Bluetooth Xamarin

Wrapping up 2020

I haven’t posted as much as I would have liked this year, because “reasons” but I thought I would put together a short post to wrap up some of the things that have been going on behind the scenes.


Back in January I announced a re-write of 32feet.NET. The library for Bluetooth Classic has been reworked for the NuGet/.NET Standard age. The intention was removing legacy code and take advantage of newer language features to make it easier to support more platforms and concentrate on getting the core functionality across as many .NET platforms as possible. It’ll also make it easier to support .NET 5/6 ahead. I’ve invested a lot of time into the macOS IOBluetooth and IOBluetoothUI bindings too and will be integrating macOS into the 32feet library in 2021.

I started to work on integrating some code I had written for iOS using the External Accessory API. This allows access to a subset of Bluetooth devices if they implement Apple’s proprietary MFi standard. This will allow devices such as mobile printers to be used with shared code across multiple platforms.

I would like to start work on Linux support (via BlueZ) too which will be another major platform ticked off.

This year I also revived original parts of 32feet.NET in separate packages so IrDA and Object Exchange functionality are available separately. IrDA is end-of-life so I won’t be investing in this area but I wanted to ensure it was made available for desktop Windows in a modern package if required. OBEX has the basic ObexWebRequest with a few minor improvements but it’s something I intend to revisit with a more powerful API soon.

Bluetooth Low Energy

Alongside the Bluetooth Classic updates I also created a new library for Bluetooth Low Energy. This currently supports Windows 10 (.NET Framework, .NET Core and UWP), Android, iOS, tvOS, watchOS, macOS. It would be nice to add Linux and possibly Tizen to this list!

The API follows the existing WebBluetooth standard closely with some additional features relevant to apps. For example not only can you display a picker to the user to choose a device but you can also programmatically discover to generate your own UI. There are some platform specific additions too such as the ability to request a particular physical layer (PHY) which facilitates longer range communication (Currently Android only). Work has also gone into round-tripping support for service UUIDs – the ability to specify by name or short and long UUIDs.


The Xamarin Forms MediaElement now has a new home in the Xamarin Community Toolkit. It’s already received a number of fixes and improvements. In October I added support for databinding to the Position property. Rather than having to write your own timer and query the position you can just bind your UI to the property which gets regularly updated as the media plays.

Happy New Year

I hope you have a happy holiday season and wish you the best for 2021.

Bluetooth Xamarin

Asyncify a method and event response

It’s common to come across a pattern where you call a synchronous method to kick off an activity then await an event which gives you the result (or an error). In the process of building 32feet‘s Bluetooth LE library this became a familiar sight both for Android and iOS APIs.

The goal was to create a single async method which starts the process and completes when the response (good or bad) is received. The API model for the resulting cross-platform API is based on Web Bluetooth but with some extensions. The Web Bluetooth API is JavaScript based and each operation returns a promise – the .NET approach is to use a Task which you can await or ContinueWith.

In an early implementation I had used a WaitHandle and had a thread blocking on this but this is not a clean async approach. I needed a solution which would handle the method and event handler and use a standard Task. Luckily there is the TaskCompletionSource type which can facilitate this. It wraps a Task and allows you to set a result (or exception) which we’ll be setting inside the event handler. Therefore each resulting method follows basically the pattern below:-

TaskCompletionSource<IReadOnlyList<GattDescriptor>> tcs = new TaskCompletionSource<IReadOnlyList<GattDescriptor>>();
CBPeripheral peripheral = Service.Device;

void handler(object sender, CBCharacteristicEventArgs args)
   peripheral.DiscoveredDescriptor -= handler;

   if (args.Error != null)
      tcs.SetException(new Exception(args.Error.ToString()));

   List<GattDescriptor> descriptors = new List<GattDescriptor>();

   foreach (CBDescriptor cbdescriptor in _characteristic.Descriptors)
      descriptors.Add(new GattDescriptor(this, cbdescriptor));


peripheral.DiscoveredDescriptor += handler;

return tcs.Task;
  • Create a TaskCompletionSource with the required return type
  • Get the native object we’ll be starting the operation on
  • Create a handler for the event. Inside the handler we
    • Remove the event handler
    • Check the response – set either positive or negative result to TaskCompletionSource.
  • Add the event handler
  • Call the method to start the process
  • Return the TaskCompletionSource Task

I hope this provides a handy pattern to follow if you’re trying to use similar APIs, if not I’m sure it will at least jog my memory. I’ll post again soon about the full Bluetooth library in more depth.

