我有一台使用bleno的蓝牙服务器，并向客户端返回可用的Wifi网络列表。 readCharacteristic的代码看起来基本上是这样的：
class ReadCharacteristic extends bleno.Characteristic {
constructor(uuid, name, action) {
super({
uuid: uuid,
properties: ["read"],
value: null,
descriptors: [
new bleno.Descriptor({
uuid: "2901",
value: name
})
]
});
this.actionFunction = action;
}
onReadRequest(offset, callback) {
console.log("Offset: " + offset);
if(offset === 0) {
const result = this.actionFunction();
result.then(value => {
this.actionFunctionResult = value;
const data = new Buffer.from(value).slice(0,bleno.mtu);
console.log("onReadRequest: " + data.toString('utf-8'));
callback(this.RESULT_SUCCESS, data);
}, err => {
console.log("onReadRequest error: " + err);
callback(this.RESULT_UNLIKELY_ERROR);
}).catch( err => {
console.log("onReadRequest error: " + err);
callback(this.RESULT_UNLIKELY_ERROR);
});
}
else {
let data = new Buffer.from(this.actionFunctionResult);
if(offset > data.length) {
callback(this.RESULT_INVALID_OFFSET, null);
}
data = data.slice(offset+1, offset+bleno.mtu);
console.log(data.toString('utf-8'));
callback(this.RESULT_SUCCESS, data);
}
}
}
（我已经尝试data = data.slice(offset+1, offset+bleno.mtu);并且喜欢这个data = data.slice(offset+1);）
客户端是一个Android应用程序，可以读取此特征。
用于阅读的Android部分如下所示：
@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status,
int newState) {
if (newState == BluetoothProfile.STATE_CONNECTED) {
gatt.requestMtu(256);
} else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
Log.i(TAG, "Disconnected from GATT server.");
mFancyShowCaseView.show();
gatt.close();
scanForBluetoothDevices();
}
}
@Override
public void onMtuChanged(BluetoothGatt gatt, int mtu, int status) {
if (status != BluetoothGatt.GATT_SUCCESS) {
Log.e(TAG, "Can't set mtu to: " + mtu);
} else {
Log.i(TAG, "Connected to GATT server. MTU: " + mtu);
Log.i(TAG, "Attempting to start service discovery:" +
mWifiProvisioningService.discoverServices());
}
}
@Override
// New services discovered
public void onServicesDiscovered(BluetoothGatt gatt, int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
Log.d(TAG, "ACTION_GATT_SERVICES_DISCOVERED");
BluetoothGattService wifiProvisioningService = gatt.getService(WIFI_PROVISIONING_SERVICE_UUID);
BluetoothGattCharacteristic currentConnectedWifiCharacteristic = wifiProvisioningService.getCharacteristic(WIFI_ID_UUID);
BluetoothGattCharacteristic availableWifiCharacteristic = wifiProvisioningService.getCharacteristic(WIFI_SCAN_UUID);
// Only read the first characteristic and add the 2nd one to a list as we have to wait
// for the read return before we read the 2nd one.
if (!gatt.readCharacteristic(currentConnectedWifiCharacteristic)) {
Log.e(TAG, "Error while reading current connected wifi name.");
}
readCharacteristics.add(availableWifiCharacteristic);
} else {
Log.w(TAG, "onServicesDiscovered received: " + status);
}
}
@Override
// Result of a characteristic read operation
public void onCharacteristicRead(BluetoothGatt gatt,
BluetoothGattCharacteristic characteristic,
int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
UUID characteristicUUID = characteristic.getUuid();
if (WIFI_ID_UUID.equals(characteristicUUID)) {
Log.d(TAG, "HEUREKA we found the current wifi name: " + new String(characteristic.getValue()));
final String currentWifiName = new String(characteristic.getValue());
runOnUiThread(new Runnable() {
@Override
public void run() {
((TextView) findViewById(R.id.currentWifiTxt)).setText(currentWifiName);
findViewById(R.id.currentWifiTxtProgress).setVisibility(View.GONE);
}
});
} else if (WIFI_SCAN_UUID.equals(characteristicUUID)) {
Log.d(TAG, "HEUREKA we found the wifi list: " + new String(characteristic.getValue()));
List<String> wifiListArrayList = new ArrayList<>();
try {
JSONObject wifiListRoot = new JSONObject(characteristic.getStringValue(0));
JSONArray wifiListJson = wifiListRoot.getJSONArray("list");
for (int i = 0; i < wifiListJson.length(); i++) {
wifiListArrayList.add(wifiListJson.get(i).toString());
}
} catch (JSONException e) {
Log.e(TAG, e.toString());
return;
}
final String[] wifiList = new String[wifiListArrayList.size()];
wifiListArrayList.toArray(wifiList);
runOnUiThread(new Runnable() {
@Override
public void run() {
((ListView) findViewById(R.id.availableWifiList)).setAdapter(new ArrayAdapter<String>(mContext, R.layout.wifi_name_list_item, wifiList));
findViewById(R.id.currentWifiTxtProgress).setVisibility(View.GONE);
}
});
} else {
Log.i(TAG, "Unexpected Gatt vale: " + new String(characteristic.getValue()));
}
if (readCharacteristics.size() > 0) {
BluetoothGattCharacteristic readCharacteristic = readCharacteristics.get(0);
if (!gatt.readCharacteristic(readCharacteristic)) {
Log.e(TAG, "Error while writing descriptor for connected wifi");
}
readCharacteristics.remove(readCharacteristic);
}
}
}
MTU调整为256字节。我在阅读列表时反映在服务器上。调用本身工作正常并返回列表但如果列表包含更多 600 字节，则Android上只有600字节可用。我在某种程度上确定JS服务器发送所有数据但由于某种原因，Android客户端只接收或缓存600字节似乎不正确。
我发现了这篇文章：Android BLE - Peripheral | onCharacteristicRead return wrong value or part of it (but repeated)
并且： Android BLE - How is large characteristic value read in chunks (using an offset)?
