如何在Xamarin Forms App中拦截Android设备的音量增大/减小按钮以打开相机？

如果您只是想在android中实现。您可以像跟随GIF一样直接打开相机。

您可以在android中拍照，然后使用消息中心在PCL中上传您的图像，例如followcode。

MainPage.xaml.cs

      public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();

        CameraImage.Source = "CameraImage.png";


        MessagingCenter.Subscribe<App, string>(App.Current, "OpenPage", (snd, arg) =>
        {
            Device.BeginInvokeOnMainThread(() => {
                CameraImage.Source = arg;
            });
        });
    }   
}

MainActivity.cs

       public static class App_test
{
    public static File _file;
    public static File _dir;
    public static Bitmap bitmap;
}

[Activity(Label = "InterceptButton", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
{
    protected override void OnCreate(Bundle savedInstanceState)
    {
        StrictMode.VmPolicy.Builder builder = new StrictMode.VmPolicy.Builder();
        StrictMode.SetVmPolicy(builder.Build());
        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;

        base.OnCreate(savedInstanceState);

        Xamarin.Essentials.Platform.Init(this, savedInstanceState);
        global::Xamarin.Forms.Forms.Init(this, savedInstanceState);


        LoadApplication(new App());

        if (IsThereAnAppToTakePictures())
        {
            CreateDirectoryForPictures();
        }
    }
    public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
    {
        Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);

        base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    public override bool OnKeyUp([GeneratedEnum] Keycode keyCode, KeyEvent e)
    {
        if (keyCode == Keycode.VolumeDown)
        {
            Toast.MakeText(this, "OnKeyUp-VolumeDown", ToastLength.Short).Show();


            return true;
        }

        if (keyCode == Keycode.VolumeUp)
        {
          //  Toast.MakeText(this, "OnKeyUp-VolumeUp", ToastLength.Short).Show();
            return true;
        }

        return base.OnKeyUp(keyCode, e);
    }
    public override bool OnKeyDown([GeneratedEnum] Keycode keyCode, KeyEvent e)
    {
        if (keyCode == Keycode.VolumeDown)
        {
            Toast.MakeText(this, "OnKeyDown-VolumeDown", ToastLength.Short).Show();
            return true;
        }

        if (keyCode == Keycode.VolumeUp)
        {
            Intent intent = new Intent(MediaStore.ActionImageCapture);
               App_test._file = new File(App_test._dir, String.Format("myPhoto_{0}.jpg", Guid.NewGuid()));
               intent.PutExtra(MediaStore.ExtraOutput, Uri.FromFile(App_test._file));
            StartActivityForResult(intent, 0);
            Toast.MakeText(this, "OnKeyDown-VolumeUp", ToastLength.Short).Show();
            return true;
        }
        return base.OnKeyDown(keyCode, e);
    }


    protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
    {
        base.OnActivityResult(requestCode, resultCode, data);
        Intent mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
        Uri contentUri = Uri.FromFile(App_test._file);
        mediaScanIntent.SetData(contentUri);
        SendBroadcast(mediaScanIntent);

        App_test.bitmap = App_test._file.Path.LoadAndResizeBitmap(100, 100);
       string path= App_test._file.Path;
        if (App_test.bitmap != null)
        {
            MessagingCenter.Send<App, string>(App.Current as App, "OpenPage", path);

            App_test.bitmap = null;
        }
        GC.Collect();
    }

    private void CreateDirectoryForPictures()
    {
        App_test._dir = new File(
            Environment.GetExternalStoragePublicDirectory(
                Environment.DirectoryPictures), "CameraAppDemo");
        if (!App_test._dir.Exists())
        {
            App_test._dir.Mkdirs();
        }
    }

    private bool IsThereAnAppToTakePictures()
    {
        Intent intent = new Intent(MediaStore.ActionImageCapture);
        IList<ResolveInfo> availableActivities =
            PackageManager.QueryIntentActivities(intent, PackageInfoFlags.MatchDefaultOnly);
        return availableActivities != null && availableActivities.Count > 0;
    }
}

所以我不知道为什么要使用DependencyService。也许您有更好的方法来实现。

有我的演示。我使用VS 2019，如果您使用VS 2017，它将有所不同。

https://github.com/851265601/InterceptButton