Hi I'm writing an Android app to connect to a BLE peripheral device. Android 4.4.2, Galaxy Nexus.
I have an LED on the device to indicate connection state.
The issue is the duration from connectGatt() call to the point of receiving onConnectionStateChange STATE_CONNECTED
are so inconsistent. Some time it is very quick but most of the time it takes 5s or more. Turning Bluetooth off/on does not have any effect.
I tried TI BTool on PC with the TI BLE Dongle and it always establishes connection very fast.
I also tried with an iPhone 5S and it is fast too.
Passing true to connectGatt() autoconnect argument requests a background connection, while passing false requests a direct connection. BluetoothGatt#connect() always requests a background connection.
Background connection (according to Bluedroid sources from 4.4.2 AOSP) has scan interval of 1280ms and a window of 11.25ms. This corresponds to about 0.9% duty cycle which explains why connections, when not scanning, can take a long time to complete.
Direct connection has interval of 60ms and window of 30ms so connections complete much faster. Additionally there can only be one direct connection request pending at a time and it times out after 30 seconds. onConnectionStateChange() gets called with state=2, status=133 to indicate this timeout.
I have verified this behavior on Nexus5 but obviously YMMV.
I should mention that there is a race condition in BluetoothGatt.java that can cause a direct connection request even if autoconnect=true is passed into BluetoothDevice#connectGatt().
我已尝试慢速连接,但只有在尝试重新连接远程设备时,第一次连接设备才没有问题,但重新连接仍然是onClientRegistered()方法中的连接
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