问题
To try out the nearbyAPI, I decided to build an app that would allow a teacher to track attendance of student in a class. Similar to what Caren Chang is doing.
I have gone through the google sample codes on rockpaperscissors and walkietalkie
But only a single device seems to connect at any one time when i test the samples using 4 phones.
I want to build a teacher and student app which the teacher advertises and discovers students, sends payloads to each and every connected device simultaneously as more devices become connected in a classroom set up.
How can i use nearby api to connect and send data to new and multiple devices simultaneously?
回答1:
That's a great use case, and one we've talked about in the past.
If it's specifically for attendance, then you don't need to form a connection. You can have each device advertise while one device constantly scans. You'll build up a list of devices quickly that way.
If you want to do more than attendance, though, such as pushing an assignment to everyone's device, you'll need to build a mesh. To start with, you'll want to use Strategy.P2P_CLUSTER. We have 3 strategies available inside Nearby Connections (CLUSTER, STAR, POINT_TO_POINT) and cluster is the most general one. With cluster, you can connect to as many devices as you want, and you can receive incoming connections from as many devices as you want. Or, almost... The Bluetooth radio inside phones is weak and can only hold 3~4 connections at a time.
To be able to connect all ~30 devices, I'd recommend forming a 'snake-like' connection. The head and tail of the device will scan and advertise at the same time (and devices that aren't connected to anyone are considered snakes of length 1). The heads and tails will keep connecting to each other (being sure not to connect to itself*), and you'll pretty quickly have a long chain of connections connecting everyone together. From there, you can forward messages down the chain to make sure everyone gets it.
- To avoid connecting to yourself, you can either assign every device a random number (eg. 1, 4, 8, 10) and each device tries to connect to the next highest number, or you can broadcast a message when connecting and disconnect if you get an echo back (because the broadcast went in a circle).
来源：https://stackoverflow.com/questions/52773197/be-able-to-send-messages-bytes-simultaneous-to-multiple-devices-using-nearby-con