有没有办法在BLE特性中只写一个字节？

Question

I am new to android. My BLE device expects only one byte(flag). I tried writing only one byte in a byte array but it never works. While writing an array to another characteristic where it expects 2 bytes works. Is there a way to solve my problem or do I have to ask for device code to be changed?

I am working with this project as my example

public void writeCustomCharacteristicStart() {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }
        /*check if the service is available on the device*/
        BluetoothGattService mCustomService = mBluetoothGatt.getService(UUID.fromString("00001813-0000-1000-8000-00805f9b34fb"));
        if(mCustomService == null){
            Log.w(TAG, "Custom BLE Service not found");
            return;
        }
        /*get the read characteristic from the service*/
        byte pom= 1;
        byte[] data= new byte[1];
        data[0]=pom;
        BluetoothGattCharacteristic mWriteCharacteristic = mCustomService.getCharacteristic(UUID.fromString("00002a4e-0000-1000-8000-00805f9b34fb"));

        if((mWriteCharacteristic.getProperties() & BluetoothGattCharacteristic.PROPERTY_WRITE)>0 ) {
            Log.w(TAG,"Writing started");
            mWriteCharacteristic.setValue(data)
        }
        if(!mBluetoothGatt.writeCharacteristic(mWriteCharacteristic)){
            Log.w(TAG, "Failed to write characteristic");
        }
        Log.w(TAG,"Writing ending");
    }

08-06 11:59:57.700 29279-29279/com.example.devicescanactivity E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.devicescanactivity, PID: 29279
    java.lang.IllegalStateException: Could not execute method of the activity
        at android.view.View$1.onClick(View.java:4084)
        at android.view.View.performClick(View.java:4858)
        at android.view.View$PerformClick.run(View.java:20167)
        at android.os.Handler.handleCallback(Handler.java:815)
        at android.os.Handler.dispatchMessage(Handler.java:104)
        at android.os.Looper.loop(Looper.java:194)
        at android.app.ActivityThread.main(ActivityThread.java:5931)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:987)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:782)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at android.view.View$1.onClick(View.java:4079)
        at android.view.View.performClick(View.java:4858) 
        at android.view.View$PerformClick.run(View.java:20167) 
        at android.os.Handler.handleCallback(Handler.java:815) 
        at android.os.Handler.dispatchMessage(Handler.java:104) 
        at android.os.Looper.loop(Looper.java:194) 
        at android.app.ActivityThread.main(ActivityThread.java:5931) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:372) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:987) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:782) 
     Caused by: java.lang.SecurityException: Need BLUETOOTH_PRIVILEGED permission: Neither user 10097 nor current process has android.permission.BLUETOOTH_PRIVILEGED.
        at android.os.Parcel.readException(Parcel.java:1549)
        at android.os.Parcel.readException(Parcel.java:1502)
        at android.bluetooth.IBluetoothGatt$Stub$Proxy.writeCharacteristic(IBluetoothGatt.java:1003)
        at android.bluetooth.BluetoothGatt.writeCharacteristic(BluetoothGatt.java:1029)
        at com.example.devicescanactivity.BluetoothLeService.writeCustomCharacteristic(BluetoothLeService.java:377)
        at com.example.devicescanactivity.DeviceControlActivity.onClickWrite(DeviceControlActivity.java:332)

Here is the error, I don't get the BLUETOOTH_PRIVILEGED error, but I don't get any errors when I write to the characteristic that accepts 2 bytes.

Answer 1

First, to use bluetooth in your app, you need to add the following permissions to your app manifest :

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />

It is possible to write a 1 byte value to a BLE characteristic, provided the device expects a 1 byte value.

If there is a length issue, you should see the error in the onCharacteristicWrite() callback:

public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {

        if (status == BluetoothGatt.GATT_SUCCESS) {
            Log.i("TAG", "onCharacteristicWrite() status: " + status + "  - UUID: " + characteristic.getUuid());
        } else if (status == BluetoothGatt.GATT_INVALID_ATTRIBUTE_LENGTH)  {
            Log.e("TAG", "onCharacteristicWrite() A write operation failed due to invalid attribute length");
        } else {
            Log.e("TAG", "onCharacteristicWrite() A write operation failed. Status = " + status);
        }
}

Also, I would move the writeCharacteristic() call inside your check for PROPERTY_WRITE to ensure the write operation can be done:

if((mWriteCharacteristic.getProperties() & BluetoothGattCharacteristic.PROPERTY_WRITE) > 0 ) {
    Log.i(TAG,"Writing started");
    mWriteCharacteristic.setValue(data)
    if(!mBluetoothGatt.writeCharacteristic(mWriteCharacteristic)){
        Log.e(TAG, "Failed to write characteristic");
    }
}

Answer 2

The problem wasn't in what I write in the characteristic it was what characteristic I used. I used HID Control Point which when tested on the computer nRF Connect app worked fine for my device but it would not work on android because of its predefined states. We just changed characteristic to Record Access Control Point and it works for now.