但两者都没有解决我的问题。我知道在开始下一次读取之前我需要等待一次读取才能返回，我需要等到MTU写入才继续读取数据。据我所知，这反映在您上面看到的来源中。我有点迷失在这里。
任何想法都是高度关注的。
非常感谢
答案 0 :(得分：1)
对于任何遇到这篇文章的人，也想知道为什么Android似乎只为长GATT特性返回600个字节，就像这个问题所要问的那样，这全都归结于Bluedroid（Android的蓝牙堆栈）如何实现其GATT客户端以及其实现方式。不合规格就我而言，我使用基于ESP32的IoT设备作为GATT服务器，并使用GATT客户端的Android（SDK 24）。
根据规范（Bluetooth Core 4.2；第3卷，F部分：3.2.9），特征值的最大大小（从ATT的属性值继承）为512字节。但是，由于某些原因，Bluedroid不会尝试执行此要求，而是决定最大大小为600；如果您深入Bluedroid源并找到设置为600（GATT_MAX_ATTR_LEN）的宏stack/include/gatt_api.h:125，则可以看到该代码。因为就我而言（似乎是您自己），我正在实现读取请求响应代码，所以我也没有看到针对特征读取对512字节的限制。
现在，重要的是要认识到Bluedroid读取特征的方式以及与MTU大小，读取的最大大小（应为512，但对于Bluedroid为600）以及如何处理比最大尺寸。 MTU大小是您可以使用的ATT级别上最大的数据包大小。因此，对于每个对BluetoothGatt.readCharacteristic的调用，您可能正在向服务器发送一个或多个读取请求，具体取决于Bluedroid是否认为特征大小超出了MTU大小。在较低级别上，Bluedroid将首先发送ATT读取请求（0x0a），如果数据包的长度为MTU字节，它将跟进ATT读取Blob请求（0x0c），其偏移量为设置为MTU大小。它将继续发送ATT读取Blob请求，直到ATT读取Blob响应的长度小于MTU字节，或者直到达到最大特征大小为止（例如，Bluedroid为600）。重要的是要注意，如果MTU大小不是大于600字节的数据的600的完美倍数，则剩余的字节将被丢弃（因为Bluedroid从未真正期望读取600字节，因为它认为GATT服务器将强制执行512字节限制特征尺寸）。因此，如果您的数据超过600字节的限制（或出于安全性考虑为512的限制），则应该期望多次调用BluetoothGatt.readCharacteristic。这是一个在Android端读取大量数据的简单示例（对不起，我未使用bleno，因此无法为您提供修复该端的代码），它依赖于首先将数据长度作为无符号32位整数发送，然后如果数据长度超过600字节，则重复调用BluetoothGatt.readCharacteristic读取数据：
private int readLength;
private StringBuilder packet; // In my case, Im building a string out of the data
@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status,
int newState) {
if (newState == BluetoothProfile.STATE_CONNECTED) {
gatt.requestMtu(201); // NOTE: If you are going to read a long piece of data, its best to make this value a factor of 600 + 1, like 51, 61, 101, 151, etc due to the risk of data loss if the last packet contains more than 600 bytes of cumulative data
}
}
@Override
public void onMtuChanged(BluetoothGatt gatt, int mtu, int status) {
gatt.discoverServices();
}
@Override
public void onServicesDiscovered(BluetoothGatt gatt, int status) {
// Kick off a read
BluetoothGattCharacteristic characteristic = gatt.getService(UUID.fromString(SERVICE_UUID)).getCharacteristic(UUID.fromString(CHAR_UUID));
readLength = 0;
gatt.readCharacteristic(characteristic);
}
@Override
public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
if (readLength == 0) {
readLength = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT32, 0);
packet = new StringBuilder();
gatt.readCharacteristic(characteristic);
} else {
byte[] data = charactertic.getValue();
packet.append(new String(data));
readLength -= data.length;
if (readLength == 0) {
// Got all data this time; you can now process the data however you want
} else {
gatt.readCharacteristic(characteristic);
}
}
}