Swift 3 CoreBluetooth后台数据接收

Question

I am trying to receive some data from an Arduino which is emitting some data when I push a button.

Now I've noticed that it does not receive after a while. I did not kill the app yet. How come? I've already added

<key>UIBackgroundModes</key>
<array>
    <string>bluetooth-central</string>
    <string>bluetooth-peripheral</string>
    <string>external-accessory</string>
</array>

My devices are both paired.

EDIT:

I am trying to do the same as the Tile App , it works really well there.

EDIT 2:

I instantiate my manager like this:

centralManager = CBCentralManager(delegate:self, queue:nil)

Answer 1

you need to initialize your CBCentralManager by passing in the option parameter your restore key identifier.

let central = CBCentralManager(delegate: self, queue: nil, options: [CBCentralManagerOptionRestoreIdentifierKey: "yourkey"])

Where "your key" can be anything you want and it is useful for you just to understand that your app was awaken to handle a BLE message. If the app was killed by the system when a new BLE message will cause it to be resurrect in your app delegate didFinish launching this key will be passed in the options parameter.

To make it work properly you'll have to implement the delegate method

optional public func centralManager(_ central: CBCentralManager, willRestoreState dict: [String : Any])

this will be called in case of app killed by the system (or by a crash.. not by the user by the way) in order to notify you that a new BLE event happened. In the will restore state dict you will have all the info you need to restore your BLE session.

Here you will find all apple's recommendations to work with the BLE in background.

Remember that the connection with your peripheral will always be severed if the user performs any actions that voluntarily causes the connection to go off, such as app killed, Blueetooth disabled. iOS will continue to awake/resurrect your app in case of crash, app in background and then killed by the system.

The events that will cause the app to be awaken are notifications from registered notifying characteristics, new peripherals discovery, read of a characteristic. (basically any callback of the CoreBluetooth delegate)

Keep in mind that a scan operation while in background has limitations.

From apple's documentation linked before:

Although you can perform many Bluetooth-related tasks while your app is in the background, keep in mind that scanning for peripherals while your app is in the background operates differently than when your app is in the foreground. In particular, when your app is scanning for device while in the background:

• The CBCentralManagerScanOptionAllowDuplicatesKey scan option key is ignored, and multiple discoveries of an advertising peripheral are coalesced into a single discovery event.
• If all apps that are scanning for peripherals are in the background, the interval at which your central device scans for advertising packets increases. As a result, it may take longer to discover an advertising peripheral.

Happy coding.