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android - 被 BluetoothGattServerCallback#onCharacteristicWriteRequest 弄糊涂了

问题描述

对 Android BluetoothGattServerCallback#onCharacteristicWriteRequest方法感到困惑。

  1. 对于参数preparedWrite，我认为实现写操作是我的责任，回调如何知道我应该何时将消息排队？

  2. 文档说An application must call BluetoothGattServer.sendResponse(BluetoothDevice, int, int, int, byte[]) to complete the request.，如果responseNeeded是假的怎么办？

  3. BluetoothGattServer#sendResponse(BluetoothDevice, int, int, int, byte[])无论我设置什么，调用似乎总是返回请求中的值value。是预期的吗？

标签： android

解决方案

用于支持长preparedWrite写入（大于 MTU）。

BluetoothGattServerCallback#onCharacteristicWriteRequest与 一起工作BluetoothGattServerCallback#onExecuteWrite以重组对等设备发送的碎片数据。要回答您的问题：

  1. 当接收到传入数据的片段时，该参数preparedWrite为真，即数据需要排队等待更多数据到达。BluetoothGattServerCallback#onExecuteWrite将在收到最终片段并且可以完全组装发送的特征值后调用。

  2. 如果responseNeeded为假，请不要调用BluetoothGattServer.sendResponse(BluetoothDevice, int, int, int, byte[]).

  3. 只发一个空的value。

    这是一个例子onCharacteristicWriteRequest：

                @Override
                public void onCharacteristicWriteRequest(BluetoothDevice device, int requestId, BluetoothGattCharacteristic characteristic, boolean preparedWrite, boolean responseNeeded, int offset, byte[] value) {
                    super.onCharacteristicWriteRequest(device, requestId, characteristic, preparedWrite, responseNeeded, offset, value);
                    Log.d("SVC", "BluetoothGattServerCallback.onCharacteristicWriteRequest with " + value.length + " bytes");
                    if(preparedWrite) {
                        handleInputFragment(device, characteristic.getUuid(), value);
                    } else {
                        handleInputMessage(device, characteristic.getUuid(), value);
                    }
                    if(responseNeeded) {
                        Log.d("SVC", "sending response to write request for characteristic: " + characteristic.getUuid());
                        if(!gattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, 0, new byte[0])) {
                            Log.e("SVC", "response to characteristic write request failed");
                        }
                    }
                }

和onExecuteWrite

                @Override
                public void onExecuteWrite(BluetoothDevice device, int requestId, boolean execute) {
                    Log.d("SVC", "BluetoothGattServerCallback.onExecuteWrite " + (execute ? "execute" : "cancelled"));
                    super.onExecuteWrite(device, requestId, execute);
                    Iterator<InputBuffer> itr = inputBuffers.iterator();
                    while(itr.hasNext()) {
                        InputBuffer buf = itr.next();
                        if(buf.device.equals(device)) {
                            itr.remove();
                            if(execute) {
                                ByteArrayOutputStream os = new ByteArrayOutputStream();
                                for (byte[] b : buf.bytes) {
                                    os.write(b, 0, b.length);
                                }
                                handleInputMessage(device, buf.characteristicUuid, os.toByteArray());
                            }
                        }
                    }
               }

InputBuffer 机制的实现类似于

    private class InputBuffer {
        final BluetoothDevice device;
        final UUID characteristicUuid;
        final List<byte[]> bytes;

        InputBuffer(BluetoothDevice device, UUID characteristicUuid, byte[] value) {
            this.device = device;
            this.characteristicUuid = characteristicUuid;
            this.bytes = new ArrayList<>();
            this.bytes.add(value);
        }
    }

    private List<InputBuffer> inputBuffers = new LinkedList<>();

    private void handleInputFragment(BluetoothDevice device, UUID characteristicUuid, byte[] value) {
        Log.d("SVC", "handling input Fragment");
        for(InputBuffer buf : inputBuffers) {
            if(buf.device.equals(device)) {
                buf.bytes.add(value);
                return;
            }
        }
        inputBuffers.add(new InputBuffer(device, characteristicUuid, value));
    }

的实现handleInputMessage是特定于应用程序的。

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