSubview(adView);
                    viewOnScreen = true;
                };

                var request = Request.GetDefaultRequest();

                e.NewElement.HeightRequest = GetSmartBannerDpHeight();
                adView.LoadRequest(request);

                base.SetNativeControl(adView);
            }
        }

        private UIViewController GetRootViewController()
        {
            foreach (UIWindow window in UIApplication.SharedApplication.Windows)
            {
                if (window.RootViewController != null)
                {
                    return window.RootViewController;
                }
            }

            return null;
        }

        private int GetSmartBannerDpHeight()
        {
            var dpHeight = (double)UIScreen.MainScreen.Bounds.Height;

            if (dpHeight <= 400) return 32;
            if (dpHeight > 400 && dpHeight <= 720) return 50;
            return 90;
        }
    }
}

That’s it. Now you have ads being served on both platforms. Any comments or suggestions you have leave them in the comments section below.

Update 30 Dec 2017

In this article we looked at displaying Banner ads and hard-coded the view size to 320 x 50 dp. If you’re looking to implement smart banners have a look at this follow-up post: AdMob Smart Banner sizing in Xamarin Forms

Update 21 Jan 2018

I finally made the courage to try and build one of my apps for iOS so I’ve updated this article to work with the latest version of AdMob for Xamarin. I’ve also included the smart sizing code mentioned in the 30 Dec update. Thanks to everyone helping out in the comments section with the iOS implementation.




Fix Xamarin error – Csc.exe exited with code -1073741790. (MSB6006)

Just run Xamarin Studio as an Administrator.

The error usually means that the process cannot access a certain resource. In my case that meant insufficient rights; but it could also mean that some file is already in use; in that case – Clean the solution & Rebuild and if that doesn’t work either do a manual cleanup of the solution bu deleting the “bin” and “obj” folders for every project in your solution.




Changing the Cordova version used by Hybrid Apps in Visual Studio 2013

Updating the Cordova version requires you to edit the platforms.js file found in:

%APPDATA%\Roaming\npm\node_modules\vs-mda\node_modules\cordova\node_modules\cord‌​ova-lib\src\cordova

You can change the version individually for each platform, tho I suggest you use the same version for all.
Also, it might be the case you’re looking to update only because of the Google Play warning regarding the high severity cross-application scripting (XAS) vulnerability found in Cordova 3.5.0. If so here you have the file updated to point to version 3.5.1 which fixes the mentioned vulnerability: platforms.js.

Note: In case you are using CTP 1 of the Hybrid Apps the path will be different:
%APPDATA%\npm\node_modules\vs-mda\node_modules\cordova\