Question

对Android BluetoothGattServerCallback#onCharacteristicWriteRequest方法感到困惑。

对于参数preparedWrite，我认为实施写操作是我的责任，回调如何知道何时应该对消息进行排队？
文档说An application must call BluetoothGattServer.sendResponse(BluetoothDevice, int, int, int, byte[]) to complete the request.，如果responseNeeded为假，该怎么办？
无论我设置为BluetoothGattServer#sendResponse(BluetoothDevice, int, int, int, byte[])，调用value似乎总是返回请求中的值。这是预期的吗？

Answer 1

preparedWrite用于支持长写（大于MTU）。

BluetoothGattServerCallback#onCharacteristicWriteRequest与BluetoothGattServerCallback#onExecuteWrite结合使用以重新组合对等设备发送的分段数据。要回答您的问题：

当接收到一部分传入数据时，参数preparedWrite为true，即数据需要排队直到更多数据到达。 BluetoothGattServerCallback#onExecuteWrite将在收到最终片段并且已发送的特性值可以完全组装之后被调用。
如果responseNeeded为假，请勿致电BluetoothGattServer.sendResponse(BluetoothDevice, int, int, int, byte[])。
只需发送一个空的value。

以下是onCharacteristicWriteRequest的示例：

                @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));
    }

handleInputMessage的实现是特定于应用程序的。

由BluetoothGattServerCallback＃onCharacteristicWriteRequest混淆

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