This blog post is a part of Louis Matos’s Xamarin Month, where this months topic is Code Snippets. For more information take a look at his blog and see the list of all the other auhtors who are participating. There will be a new post each day of the month, which is super cool!

Let me share some code snippets that I often use. All of these snippets will be available in my XamarinSnippets code repository on GitHub, for import in Visual Studio 2019, ReSharper, Rider and Visual Studio for Mac. Instructions provided in the repository Readme file.

When writing an Application using MvvmCross, or even with other frameworks, there are some pieces of code that you have to repeat again and again. As programmers we are usually a bit lazy and don’t want to type all that code over and over again. Fortunately, we can have code snippets ready to help us, a nice feature built into our IDE’s. Some of the snippets I use often are as follows.

mvxprop

I often have to create a property in my ViewModels which raise the PropertyChanged event in the MvvmCross flavor. For that I simply type mvxprop and press Tab, and magically I get the following code.

private int propertyName;
public int PropertyName
{
    get => propertyName;
    set => SetProperty(ref propertyName, value);
}

I have considered making some flavors for some common types that I use, such as string, bool, int. Not sure how much I would use them.

mvxcom

Creating commands is also something that I often do. However, I’ve gone away from using this pattern where I lazily initialize commands, some of you may find them useful. What I prefer instead is initialize them in the constructor of the ViewModel instead.

private MvxCommand _command;
public MvxCommand Command =>
    _command ??= new MvxCommand(DoCommand);

private void DoCommand()
{
    // do stuff
}

I have a couple of variants of this snippet, which creates different types of MvxCommand. mvxcomt for the generic MvxCommand<T>, mvxcomasync for the async version MvxAsyncCommand and similarly the generic version of that mvxcomtasync which creates a MvxAsyncCommand<T>.

mvxbset

Last but not least, a snippet for creating a binding set for binding views on both Android and iOS. I’ve taken the liberty to make the assumption that View and ViewModel names follow each other. So PeopleView will usually have a corresponding ViewModel PeopleViewModel. This ends up creating something like this:

using var set = this.CreateBindingSet<PeopleView, PeopleViewModel>();

Get all the snippets along with instructions in my XamarinSnippets repository on GitHub.

Do you have any cool related MvvmCross snippets to share? Put them in the comments or make a Pull Request on the repository.

Some years ago I ported over some Java color pickers to work with Xamarin. However, it didn’t really cross my mind to release these as a NuGet package. However, due to demand I finally took the time to do so.

You can find the NuGet package here: https://www.nuget.org/packages/Cheesebaron.ColorPickers/

The source code as usual is available on GitHub: https://github.com/Cheesebaron/Cheesebaron.ColorPickers

Here is a couple of screen shots of what you can expect from the library.

I just had the task to figure out why we had an App crashing randomly on us all of the sudden. The App is distrubuted through AppCenter only, as a Enterprise build. Turned out that the provisioning profile had expired hence it started to fail.

Now, I did not want to build a new version of the App, since the one distributed and in production is much older and we are in the middle of moving CI for the App into a new enviroment. So the only option was to sign the App again with the new provisioning profile. So here are some notes on how I did that.

The prerequisites for this are:

• IPA file to re-sign
• Distribution certificate is installed in your KeyChain
• mobileprovision file to sign with
• a macOS installation with Xcode installed (I used 11.3.1)

The following snippets are commands you would run in your preferred commandline.

First we need to unizp the IPA file. It will contain a Payload folder with AppName.app inside, where AppName is your App’s name. I will use MyApp as an example throughout this post. Yours will of course be different.

unzip MyApp.ipa

You should now have a Payload folder where contents were extracted.

First we need to extract the entitlements for the App, we will need them later when we are signing the App again.

codesign -d --entitlements :- "Payload/MyApp.app" > entitlements.plist

This will create a entitlements.plist file in the folder you are currently in. If you are re-signing the App with a distribution certificate for another team, remember to change identifiers in the entitlements file to match your distribution certificate.

Now before we re-sign the App, we need to remove the old code signing.

rm -r Payload/MyApp.app/_CodeSignature

We also need to replace the provisioning profile. Assuming your profivisioning profile file name is called MyApp.mobileprovision and is located in the same folder we are in.

cp MyApp.mobileprovision Payload/MyApp.app/embedded.mobileprovision

We should now be ready to re-sign the Application.

codesign -f -s "iPhone Distribution: your team name here" --entitlements entitlements.plist Payload/MyApp.app

Now we just need to zip the folder and we are ready to distribute it.

zip -qr MyApp-resigned.ipa Payload

That is it! You are ready to ship your re-signed App!

Android 10 was recently released and it introduces a bunch of changes in terms of Privacy. This means that access to /proc/net from the Linux sub-system has been restricted, which requires you to use NetworkStatsManager and ConnectivityManager to get VPN information.

It adds restrictions to who is allowed to enable/disable WiFi. We could previously use WifiManager.SetWifiEnabled(), but not anymore, this method will return false. You will need to show one of the new Settings Panels, which shows a slice of the Android Settings within your App.

