What I want to know:
I'm wondering how the android system (like Android smart phone) auto-connects to devices which is paired before.
For example, I pair my bluetooth headset with my android smartphone in the procedure of "turn on scanning/advertising -> click pairing" on day 1. And when I turn on advertising on my headset, it connects automatically on day 2, 3, 4, and so on. The point is, I don't have to make my smartphone scan again to find my (paired) headset.
I can't understand how android system finds that the paired device is turned on. Does the android system scan periodically in background? Even if I don't click "scan" button?
Why I ask:
I want to make my app autoconnect to customized BLE device, after make pairing. I succeed to make pairing(bonding) with createBond() method, but after that, I couldn't find how to make autoconnect. I know I can turn on autoconnect function like this way, connectGatt(XXX, true, XXX), but this autoconnect function doesn't work when the BLE device is disconnected a few days.
So I want to make my app works like android system and Bluetooth headset. But I couldn't find how android bluetooth system works even I dig AOSP codes.
I found many questions (here and here) about problems like mine but there were no answers.
Thanks in advance.
Update:
I found that bt_btif gets activated (with LG smartphone and Nexus 5) when the paired headset is turned on (start advertising). But bt_btif doesn't get activated with my custom BLE device... What can I do?
Executing connectGatt(XXX, true, XXX) is the correct way to go. gatt.connect() also starts an auto connect. Once you execute that, your phone will scan for the device and once it appears it connects to it. The only thing that interrupt this call is either if Bluetooth is turned off on the phone or if your app process is being killed. To avoid your process from being killed, let your app have a foreground service.
One gotcha however, there is a bug in Android which will sometimes make auto connect do a direct connect instead (and cancel after 30 seconds or so). See https://code.google.com/p/android/issues/detail?id=69834. You need to use reflection to avoid this bug.
I think I found a solution.
First, the solution for my question: Android smartphone seems to detect state changes of nearby bluetooth devices from the hardware sides. When the paired bluetooth headset starts to advertise, a callback in HAL (I think) is called.
So I made my app to connectGatt with autoConnection=True to the device that I want, by using MAC address, when the activity is started (in onResume() of MainActivity).
The connection would fail if my BLE peripheral device is not advertising. But the device auto-connects when it starts advertising, because the autoConnection parameter was set to true.
I've done a similar app and i didn't have problem with that. As a last resort for your problem, i would suggest writing the BLE Device Address in a simple DB table and manually connect to it. I've made something like this here
. It is no best practice code, but i hope you can find ideas for you solution.
Related
We want Android to automatically connect to our custom made BLE peripheral.
Our peripheral should regularly (but infrequently) advertise and attempt to Indicate some time-sensitive sensor data to the phone. Thus we want the phone to be ready to connect at any time.
Generally, you can pair a smart watch with an Android, and Android will then automatically connect to the smart watch whenever it is in range. So we believe our use case should be feasible.
I read a lot of answers that advise to set the "autoconnect" parameter to true when connecting. I have tried that and the reconnections don't persist through a reboot or even after disabling and re-enabling Bluetooth on Android. This answer by Brian says I should scan in the background, but Android made this unrealistic. If I use a foreground service, my users will hate the app. If I use a background service, I may miss the peripheral's attempts to connect during Android's Doze and the code becomes error prone.
Ideally, I want to do something like what Emil said in his answer here. Please read the follow up question and response.
However, we can't see our app through Android's Bluetooth settings. We can only connect to the peripheral and pair with it using our app (or nrf Connect). In desperation, I tried modifying the peripheral's advertising flags. Then I could see it in Android's Bluetooth settings. But when I try to pair using Android's settings, the attempt fails because the peripheral is not in "pairing mode".
We are building both the app and the peripheral, so we can change both. I want to know if our use case is possible and what we need to do to get it working. We are using the STM32WB for our peripheral.
Use a combination of these techniques:
Bond the device. This might be needed due to the crappy Android Bluetooth LE API design that doesn't take the "address type" as an extra parameter when connecting to a device. When you connect using the Bluetooth device address, it looks up a device with this address in the bonding info, and uses the corresponding address type (random or public).
Use connectGatt with autoConnect set to true. This means no timeout, as well as auto-reconnect if the connection drops. Even if it takes days or weeks until the peripheral starts advertising, it will still work.
Listen to https://developer.android.com/reference/android/bluetooth/BluetoothAdapter#ACTION_STATE_CHANGED and restart your connections when Bluetooth is re-started.
Use a Foreground Service in your application's process to prevent the OS from killing the process. Users can nowadays hide the annoying notification in Android settings if they want to.
Listen to https://developer.android.com/reference/android/content/Intent#ACTION_BOOT_COMPLETED to start your app after boot, including your Foreground Service.
