How android system autoconnects to paired device?

问题:

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?

回答1:

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.

回答2:

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.

回答3:

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.