What this post will focus on is the restrictions to access to configured networks and connecting to networks. A bunch of the network API has changed, so let us look a bit into what we have available now.

Suggesting networks

Something new to Android 10 is suggesting networks to connect to. These are just hints to the platform that the networks you provide it, can be connected to and it might decide to chose one of them. When any of these networks are detected nearby, Android will show a notification to the user the first time, which is how they allow connecting to a suggested network.

This could be useful for an Enterprise App, which can allow Access to networks depending on the logged in user or suggest a separate network for guests.

You can suggest networks like so.

var guestUsers = new WifiNetworkSuggestion.Builder()
    .SetSsid("GuestNetwork")
    .SetWpa2Passphrase("hunter2")
    .Build();

var secretEnterpriseNetwork = new WifiNetworkSuggestion.Builder()
    .SetSsid("Cyberdyne")
    .SetWpa2Passphrase(":D/-<")
    .Build();

var suggestions = new[] { guestUsers, secretEnterpriseNetwork };

var wifiManager = this.GetSystemService(Context.WifiService) as WifiManager;
var status = wifiManager.AddNetworkSuggestions(suggestions);

if (status == NetworkStatus.SuggestionsSuccess)
{
    // We added suggestions!
}

The suggestions you provide can only be added once. If you try add the same suggestion again, the status from AddNetworkSuggestion will return SuggestionsErrorAddDuplicate. If you need to modify a suggestion, you need to remove it first with  RemoveNetworkSuggestion, then add the modified version of it again. Additionally, in order to add these suggestions you will need to add the CHANGE_WIFI_STATE permission to your AndroidManifest.xml.

<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />

Note: Some of the options on a WifiNetworkSuggestion requires you to request Fine Location permission in order to work. Make sure to consult the Android documentation to be sure.

After you run AddNetworkSuggestion don’t expect something to happen immediately. There will eventually be a notification in the notification drawer. Which looks like something in the image below. Choosing “Yes” on the notification, won’t necessarily automatically connect to the network. However, going to Wifi Settings on your device, it should now know how to connect to that network.

Connecting to Specific Networks

Prior to Android 10, we could explicitly tell Android to connect to a specific Network and it would simply do it. This was done using WifiManager.AddNetwork() where you provided a WifiConfiguration. Now this API has been deprecated and replaced with ConnectivityManager.RequestNetwork, where you build a Network Request with a Wifi Specification, similar to the suggestions I showed you above. However, it also allows you to specify, whether the connection requires Internet, cannot be VPN, has to be WiFi and more.

You will also need to provide a ConnectivityManager.NetworkCallback, which will let you know whether connection to the network was successful or not. The idea here is that you are awaiting the requested network to connect and only when that is successful you can continue with your execution.

This is excellent for scenarios, where you need to be on a specific network to do some specific operations. One thing to note is that you will only be able to connect to this network while the App is open. As soon as the App is closed, it disconnects the network.

Note: this connection will not have any Internet connectivity according to the Google docs. See the notes in the Android documentation about WiFi Bootstrapping. For Internet connectivity Google suggests to use the Network Suggestions described above.

Let us look at some code.

var specifier = new WifiNetworkSpecifier.Builder()
    .SetSsid("cyberdyne")
    .SetWpa2Passphrase("ill be back")
    .Build();

var request = new NetworkRequest.Builder()
    .AddTransportType(TransportType.Wifi) // we want WiFi
    .RemoveCapability(NetCapability.Internet) // Internet not required
    .SetNetworkSpecifier(specifier) // we want _our_ network
    .Build();

var connectivityManager = this.GetSystemService(Context.ConnectivityService) as ConnectivityManager;

connectivityManager.RequestNetwork(request, callback);

Here is an example of wanting to connect to the network with SSID “cyberdyne” and passphrase “ill be back”. The transport type specifies what kind of network you want to connect to. In this case it is a WiFi network. We do not require any Internet capability on the network. Per default it requests Internet, Not VPN and Trusted capabilities.

The callback looks something like this.

private class NetworkCallback : ConnectivityManager.NetworkCallback
{
    public Action<Network> NetworkAvailable { get; set; }

    public override void OnAvailable(Network network)
    {
        base.OnAvailable(network);
        NetworkAvailable?.Invoke(network);
    }
}

var callback = new NetworkCallback
{
    NetworkAvailable = network =>
    {
        // we are connected!
    }
};

You can also use OnUnavailable to detect that we could not connect to the network in particular, or the user pressed cancel.

If you connect once to a network request, any subsequent requests will automatically connect if you are in range of the network and the user will not have accept it again. Also, while the App is open and you want to disconnect from the requested network, you simply call ConnectivityManager.UnregisterNetworkCallback() on the callback.

This hopefully gives you an idea about how to connect to WiFi Networks with Android 10.

You can check out the code from my Android 10 WiFi Repository on GitHub and have a go at playing with it yourself.