Listen to https://developer.android.com/reference/android/content/Intent#ACTION_PACKAGE_REPLACED to automatically restart your app after an app update. See https://stackoverflow.com/a/2134443/556495 for some instructions.
The best approach is to make sure your peripheral can be bonded. Once you have bonded with it you can ALWAYS use autoconnect because Android stores info about bonded devices and you don't have to scan for it anymore. Hence you avoid the issues with scanning in the background.
Although that resolves the need for scanning, you still need to deal with your app being killed once it is in the background. Using a Foreground Service is still the best solution to my knowledge. I don't think you users will hate your app for it...
I need to estabilish a brief communication between a BLE master and a BLE peripherical device.
Until I have to use the mobile phone I have no problems, since I select the bluetooth device from Android, I can connect to it and then I can receive data simply writing them to HM-10.
BUT, this approach is battery expensive, since the BLE scanning is more battery expensive than the BLE advertising (right?). So I want that the mobile becames an advertisor and the HM-10 takes the ROLE of Slave.
BUT, I still don't understand if it is possible to connect to other devices: I tried AT+CONN and it always fails (AT+CONF). Is there any way to send brief data from the HM-10 to the advertisor, even without connecting?
Even my firmware looks slightly different from the readme! For example,AT+LN is present in the documentation but it appears to not exist. Does a better version of the firmware for this device exist, or, are sources available somewhere? I installed the HM-Soft V705 (lastest).
Thanks
UPDATE 1:
#ChrisStratton Ok, I have some intresting news: I was already following your idea (phone advertise sometimes, while HM-10 always scan and tries to connect to the service if it sees it, then HM-10 send some data, phone answer). I managed to make the phone visible from HM-10 (don't ask me how!). I am using the BLE Tool Android app to test everything. Now, the central problem now is that HM-10 won't connect to the android service even if the advertisor is connectable (see attached images).
I am sospicious that the AT+CO (that I discovered is a kind a replacement of AT+LN, but I am not sure) can help, but i don't know how.
My goal is to get the Android device to reconnect to a BLE device that it has previously connected to without user intervention in the same way it does for a classic BT paired device does (even works through power cycles).
One of the ideas of BTLE devices is that one saves service, bonding, and enabling states such that a reconnect is VERY fast and consumes very little power on the peripheral.
What I have done seems to work but it works poorly.
The first step is to connect or pair and connect to a new device setting the 'autoconnect' parameter to 'true'.
When the device disconnects, do not call gatt.close(). Everywhere I look I see that one should call gatt.close(). But if I do call gatt.close() the Android central app never reconnects. I have tested this many times.
If I have not called gatt.close() and have not power cycled the Android, the auto-connection usually happens. Sometimes it can take a long time, especially after version 5.0. It is, however, unreliable and it may be unreliable due to a very low-duty scan cycle and the device quitting advertising before a scan cycle actually detects the advertisement. I am not sure because there is no way to detect the scanning operation like there is advertisements! It is also possible the scanning stops after a certain amount of time but there is no documentation on that.
So what I think I need to do is to somehow set the background scan rate used by the Android to a higher duty cycle (only possible in 5.0 and up) when auto-connect has been set but I do not know how to do this. I do not want to start my own scan but somehow set the background scanning rate used by Android for the reconnect. Does anyone know how to do this? Does anyone really know how autoconnect and gatt.close() are to work?
Maybe the auto-connect was NOT meant to re-connect as I indicated above?
Well after many trials and tribulations this is how I best get the Android to auto connect with the only user action being to first select the device (if using the settings menu then first pairing).
You have to trap the paired event in a BroadcastReceiver and do a BluetoothDevice.connectGatt() setting the autoconnect to true. Then when the device disconnects, invoke a gatt.connect().
Update: While the above approach works in general, it is sometimes agonizingly slow probably because the pending connection uses extremely conservative scan rates. The other downside is that for each device you want to auto-reconnect to you have to keep a BluetoothGatt object performing a pending connection. In the embedded world this is insane. Instead what one does is continuously scan and connect to a desired device by inspecting its advertisement. One saves only the minimal amount of data about the device (the services, its paired state and keys, etc.). When an advertisement is captured you see if it is one of your known devices and connect to if it is.
I tried the equivalent on Android. Scan all the time (low power rate) and connect to advertisements of interest, and maintain a class representing a known device. There are some annoying details in this approach (like turning off scanning while connecting and restarting after connected) but it basically works without the overhead of maintaining connections. BUT there is one exception I do not understand. One pre-paired device's advertisements are never seen by the scanner. However, if I invoke a pending connection to this device, I re-connect. I do not understand this at all. On my embedded platforms it works as it should.
If anyone else has tried this approach for auto-reconnecting, please share your experiences!