Bluetooth Xamarin

Changes coming in Xamarin Android BluetoothSocket

While debugging an app I came across this new message in the Output window:-

"Not wrapping exception of type Java.IO.IOException from method `Read`. This will change in a future release."

When working with an RFComm service on Android you use a BluetoothSocket which in turn owns two Streams – one for Input and one for Output. Currently when there is a failure such as a broken connection the Read or Write operation will throw a Java.IO.IOException even though these streams are exposed as regular .NET Streams. It looks like Microsoft plan to change this behaviour and I presume this will be to wrap the exception in the equivalent System.IO.IOException. This makes a lot of sense and will improve consistency with other .NET flavours but it is something you need to be aware of as it will require code changes in your exception handling.

Desktop Code Windows Xamarin

WPF Activity in Xamarin Forms

I’ve recently been working on extending an existing product written in Xamarin Forms to Windows desktop and this has meant learning a lot more about the WPF implementation. Originally Xamarin Forms was created for mobile platforms such as Android and iOS (and Windows Phone – more on that later) but has since expanded to desktop platforms such as macOS, GTK and WPF on Windows (UWP originally straddled mobile and desktop). WPF is a more recent addition to Xamarin Forms and so understandably the implementation is less mature. It does follow the native look and feel closely but I soon found quite a jarring omission.

Our app uses the ActivityIndicator to represent indeterminate activity. On Android et al. this displays a rotating circular spinner but on WPF it was using an indeterminate progress bar squashed into a rectangular space which looked odd. This is understandable as there is no native control in WPF. I then had a flash of deja vu when thinking about something I wrote some time ago for Windows Phone!

Windows Phone 8.1 introduced the WinRT application model (from Windows 8) alongside the existing Silverlight approach but there were some complications and some types of apps could only be written in Silverlight. I created a library called Charming which added some of the Windows 8 style APIs to the Silverlight app model. This was later expanded and became Pontoon. One of the APIs was a Silverlight ProgressRing control which looked just like the Windows 8 equivalent. The progress ring still regularly appears in the Windows shell and is implemented in UWP. Since Silverlight is a subset of WPF it was straightforward to port it into the Xamarin Forms WPF platform library and modify the renderer for ActivityIndicator. Voila! A nice native looking ActivityIndicator for Windows desktop apps. This has now been merged into the Xamarin Forms code and should appear in a preview release soon alongside the MediaElement that also got merged recently. Until it hits a stable release you can add it via InTheHand.Forms too.

Bluetooth Windows Xamarin

New Year, New 32feet.NET Library

I’ve been working away on the Bluetooth code for some time. I’d been meaning to modernise the code and build with .NET Standard and NuGet in mind and ideally support more platforms. However the project has stopped and started a few times and gone down a few dead ends.

I had planned to rework the library into a full implementation of the UWP API set for Windows.Devices.Bluetooth covering Bluetooth Classic and Bluetooth LE but while I got quite far and had a set of working code for Windows, Android and iOS I decided that that wasn’t the right approach. Some aspects are clearly tied to underlying Windows concepts – the whole device finding/picking process uses magic strings and is not intuitive from a purely Bluetooth perspective. Also there was no real need to integrate both Bluetooth Classic and Bluetooth LE into a single library because there was very little shareable code and these could be maintained separately.

Instead I decided to go with a much simpler option. Build a new library around fundamentally the same API which the library has used for 16 years with a few necessary tweaks, cutting out a few of the more complex features and making it easy to do the most common tasks with Bluetooth Classic. The Bluetooth LE functionality will be revisited in 2020 with a more logical cross-platform API and packaged in a separate NuGet package (but still part of the 32feet.NET project).

What has changed

The library has been renamed InTheHand.Net.Bluetooth to make clear it is a different package and also to make clear the focus as I’ll no longer be including the IrDA functionality. It includes .NET Standard, Classic .NET Desktop, Android and iOS versions. As you can imagine there will be no new version for .NET Compact Framework. The iOS version is not yet complete but will have some limitations as imposed by the platform. I’ll discuss the details further in a separate blog post because it does some fun things over Apple’s quirky API and should make cross-platform Bluetooth Classic a lot easier for iOS.

