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
Related
I'm working with app development for a HW-driven device company. Our current (BLE) HW-devices communicates with the phone, and when the HW is considered "active" (triggered by a physical activation on the HW), the phone retrieves continuous data from said HW.
In order for the application to be ready for the HW to become active, it is holding a permanent "binding" to the HW. On Android, the application holds a permanent ("sticky") notification, that prevents the app from being killed, and lets the background process stay alive at all times and listens for the HW to announce that it is active. On iOS, I believe that the HW is spamming "ping-requests" to the app in order to keep it alive.
All in all, the setup works, but is not ideal. I've tried to wrap my head around the different "peripheral modes" and master/slave-setups, but have yet to understand how to setup a relationship between phone and HW that behaves as we would like:
We would like to have the HW to behave like (for instance) BT-headphones work, i.e. when I open the case for my headphones, the application starts up (in the case of my earplugs, a battery status notification is shown) and communication starts. When I put the plugs back in the case, the application closes (at least as far as the user can tell). Once the HW is in range again and is activated (i.e. equivalent to opening the case) the app/communication is ready/resumed.
Is the described functionality reserved for headphones only, or is this a setup that can be achieved by any BLE-device?
tl;dr I don't want to have a persistent notification on Android to make sure that the app is always ready when the device is "active", but without it, the app is easily killed in the background.
It is tricky when you ask a question that covers both Android and iOS as both platforms will have different approaches.
I will answer for iOS as that is what I know, but I presume there is something similar on Android.
First, a little on how the BLE GATT service works in simple terms;
In BLE there are two roles; central and peripheral. 99% of the time, the peripheral advertises services and the central discovers peripherals advertising the services it is interested in. The central can then connect to the peripheral.
Once connected, the central can initiate reads from and writes to the peripheral. The central can also register for notifications from the peripheral. This allows the peripheral to send new data when it has it, rather than waiting until the central reads it.
Now, for how you can use all of this in iOS Core Bluetooth.
Your initial task is to discover the peripheral you are interested in. You do this by scanning for a peripheral advertising your specific service. You typically need to provide some sort of UI to show what has been discovered and allow the user to select the device to which they want to connect, but this depends on your specific use case.
Once you have discovered (and the user has selected) a peripheral, you can store the identifier that Core Bluetooth provides. This is a unique value for this peripheral on this iOS device; It is not the peripheral MAC address and it cannot be used on a different iOS device to identify the same peripheral.
In future you can use this identifier to try and retrieve a CBPeripheral object from Core Bluetooth without needing to scan for it.
Once you have a CBPeripheral you can issue a connect. Once the connect completes you can read/write data or register for notifications.
If you have enabled Core Bluetooth background mode in your app then the connect and notification events will be delivered to your app even when it is in the background. You can also issue read and write commands in the background in response to the connect and notification events, although you only have a few seconds execution time.
At some point it is likely that the peripheral will go out of range/be turned off and the connection will disconnect. Again, this event may be delivered in the background or foreground.
In response your app should immediately issue a new connect. If the device is not currently available then this connect will be pending and will complete when/if the peripheral is seen again. iOS will deliver this to your app in the foreground or background.
The final point to consider is the case where your app is not currently in the suspended state; Perhaps the iOS device has been rebooted or your app has been offloaded from memory by iOS because it hasn't been active for some time.
Your app can opt in to Core Bluetooth State Restoration by providing a n identifier parameter when it creates its CBCentralManager. This will relaunch your app.
On relaunch you need to re-establish your CBCentralManager with the same identifier and then you will receive the connect callback that triggered your apps relaunch. From this point you can proceed normally.
This is an article from Apple that, while old, explains Core Bluetooth background and state restoration pretty well.
Note that all of this can happen without pairing/bonding. If your peripheral requires encryption for any of its attributes then iOS will automatically present a pairing dialog the first time you connect. Once this pairing is in place it is not required again. Pairing/bonding only exchanges keys for encryption. It is not explicitly required to allow future connections.
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...
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.
I would like to ask for your valuable opinion about the following:
I'm developing an Android app that needs to connect with the BLE device (whose name I know) automatically on starting the app. Furthermore, the app should be able to keep the phone connected to the BLE device (there will be no "connect" and "disconnect" buttons on the app screen). In other words, if established connection is lost for some reason, the app should be capable to detect this and to re-connect again. The user of the app don't need to be aware of this background process. For him/her, it is only important that the phone is connected with the BLE device.
Simply speaking, I know the name of the BLE device (it is called 'HMSoft'). How should I keep the app connected with this BLE module? Should I use some Thread that will do the job for me in parallel with my other activities within the app MainActivity? Or there is some better approach? Did you have similar experience in the past?
Thank you very much for your time and effort. I really appreciate it.
Sincerely,
Bojan.
You will want to wrap the BLE implementation up in a Service. Within that Service, you will have to implement the logic to connect to the BLE device and keep trying to connect to that BLE device if it becomes unavailable.
I am writing an android app to connect with BLE heart rate monitor devices.
All works well until another application tries to connect to the same service on the BLE device.
Only one application seems to be able to connect at a time.
Is it possible to connect the same BLE service to multple apps at once?
I set up my Bluetooth service in accordance with Link :
You can use BluetoothManager.getConnectedDevices() to get a list of Bluetooth devices, even ones not connected by your app. Unfortunately, you don't also get the service information for those devices.
What you can do then, is connect to the ones not already connected by your app and perform service discovery on them (BluetoothDevice.connectGatt() / BluetoothGatt.discoverServices()), then disconnect those that you aren't interested in. It's a bit cumbersome, but it seems to work for us.
No you can not do that, this is something is not supported in BLE.