如何授予使用usb管理器打开usb设备的权限？openDevice()总是返回null

Question

我想在下面的代码中使用usb设备。它成功地列出了usb设备并对它们进行了迭代。在下面的代码中，object "device“是我需要打开的usbdevice。除了总是返回空值的OpenDevice()方法之外，一切看起来都很好！

[Activity(Label = "TestApp", MainLauncher = true, Icon = "@drawable/icon")]
[IntentFilter(new[] {UsbManager.ActionUsbDeviceAttached})]
[MetaData(UsbManager.ActionUsbDeviceAttached, Resource = "@xml/device_filter")]

public class MainActivity : Activity
{
    int count = 1;
   {
       base.OnCreate(bundle);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        UsbManager manager = (UsbManager)GetSystemService(Context.UsbService);
        UsbDevice device = null;
        foreach (var dev in manager.DeviceList)
        {
            if (dev.Value.VendorId == 5401)
            {
                device = dev.Value;
            }
        }
        var connection = manager.OpenDevice(device);
        // Read some data! Most have just one port (port 0).
    }

device_filter.xml包含以下行：

<?xml version="1.0" encoding="utf-8" ?>
<resources>
   <usb-device product-id="8704" vendor-id="5401" />
</resources>

当我尝试bool hasPermision =manager.HasPermission(设备)时，我发现hasPermission是假的。有人能告诉我怎么才能允许我在xamarin打开usb设备吗？谢谢你的帮助。

Answer 1

经过几个建议，我终于把它修好了。我发现在清单中添加意图筛选器并不能解决未知原因的权限问题。听起来像是萨马林的窃听器。说实话，Xamarin usb名称空间似乎太天真了，最好不要浪费时间用于USB管理和连接。它还有其他一些恼人的限制。例如，看看这里。(所以我建议用java编写低级别的usb通信代码，并通过JNI用xamarin导入jar文件，我对此感到厌倦，应该说比第一次看起来容易得多)

要授予打开usb设备的权限，您必须使用grantPermission()方法。下面的代码显示了如何使用该方法和BroadcastReceiver来弹出一个对话框，请求用户允许usb使用。

 public class MainActivity : Activity
{
    private static String ACTION_USB_PERMISSION = "com.android.example.USB_PERMISSION";
    UsbDevice device; 
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        UsbManager manager = (UsbManager)GetSystemService(Context.UsbService);

        UsbReciever usbReciever = new UsbReciever();
        PendingIntent mPermissionIntent = PendingIntent.GetBroadcast(this, 0, new Intent(
            ACTION_USB_PERMISSION), 0);
        IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
        RegisterReceiver(usbReciever, filter);

        foreach (var dev in manager.DeviceList)
        {
            if (dev.Value.VendorId == 8192)
            {
                device = dev.Value;
            }
        }
        manager.RequestPermission(device, mPermissionIntent);
        bool hasPermision = manager.HasPermission(device);

        UsbDeviceConnection connection = manager.OpenDevice(device);
        if (connection == null)
        {
            return;
        }
        Button button = FindViewById<Button>(Resource.Id.MyButton);
    }
    class UsbReciever : BroadcastReceiver
    {
        public override void OnReceive(Context context, Intent intent)
        {
            String action = intent.Action;
            if (ACTION_USB_PERMISSION.Equals(action))
            {
                lock (this)
                {
                    UsbDevice device = (UsbDevice)intent
                            .GetParcelableExtra(UsbManager.ExtraDevice);

                    if (intent.GetBooleanExtra(
                            UsbManager.ExtraPermissionGranted, false))
                    {
                        if (device != null)
                        {
                            // call method to set up device communication
                        }
                    }
                    else
                    {

                    }
                }
            }
        }
    }


}