In due course it will be joined by UWP, Linux and macOS implementations. Linux will require reworking for BlueZ 5 and macOS will be built on top of the IOBluetooth and IOBluetoothUI framework wrappers which are slowly taking shape. Potentially there may be other platforms too, such as Tizen, and hopefully we can grow the community around the project to target other platforms.

There is no BluetoothListener support in this release but it will be included in future builds (except on iOS). 95% of 32feet.NET users are using BluetoothClient to connect out to other devices so this wasn’t a top priority for this release.

The SelectBluetoothDeviceDialog which was heavily tied to WinForms has been replaced with the BluetoothDevicePicker. This offers a similar level of functionality but in a way which feels comfortable from any UI stack and supports async/await.

DiscoverDevices again is vastly simplified to remove concepts which only make sense in the Win32 implementation. Instead this method is used to discover “live” devices in the vicinity. It is accompanied by the new property PairedDevices which offers a more efficient way of retrieving paired devices without doing a live discovery.

When I started working on a modified version of the library for Unity development there was an odd problem. The Sockets stack in the Mono runtime doesn’t correctly support Bluetooth sockets and so the workaround was to P/Invoke the native Sockets APIs directly. In order to simplify the code the Win32 version uses a single implementation which calls the relevant Win32 APIs rather than using System.Net.Sockets. The library defines its own version of NetworkStream to be able to provide a familiar API which also not being tied to .NET Sockets. This makes it easier to support other platforms such as Android where the underlying implementation is completely different but can be wrapped inside a familiar .NET Stream. All of this means that the desktop .NET version of the library will work for the Windows target of a Unity project.

Where is it going?

The aim with this new version is to have a modern code base able to run on the current spectrum of .NET platforms – From IoT devices, to mobile phones to desktop PCs. Alongside the current Bluetooth Classic library and the Bluetooth LE library I’m also planning for other technologies to join the 32feet.NET project all aimed at handling short range device to device communications.

View the project on GitHub

See the latest InTheHand.Net.Bluetooth NuGet package


MediaElement Enhancements

In the process of digging into a native exception when disposing the MediaElementRenderer on iOS I came across the closest thing I would get to the source of the problem.

Do not subclass AVPlayerViewController

Because the resulting stacktrace contains redacted elements there was no hope of fixing the issue, so in the end I decided to rewrite the renderer again using the original (non-FastRenderer) approach. The control now behaves correctly and can be created/disposed repeatedly without causing any unexpected problems. I can’t say you can notice any performance hit in not using a fast renderer for this. Though this may be more of a problem if you’re trying to use multiple instances of the control or have some frequently changing screen layout.

I was also able to fix a small issue on Android where depending on the aspect ratio of the video and the size allocated to the control the content may appear left-aligned rather than centered. The result is the latest version of the package on NuGet. At some point I may need to roll these changes into the Xamarin Forms Pull-Request although this seems to have lost all traction after 18-months and is now the second oldest open pull request in the Xamarin Forms project. For the foreseeable future it looks like I’ll be maintaining the standalone control.

Windows Xamarin

Xamarin Forms MediaElement now available on WPF

Continuing from my last post on macOS, there is now a MediaElement renderer for WPF in InTheHand.Forms 2.0.2019.913. The native control in WPF is a lot more limited and is missing two big features – no system provided transport controls, and no support for HTTPS sources. The latter is an unfortunate limitation though it looks as though this is being fixed for WPF on .NET Core.

A possible workaround for these limitations is to have the WPF renderer use the UWP MediaPlayerElement when running on Windows 10 1903 and above. This is currently available as a WPF control as part of the Windows Community Toolkit.

macOS Xamarin

Xamarin Forms MediaElement now available for macOS

As the Pull Request to Xamarin Forms continues in its holding pattern I thought about what else I can do with it. I had neglected the original codebase for a while but I’ve spent some more time with it recently providing some much needed updates. The latest which was added to NuGet today adds a whole new platform – macOS.

The functionality is almost exactly the same as that available on iOS with the exception of the KeepScreenOn property as I haven’t found the equivalent macOS APIs to implement this. The sample app code has been updated to include a macOS version demonstrating the basic functionality including toggling different Aspect settings.

