Getting into detail

The Android application is a  simple demo that toggles a LED connected to the accessory (the Arduino UNO). The Android code can be found under the java directory in the source git:

git clone git://github.com/joekickass/Arduino-Uno-Android-Accessory.git

Below I will highlight parts of the code. Most of the information given can be found in the Google Accessory Dev Guide.

AndroidManifest.xml

The are a few important lines to notice:

<intent-filter>
    <action android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED" />
</intent-filter>

Declare that the Activity wants to receive events when an accessory is connected.

<meta-data android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"
           android:resource="@xml/accessory_filter" />

In order to specify what particular accessory to connect to, an additional resource must be created (see accessory_filter).

<uses-library android:name="com.android.future.usb.accessory" />

Classes that support USB accessory mode where added to the android.hardware.usb namespace in Android 3.1. It was also backported to Android 2.3.4 in the Google APIs add-on library (which you downloaded in Part I) in the com.android.future.usb namespace. This application is written for version 2.3.4 and thus the library needs to be declared in the manifest.

accessory_filter.xml

Also, a resource specifying what resource to connect to is required. Add the following accessory_filter.xml file to the res/xml/ directory in the Android project:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <usb-accessory manufacturer="JoeKickass" model="AndroidApkTakeTwo" version="1.0" />
</resources>

The manufacturer, model and version attributes should match the information given to the ADK class in the Arduino firmware.

ToggleLedActivity

The application opens a connection to the accessory when the ACTION_USB_ACCESSORY_ATTACHED intent is received. The user can then toggle the LED by pressing the button in the application. Whenever the application is paused, the connection is closed. However, application takes many shortcuts in order to keep the code small (but it is still slightly verbose to be more readable). First of all, it does not handle disconnects/reconnects particularly well. Also, it doesn’t obtain permission to connect to device, which requires the user to actively accept the accessory the first time it is connected.

The following lines are using the Google APIs Add-on library to create a UsbManager and open a UsbAccessory:

private UsbManager mUsbManager = UsbManager.getInstance(this);

private UsbAccessory mAccessory;

    @Override
    protected void onResume() {
        super.onResume();
        setupDetachingAccessoryHandler();
        reOpenAccessoryIfNecessary();
    }

    private void reOpenAccessoryIfNecessary() {
        setStatusWaiting();
        if (mOutputStream != null) {
            setStatusConnected();
            return;
        }
        Intent intent = getIntent();
        String action = intent.getAction();
        if (UsbManager.ACTION_USB_ACCESSORY_ATTACHED.equals(action)) {
            mAccessory = UsbManager.getAccessory(intent);
            openAccessory();
        }
    }

    private void openAccessory() {
        try {
            mFileDescriptor = mUsbManager.openAccessory(mAccessory);
            if (mFileDescriptor != null) {
                FileDescriptor fd = mFileDescriptor.getFileDescriptor();
                mOutputStream = new FileOutputStream(fd);
                setStatusConnected();
            }
        } catch (IllegalArgumentException ex) {
            // Accessory detached while activity was inactive
            closeAccessory();
        }
    }

The next section shows how to register a BroadcastReceiver to listen for detaching accessories. By declaring and registering the receiver in the Activity instead of in the Manifest, detaching accessories are only handled when the  Activity is running.

    private void setupDetachingAccessoryHandler() {
        mReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String action = intent.getAction();
                if (UsbManager.ACTION_USB_ACCESSORY_DETACHED.equals(action)) {
                    closeAccessory();
                    finish();
                }
            }
        };
        IntentFilter filter = new IntentFilter(UsbManager.ACTION_USB_ACCESSORY_DETACHED);
        registerReceiver(mReceiver, filter);
    }

Interaction with the user is handled in the following piece of code. When the toggle button is clicked, the TOGGLE_LED_COMMAND is sent to the accessory.

    private void setupToggleButton() {
        Button button = (Button) findViewById(R.id.toggle_button);
        button.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                sendCommand(TOGGLE_LED_COMMAND);
            }
        });
    }
     private void sendCommand(byte command) {
        if (mOutputStream != null) {
            try {
                mOutputStream.write(command);
            } catch (IOException e) {
                // Do nothing
            }
        }
    }

That’s the main components of the java code.

The last step describes the Arduino code; Step III.

Here are all posts in the series:

• Step I
• Step II
• Step III