I have a colleague who showed me these lovely Xiaomi Mijia high precision temperature sensors, which can tell you the temperature and air humidity. They are battery powered and they broadcast their information through Bluetooth Low Energy (BLE).

My colleague had some issues reading out the values the sensors broadcast over BLE, so I took it up as a small challenge to figure out how to read the values out in a Xamarin.Android App.

For the task I chose to use NuGet package Plugin.BLE, which I have used before for some other projects with great success. Lots of praise to Sven-Michael Stübe, who is the author behind the plugin. A lot of work has gone into making it and he made it very nice to work with, providing a an easy to use API, which works on both Xamarin.Android and Xamarin.iOS.

So first things first. In order to be allowed to scan for BLE devices on Android, you have to request the Location permission. For this I employ the trusty Permissions Plugin. Here I can request the permission like so:

private async Task<bool> RequestPermissions()
{
    if (DeviceInfo.Platform != DevicePlatform.Android)
        return true;

    try
    {
        var status = await CrossPermissions.Current.CheckPermissionStatusAsync(Permission.Location);
        if (status != PermissionStatus.Granted)
        {
            var requestStatus = await CrossPermissions.Current.RequestPermissionsAsync(Permission.Location);
            if (requestStatus.ContainsKey(Permission.Location))
            {
                return requestStatus[Permission.Location] == PermissionStatus.Granted;
            }
        }
    }
    catch (Exception ex)
    {
        //Something went wrong
    }

    return false;
}

Now that we have a way to get permission to scan for BLE devices, we can start scanning for BLE broadcasts.

var ble = CrossBluetoothLE.Current;
var adapter = CrossBluetoothLE.Current.Adapter;
adapter.DeviceDiscovered += OnDeviceDiscovered;

As you can see, in 3 lines of code we have set up our code to get discovered devices. In the OnDeviceDiscovered method we get a model describing the BLE device, including advertisement records and much more. We will use the advertisement records later to get the values from the Mijia Sensor.

Since this App is only interested in Xiaomi Mijia devices, we can provide a filter for the BLE plugin, such that we limit our search to such devices only. The Xiaomi Mijia devices I have all advertise themselves as “MJ_HT_V1”. Hence, I created a simple filter, checking for that name.

private bool DeviceFilter(IDevice device)
{
    if (device.Name?.StartsWith("MJ_HT_V1") ?? false)
        return true;

    return false;
}

Then we can start scanning for devices by calling:

adapter.ScanMode = ScanMode.LowLatency;
await adapter.StartScanningForDevicesAsync(null, DeviceFilter, cancellationToken: token);

I have ensured that the ScanMode is Low Latency, just notice that this scan mode uses a lot of power.

Scanning for devices start popping up the devices I have. Now to the fun part parsing the advertisement records the Xiaomi Mijia is sending. From reading a bunch of other clever people reverse engineering the device protocol I figured out that the Xiaomi Mijia sends out 4 different kinds of advertisement records. These can be identified by reading the 14th byte in the advertisement record. Then reading the subsequent bytes and piece them together to a value.

The read values have to be combined and then divided by 10.0 for battery and humidity to get the values in degrees Celsius and humidity percentage.

So typically these advertisement records would look like:

95 FE 50 20 AA 01 57 29 AE 33 34 2D 58 0D 10 04 DB 00 CC 01 which corresponds to 21.9 degrees C and 46% humidity.

95 FE 50 20 AA 01 5E 29 AE 33 34 2D 58 06 10 02 CB 01 which is 45.9% humidity.

Parsing the data I ended up with a method looking something like this:

private (double battery, double? temperature, double? humidity) ReadServiceData(byte[] data)
{
    if (data.Length < 14) return (-1, null, null);

    double battery = -1;
    double? temp = null;
    double? humidity = null;

    if (data[13] == 0x04) //temp
    {
        temp = BitConverter.ToUInt16(new byte[] { data[16], data[17] }, 0) / 10.0;
    }
    else if (data[13] == 0x06) //humidity
    {
        humidity = BitConverter.ToUInt16(new byte[] { data[16], data[17] }, 0) / 10.0;
    }
    else if (data[13] == 0x0A) //battery
    {
        battery = data[16];
    }
    else if (data[13] == 0x0D) //temp + humidity
    {
        temp = BitConverter.ToUInt16(new byte[] { data[16], data[17] }, 0) / 10.0;
        humidity = BitConverter.ToUInt16(new byte[] { data[18], data[19] }, 0) / 10.0;
    }

    if (temp.HasValue)
        System.Diagnostics.Debug.WriteLine($"Temp: {temp.Value}");
    if (humidity.HasValue)
        System.Diagnostics.Debug.WriteLine($"Humidity: {humidity.Value}");

    System.Diagnostics.Debug.WriteLine($"Battery: {battery}");

    return (battery, temp, humidity);
}

Piecing all of this together in a little MvvmCross App, yielded me something like seen in this screenshot.

You can find the entire source code for the App on GitHub.