如何与USB设备通信？

Question

我正在尝试作为主机建立USB通信。 我正在遵循这里的示例http://developer.android.com/guide/topics/usb/host.html，但是我无法使其正常工作。 这是我的代码：

private static final String ACTION_USB_PERMISSION = "com.multitools.andres.LCView";
    UsbDevice device;
    //Pide permisos al usuario para comunicacion con el dispositivo USB
    private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            if (ACTION_USB_PERMISSION.equals(action)) {
                synchronized (this) {
                    UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
                    if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                        if(device != null){
                            //call method to set up device communication
                        }
                    } 
                    else {
                        Log.d(TAG, "permission denied for device " + device);
                    }
                }
            }
        }
    };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if(DEBUG) Log.i(TAG, "onCreate() -> MainMenu");

        actionBar = getActionBar();                     //obtengo el ActionBar
        actionBar.setDisplayHomeAsUpEnabled(true);      //el icono de la aplicacion funciona como boton HOME
        //Menu
        setListAdapter(new ArrayAdapter<String>(MainMenu.this, android.R.layout.simple_list_item_1, MenuNames));

        //USB
        if(DEBUG) Log.i(TAG, "Setting UsbManager -> MainMenu");
        UsbManager mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE);
        PendingIntent mPermissionIntent;

        if(DEBUG) Log.i(TAG, "Setting PermissionIntent -> MainMenu");
        mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
        if(DEBUG) Log.i(TAG, "Setting IntentFilter -> MainMenu");
        IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
        if(DEBUG) Log.i(TAG, "Setting registerReceiver -> MainMenu");
        registerReceiver(mUsbReceiver, filter);
        if(DEBUG) Log.i(TAG, "Setting requestPermission -> MainMenu");
        mUsbManager.requestPermission(device, mPermissionIntent);

    }

当我取消注释mUsbManager.requestPermission(device, mPermissionIntent);这一行时，会弹出“Force Close”对话框。如果我将其注释掉，则不会弹出Force Close，但它不起作用。我认为问题出在：

private static final String ACTION_USB_PERMISSION = "com.multitools.andres.LCView";

在谷歌的示例中，它如下所示：

private static final String ACTION_USB_PERMISSION = "com.android.example.USB_PERMISSION";

但我不知道我到底应该放什么，也找不到任何关于这个的解释。我该放什么或者我的错误在哪里？下面是我启动应用程序时得到的LogCat：

04-17 00:57:50.944: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:51.331: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:51.981: I/(1362): onCreate() -> MainMenu
04-17 00:57:52.013: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:52.151: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:52.570: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:52.731: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:53.122: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:53.231: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:53.390: I/(1362): Setting UsbManager -> MainMenu
04-17 00:57:53.451: I/(1362): Setting PermissionIntent -> MainMenu
04-17 00:57:53.470: I/(1362): Setting IntentFilter -> MainMenu
04-17 00:57:53.470: I/(1362): Setting registerReceiver -> MainMenu
04-17 00:57:53.511: I/(1362): Setting requestPermission -> MainMenu
04-17 00:57:53.660: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:53.791: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:54.311: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:54.401: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:54.531: I/(1362): onCreateOptionsMenu() -> MainMenu
04-17 00:57:54.683: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:54.772: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:55.186: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:55.291: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:55.661: I/dalvikvm(1362): threadid=3: reacting to signal 3
04-17 00:57:55.751: I/dalvikvm(1362): Wrote stack traces to '/data/anr/traces.txt'
04-17 00:57:55.791: D/gralloc_goldfish(1362): Emulator without GPU emulation detected.
04-17 01:11:47.323: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:47.720: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:48.124: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:48.291: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:48.452: I/(1459): onCreate() -> MainMenu
04-17 01:11:48.691: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:48.813: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:49.172: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:49.321: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:49.653: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:49.821: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:49.901: I/(1459): Setting UsbManager -> MainMenu
04-17 01:11:49.931: I/(1459): Setting PermissionIntent -> MainMenu
04-17 01:11:50.021: I/(1459): Setting IntentFilter -> MainMenu
04-17 01:11:50.031: I/(1459): Setting registerReceiver -> MainMenu
04-17 01:11:50.051: I/(1459): Setting requestPermission -> MainMenu
04-17 01:11:50.071: D/AndroidRuntime(1459): Shutting down VM
04-17 01:11:50.133: W/dalvikvm(1459): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
04-17 01:11:50.231: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:50.331: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:50.401: E/AndroidRuntime(1459): FATAL EXCEPTION: main
04-17 01:11:50.401: E/AndroidRuntime(1459): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.multitools.andres/com.multitools.andres.MainMenu}: java.lang.NullPointerException
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.os.Looper.loop(Looper.java:137)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread.main(ActivityThread.java:4424)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at java.lang.reflect.Method.invokeNative(Native Method)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at java.lang.reflect.Method.invoke(Method.java:511)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at dalvik.system.NativeStart.main(Native Method)
04-17 01:11:50.401: E/AndroidRuntime(1459): Caused by: java.lang.NullPointerException
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.os.Parcel.readException(Parcel.java:1333)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.os.Parcel.readException(Parcel.java:1281)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.hardware.usb.IUsbManager$Stub$Proxy.requestDevicePermission(IUsbManager.java:535)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.hardware.usb.UsbManager.requestPermission(UsbManager.java:361)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at com.multitools.andres.MainMenu.onCreate(MainMenu.java:80)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.Activity.performCreate(Activity.java:4465)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
04-17 01:11:50.401: E/AndroidRuntime(1459):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
04-17 01:11:50.401: E/AndroidRuntime(1459):     ... 11 more
04-17 01:11:50.751: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:50.851: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:51.331: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:51.403: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'
04-17 01:11:51.774: I/dalvikvm(1459): threadid=3: reacting to signal 3
04-17 01:11:51.961: I/dalvikvm(1459): Wrote stack traces to '/data/anr/traces.txt'

