Xamarin Forms - Send event to native and retrieve value

Question

I'm currently making a custom view to preview camera of the device, and trying to trigger a "take picture" event.

Right now, i can trigger the event, but i don't find a solution to return the picture to Xamarin Forms Shared Code.

Here's what i'm doing from the Xamarin Forms custom control :

public class CameraPreview : View
{
    public event EventHandler TakePictureRequested;

    public void TakePicture()
    {
        TakePictureRequested?.Invoke(this, EventArgs.Empty);
    }
}

And here from Native code for Android for example :

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

        if (e.OldElement != null)
        {
            e.OldElement.TakePictureRequested += OnTakePictureRequested;
        }
        if (e.NewElement != null)
        {
            if (Control == null)
            {
                cameraPreview = new CameraPreviewView(Context);
                SetNativeControl(cameraPreview);
            }
            Control.Preview = Camera.Open(getBackCameraId());

            e.NewElement.TakePictureRequested += OnTakePictureRequested;
        }
    }

    void OnTakePictureRequested(object sender, EventArgs e)
    {
        cameraPreview.TakePicture();
    }

The idea is that "OnTakePictureRequested" from Native Code can return the value to the Xamarin Forms.

Anyone has an idea ? Thanks

Take a look at TaskCompletionSource. Its pretty straight forward, if you still need help we can take a look at what you've tried — shanranm
Hello @shanranm , I already tried it, but it seems i can't return something from an Invoke — Prozedsq
You need to await the Task returned by TaskCompletionSource — shanranm
Yes but it can't work for an Event Handler. Invoke is a void function and cannot be awaited. What i want, is retrieving the information in my Custom control. — Prozedsq

Morse Morse · Accepted Answer · 2019-07-18T02:45:11

In your native view access Xamarin Forms view by

var xamarinFormsView = e.NewElement as CameraPreview;
xamarinFormsView .DoSomethingAfterPictureTaken("blahblah");

and in CameraPreview add a method to access and do something with the value you want to send over

void DoSomethingAfterPictureTaken(string valueFromNative){
//access valuefromNative as you wish
}

Xamarin Forms - Send event to native and retrieve value

2 Answers