how to wait for the android activity to fully resume after calling ZXing qr scanner - xamarin

Question

I have an application which when starting requests a qr code from the user, and according to the qr scanned, a different fragment is loaded in the activity
I am using ZXing mobile scanner to do this
unfortunatelly the scanner returns a reply way before it shuts down and returns to the calling activity
this means that when I call the transaction code to replace the current fragment with the new one, the activity is not yet in the foreground so nothing happens
To solve this I created a ManualResetEvent (I'm using Xamarin, but I will use a Semaphore when I have to convert this to Android Studio) that I set before starting the scan , and then reset in the OnResume part of the activity
this seems to solve the problem, but it feels like there is a much better solution
Is there something I am missing?
thanks in advance for any help you can provide
Edit: the code I am currently using
public class MyActivity : Activity {
ManualResetEvent _has_resumed = new ManualResetEvent(false);
.....
protected override void OnResume() {
base.OnResume();
_has_resumed.Set();
}
......
void scan_qr(Action<string> finished_callback) {
#region initialize the scanner
MobileBarcodeScanner scanner = new MobileBarcodeScanner();
MobileBarcodeScanningOptions options = new MobileBarcodeScanningOptions();
options.UseNativeScanning = true; //use native scan
options.AutoRotate = false;//do not rotate the screen
options.PossibleFormats = new List<BarcodeFormat> { BarcodeFormat.QR_CODE }; // only allow qr_codes;
#endregion
#region perform the actual scan, when it finishes return to the main thread and execute the callback
_has_resumed.Reset();
scanner.Scan(this, options) //do scan
.ContinueWith(result => {
_has_resumed.WaitOne();
return result.Result;
})//wait until the activity has resumed
.ContinueWith((task_result) => { //then return the result
Result result = task_result.Result;
if (result == null) {
show_toast( Resource.String.questions_select_error_no_qr_scanned );
} else {
finished_callback(result.Text);
}
}, System.Threading.Tasks.TaskScheduler.FromCurrentSynchronizationContext());
#endregion
}
}