I have discovered the reason the pre-paired device is not seen by Android. Android only reports scan results IF the device responds to a scan request. Once paired, this device only emits advertisements and ignores scan requests, so the Android system does not pass up its advertisements in the ScanCallback. Thus in order to work using the scan approach, I have to use the pending connect approach for those specific devices. It just seems like you can't win!
============= UPDATE 2020
Many years have passed and I have a lot more experience with the background scan approach. If one keeps the supported platforms 5 and up, one can use only the newest scanner APIs and use filters, eliminating the need to decode the raw advertisements yourself. I have also found that connection and re-connection is snappier if you DONT turn off scanning while connecting. I know it goes against all documentation, but it works and on some platforms allowed connections to happen that otherwise did not. Also, to date, I have found only one (health) device that needs pending connects. Disclaimer: All I have ever worked with is health devices.
This is how I was able to do it for my application.
I first stored the address of the device in a SharedPreference then in gattClientCallback funtion of my BluetoothLeService
else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
intentAction = ACTION_DISCONNECTED;
DeviceActivity.runOnUI(() -> {
sharedPreferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
String name_dev_1 = sharedPreferences.getString("Dev_1", null) ;
connectToDevice(name_dev_1);
disconnectGattServer();
});
}
}
What this does is if your device is disconnected it will keep on trying to connect to it until a connection is established.
I have an android app i am developing. It's a BTLE app, i have some BTLE tags i need to talk to.
I do all by the book, discover device, GATT connect (subsequent bonding), write/read characteristics... it works.
Now the weird is that it will stop working if i close and open the application two times.
Let's say i have already one tag correctly connected, bonded and working properly.
phone reboot.
Open app the first time -> everything works.
close app, open again -> everything works.
close app again, open -> unable to receive any readCharacteristic from the GATT callbacks.
In the logs, there is NOTHING DIFFERENT from each attempt. I log each call and print return statuses: no difference. Just the readChar callback is never called.
Android is 4.3, same issue on different phones (S3, S4, Note3)
(note: on app shutdown i properly disconnect all the GATT stuff)
Do you have any ideas on what to check? Or is this a known android bug? I have searched, but could not find anything....
The bluetooth stack on Android 4.3(at least) specially for BTLE definitely has quite some issues. I don't know if it's a specific vendor implementation, driver issues, or what else, but it's honestly a nightmare for developers.
This is what i found out does help to improve things:
- disable Wifi
- forcefully enable/disable bluetooth by code (BluetoothAdapter methods enable() and disable())
In fact, cyclying bluetooth on and off when things start getting "unstable" do fix it. This requires the BLUETOOTH_ADMIN capability and might clearly not be desired, since it will briefly shut down ALL you bluetooth conencted gadgets AND will popup some dialogs to the user where he/she can disable the ongoing restart.
Now i am getting more stable results.
Still, shame on Google (or whoever i don't know) for letting such poor software out in the wild.
I am no Bluetooth specialist and wondering what possibilities are available to find already paired Bluetooth devices automatically when they are range of each other.
Background:
In our case an Android application needs to connect to a dedicated accessory via Bluetooth (Rfcomm). Both devices are known to each other (they are paired). The Android application registers a broadcast receiver. During the startup of the application, the app initiates a discovery to find the dedicated accessory. If the accessory is in range everything works great.
Problem:
The user starts the application outside the range of the dedicated accessory. The Android application tries to discover the accessory without success. Then the user goes into the range of the Bluetooth accessory. The broadcast receiver won’t get notified about the accessory that is in range now.
Similar Thread / Possible Solutions
Similar questions were already asked on stackoverflow (e.g. autoconnect to bluetooth device when in range).
But continuously trying to discover Bluetooth devices in range isn't what I am looking for because this would cause too much battery drain of the Android device.
Another solution would be to try to connect to the paired device in the onResume method of the Activity. This would work but has the disadvantage that the application can’t run in the background. So the user had to bring the application at least once to the foreground to initiate the connection.
A third idea I thought about is to implement a server socket into the Android application too. When the android application is started and the discovery finished without success, the Android application could create server socket and to listen to incoming notifications of the accessory. This would help in some scenarios (e.g. the user starts his application, approaches the accessory, activates the accessory and the accessory notifies the application on startup that it is in range now). But this is still no 100% solution because both devices can start outside the range of each other. Also it would be mandatory to implement additional functionality (Bluetooth server socket in the Android device…).
So I am wondering if better solutions exist. I am looking for a solution where no additional ServerSockets are required and I always get the notification that the two already paired devices are in range of each other :-)
Thanks for any help!
After connecting the device for the first time, keep the mac address in a local list.
On disconnect, use connectGatt with autoconnect set to true to automatically re-connect when you are in range.
Not a full solution, but maybe it's sufficient for your app to poll the accesory's presence whenever the screen is turned on? In that case, this may be helpful: Start Activity on wake up/sleep in Android