与外设交互

当开发人员直接将硬件外设连接到应用程序时，Android Things的强大功能就实现了。

在本课中，您将学习如何使用基本的外设 I / O API来发现和与通用输入输出（GPIO）端口进行通信。

列出可用的外围设备

负责管理外设连接的系统服务是 PeripheralManagerService。您可以使用此服务列出所有已知外设类型的可用端口。

以下代码将可用的GPIO端口列表写入 logcat：

public class HomeActivity extends Activity {
    private static final String TAG = "HomeActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        PeripheralManagerService service = new PeripheralManagerService();
        Log.d(TAG, "Available GPIO: " + service.getGpioList());
    }
}

处理按钮事件

当按下连接到GPIO的按钮时接收事件：

public class ButtonActivity extends Activity {
    private static final String TAG = "ButtonActivity";
    private static final String BUTTON_PIN_NAME = ...; // GPIO port wired to the button

    private Gpio mButtonGpio;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        PeripheralManagerService service = new PeripheralManagerService();
        try {
            // Step 1. Create GPIO connection.
            mButtonGpio = service.openGpio(BUTTON_PIN_NAME);
            // Step 2. Configure as an input.
            mButtonGpio.setDirection(Gpio.DIRECTION_IN);
            // Step 3. Enable edge trigger events.
            mButtonGpio.setEdgeTriggerType(Gpio.EDGE_FALLING);
            // Step 4. Register an event callback.
            mButtonGpio.registerGpioCallback(mCallback);
        } catch (IOException e) {
            Log.e(TAG, "Error on PeripheralIO API", e);
        }
    }

    // Step 4. Register an event callback.
    private GpioCallback mCallback = new GpioCallback() {
        @Override
        public boolean onGpioEdge(Gpio gpio) {
            Log.i(TAG, "GPIO changed, button pressed");

            // Step 5. Return true to keep callback active.
            return true;
        }
    };

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // Step 6. Close the resource
        if (mButtonGpio != null) {
            mButtonGpio.unregisterGpioCallback(mCallback);
            try {
                mButtonGpio.close();
            } catch (IOException e) {
                Log.e(TAG, "Error on PeripheralIO API", e);
            }
        }
    }
}

闪烁LED

要在连接到GPIO的LED上执行闪烁模式：

public class BlinkActivity extends Activity {
    private static final String TAG = "BlinkActivity";
    private static final int INTERVAL_BETWEEN_BLINKS_MS = 1000;
    private static final String LED_PIN_NAME = ...; // GPIO port wired to the LED

    private Handler mHandler = new Handler();

    private Gpio mLedGpio;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Step 1. Create GPIO connection.
        PeripheralManagerService service = new PeripheralManagerService();
        try {
            mLedGpio = service.openGpio(LED_PIN_NAME);
            // Step 2. Configure as an output.
            mLedGpio.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);

            // Step 4. Repeat using a handler.
            mHandler.post(mBlinkRunnable);
        } catch (IOException e) {
            Log.e(TAG, "Error on PeripheralIO API", e);
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // Step 4. Remove handler events on close.
        mHandler.removeCallbacks(mBlinkRunnable);

        // Step 5. Close the resource.
        if (mLedGpio != null) {
            try {
                mLedGpio.close();
            } catch (IOException e) {
                Log.e(TAG, "Error on PeripheralIO API", e);
            }
        }
    }

    private Runnable mBlinkRunnable = new Runnable() {
        @Override
        public void run() {
            // Exit if the GPIO is already closed
            if (mLedGpio == null) {
                return;
            }

            try {
                // Step 3. Toggle the LED state
                mLedGpio.setValue(!mLedGpio.getValue());

                // Step 4. Schedule another event after delay.
                mHandler.postDelayed(mBlinkRunnable, INTERVAL_BETWEEN_BLINKS_MS);
            } catch (IOException e) {
                Log.e(TAG, "Error on PeripheralIO API", e);
            }
        }
    };
}