Other recent updates include moving the dependencies to a more recent Xamarin Forms version and implementing fast renderers for iOS and Android.

I’ve begun back-porting the WPF renderer too and hope to add this soon. This will increase the platform coverage to Android, iOS, UWP, macOS and WPF. I haven’t really worked with Xamarin Forms on Tizen but this is another possibility for the future.

Bluetooth Xamarin

Xamarin macOS Binding Libraries

In creating an IOBluetooth binding for Xamarin Mac I learned about Objective Sharpie and binding libraries. There is little documentation on this but it is fairly similar to Xamarin iOS and for that there is a lot more source material. The output from Objective Sharpie gives you binding definitions which you can use in a dll project to produce a binding library you can then call from any other Xamarin dll or app. This was fine to a point but there are issues with some of the complex types used and these cannot always be marshalled automatically. This left me with an API with a few missing bits as I tried and failed to manually adjust the binding via trial and error.

The project sat around untouched for some time but recently I’ve begun to revive it and hope to sort out these bits so it can be released as a complete functioning API for Xamarin Mac. At first I thought I was going to have to create two libraries – one with the raw API calls and another with a clean API over the top but I had missed something buried in the docs and it turns out there is an easier solution.

When you have a binding library it will, by default, have two files – ApiDefinition.cs which contains all the API calls and has a Build action of BindingApiDefinition and a StructsAndEnums.cs which contains (well I’ll let you guess from the name) and this has a Build action of ObjcBindingCoreSource. When the classes are generated from the interface definitions in ApiDefinition.cs they are actually partial classes. This means you can extend them and have additional functionality built cleanly into the library. If you have a particularly messy API call you can mark it as internal and then surface it in a more friendly way from a partial class. To do this add another source file to the library project (I’ve called it Extra.cs because I saw that in a sample but the name isn’t important) and set the Build action to Compile. Here you’ll need to create a partial class with the same name and namespace as the “interface” you want to extend from ApiDefinition.cs, and then add methods, properties etc.

The first time I added this my build failed. I subsequently found out that there is one additional step to tell the binding compiler to ignore this file. Open the project properties, under Build select the Objective-C Binding Build page. Here in Additional btouch arguments box add -x:Extra.cs (or replace with your own filenames). This stops the initial binding compilation from using the partial class, which then gets built normally in the subsequent managed code build. The project should now build and expose the combined functionality. I did find that intellisense often gets confused when editing the partial class because there isn’t another definition of a partial class at this time (remember in ApiDefinition.cs it’s actually an interface). However it seems you can safely ignore this!

This in theory allows you to completely change the API surface which you expose to Xamarin from whatever you started with. I don’t want to go too crazy with IOBluetooth – my feeling is that it should match the native API with a few tweaks for C# naming conventions, using namespaces rather than huge class names, and .NET friendly types where appropriate. Objective Sharpie struggled with some of the enum/constant definitions and so these still require a bit of massaging. It should be obvious how it maps to the native API.

If you have feedback on the API or would like to get involved in getting the library up to release standard please let me know. All the current code is on GitHub in the IOBluetooth and IOBluetoothUI folders.


Update to Xamarin Forms MediaElement

Things are progressing with the Xamarin Forms Pull Request but it’s a big change and I’ve had feedback on how quickly (or not) it’s going. For this reason I’ve decided to post an update to InTheHand.Forms to port some of the advances and compile for Xamarin Forms 4.0. The new release is marked as a pre-release to avoid anyone automatically updating at this stage and hitting problems because there are a few API changes to ease the transition to the future official version.

It takes advantage of fast renderers on iOS and Android and has a cleaner communication method between the control and the renderers. I haven’t changed the Source property which is still based on UWP style Uris for special file locations. I haven’t added a WPF renderer even though the Xamarin Forms version will have this (minus some missing platform functionality like HTTPS and overlaid media playback controls). There are some improvements to the API including a Volume property. The original PCL library is removed along with Windows 8.1 and Windows Phone platforms. I will probably continue to use this package as a method to test other renderers (e.g. Mac and Tizen) before embarking on another Pull Request to Xamarin Forms.

I think the plan with the Xamarin Forms pull request was to integrate in 4.1 but it hasn’t progressed into the current preview releases. I’ll keep you posted as I know more.