谢谢你 :)

- Andres

---

你的 MainMenu.java 文件的第80行是什么？那一行似乎抛出了一个 NullPointerException。 - chandsie

mUsbManager.requestPermission(device, mPermissionIntent); 这一行代码如果加上注释，应用程序就不会崩溃。我认为这是因为私有静态最终字符串 ACTION_USB_PERMISSION = "com.multitools.andres.LCView" 的原因，因为我不知道该放什么。 - Andres

Answer 1

我认为这个字符串只是一个标记，以便在调用registerReceiver(mUsbReceiver, filter);时你能够识别返回的意图。这不是问题所在。

我认为问题在这里:

UsbManager mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE);

这行代码应该会给你提供USB管理器，但据我所知，并非所有手机都必须支持Android USB Accessory API。这行代码可能只是将null分配给mUsbManager，这会在稍后的代码中调用其方法时导致NullPointerException。在调用之前，请尝试检查它是否为null。

更多信息，请查看以下链接：

1. http://developer.android.com/guide/topics/usb/index.html
2. http://developer.android.com/guide/topics/usb/accessory.html

编辑:

我想我现在明白问题所在了。你说得对，问题不在于USB管理器，而是UsbDevice对象(device)。在你的代码中，它从未被初始化。就像这一行:

mUsbManager.requestPermission(device, mPermissionIntent);

您基本上是在发送一个意图，询问用户是否允许您使用由UsbDevice对象device表示的设备。但是当这个调用被触发时，device还没有被初始化（因此具有null的默认值）。因此，当您尝试请求权限时，您最终会得到一个NullPointerException而不是预期的结果。要解决这个问题，您需要找出要连接的设备，并将其分配给device。在这里中查看“枚举设备”，以找出不同的方法。一种方法是，在检索到Usb Manager后，如果您知道要连接的设备的名称，则可以进行以下调用：

HashMap<String, UsbDevice> deviceList = mUsbManager.getDeviceList();
device = deviceList.get("<deviceName>");

（显然，您需要将<deviceName>替换为实际设备的名称。）

- chandsie

---

我使用Log来了解mUsbManager的值，我得到的值是：android.hardware.usb.UsbManager@412b8038，所以它不是null。我在模拟器上进行测试，因为我没有具备USB主机功能的设备:/所以字符串可以是任何名称，这不是问题吗？ - Andres

谢谢您 :) 您是正确的。我正在模拟器上测试，现在我放入这段代码来测试USB设备：HashMap<String, UsbDevice> deviceList = mUsbManager.getDeviceList(); Log.i(TAG, "Device List: " + deviceList); 在LogCat上出现了"Device List: {}"，这意味着我没有连接任何设备？ - Andres

是的，没错。您需要将一些设备连接到模拟器上。这里有一个关于如何做到这一点的问题链接：https://dev59.com/KnI-5IYBdhLWcg3wVWpi。 - chandsie

Answer 2

minSdkVersion 必须 >= 12

要启用 USB 主机 API 支持，您需要添加一个名为 android.hardware.usb.host.xml 的文件，并包含以下行：

<permissions>
 <feature name="android.hardware.usb.host"/>

- dajun

---

关于如何启用USB主机API支持的更多细节，请参见https://github.com/452/USBHIDTerminal/wiki/How-to-enable-USB-host-API-support。 - pantos27