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ENGINEERING

5min read

The best iOS solution to code Bluetooth devices

When our awesome team of Android dev released the RxAndroidBLE Bluetooth library, it quickly gained attention in the community. That made us real proud, but also we realised that there was a high demand for such tool. With the raise of the bluetooth tech in app development, we needed to create a iOS version, backed with Swift: the RxBluetoothKit . In the following post, I want to introduce you to what motivated us to create this library and I will take you through the Polidea’s developing process we implemented for our open source projects.

Motivation

As we had just started a new main project involving the use of Bluetooth Low Energy tech to back the device we were making communication with. It was big time to make some high level architectural design decisions. We reevaluated the solutions that we had previously used and we researched what new and popular in community and if it’s worth to try. Having all this in mind, we decided that the project will be coded using Swift Programming Language and RxSwift library. This choice of technologies was the reason to create RxBluetoothKit library, and I will try to explain to you why we thought it’s a good idea to build it

iOS Bluetooth

Nowadays, we see that the IoT trend gained lots of attention and inside company we’re doing lots of communication with the external devices. Bluetooth, because of its low energy consumption is obvious candidate for communication protocol. Communicating with Bluetooth on iOS, despite that it’s less problematic than on Android - it’s still not easy. Apple provides Core Bluetooth framework that does the heavy lifting for the developers. Core Bluetooth, like most of the Apple frameworks is built on the delegation pattern. Even the simplest communication with BLE device, which is reading any value from it requires couple of steps:

  • scanning desired device
  • connecting to it
  • discovering service that contains characteristic to read value from
  • discovering characteristic on that service
  • reading value from characteristic

Every one of these steps is linked to the delegate method which Core Bluetooth provides. Moreover - every one can fail and error has to be handled properly. Multiplicity of places to handle error is not a good sign - usually it means that somewhere error is not handled properly and it’s one of the causes of bugs in software. Also, code in which fail points are all over the place is just not readable for you and your coworkers.

Swift

Whole iOS community is now in the middle of transition from Objective-C to Swift - Apple’s new and shiny programming language. Inside company, we adopted Swift pretty quickly and now it’s ours default language of choice for new iOS projects. Swift enables writing clean and very readable code, so whenever we have complex Objective-C backed libraries to cooperate with (mentioned like Core Bluetooth), we are wrapping it in Swift layer. Thanks to this - every time we use it we have clean, Swifty interfaces, without @objc everywhere.

RxSwift

RxSwift is a library that provides abstraction for making asynchronous operations feel synchronous. It’s a Swift port of Reactive Extensions which were originally created by Microsoft and are widely used in many environments by thousands of developers on the world.

RxSwift gives developers abstraction of computation - mechanism called Observable. Every Observable instance contains definition of creation values. You should note that it’s just a definition - just creating it doesn’t do anything. In order to perform defined computation, you should call subscribe to Observable instance.

Every subscription could deliver a stream of events. Event could be:

  • Next - Emitted new computation element
  • Error - Error has occurred on stream, and no more values will be emitted
  • Completed - stream completed successfully, and no more values will be emitted

Concept of observables becomes very useful, when we’re coming from single Observable to operators that are combining multiple Observable instances. The best thing about RxSwift in RxBluetoothKit was that once error in any Observable that is part of sequence occurs - someone who subscribed to it immediately received error.

This is such an opportunity for our Bluetooth case study - we could combine scanning, connecting, discovering and reading in one sequence of Observables and handle error in just one place.

Creation process

Almost all of the library code was a result of pair programming sessions. It helped us a lot in reaching desired code quality. Also, thanks to that process of making architectural decision for the library was much easier. We were constantly interchanging ideas and it was crucial for building this library.

The second most important decision was to remove implementation code at some moment and write tests for all of the crucial code. After doing that, we were replacing empty spaces in code following by the Red, Green, Refactor principle. That helped in discovering bugs and resulted in high quality code.

RxBluetoothKit

RxBluetoothKit is now live and we’re very satisfied with the result. Library had great community response, showed up in couple of newsletters and now we also have contributors outside of the company.

We’re using it internally in our iOS Bluetooth projects and we’re thrilled with the result - now we have simple, readable code that is also very flexible. Here’s the example of reading value of characteristic from the library:

manager.scanForPeripherals([serviceIds])
.take(1)
.flatMap { $0.peripheral.connect() }
.flatMap { $0.readValueFromCharacteristicWithIdentifier(DeviceCharacteristic.ManufacturerName) }
.subscribeNext {
  let data = $0.value
}

As you see, code is understandable - every more complex operation is a sequence of the small ones, and by reading line by line developer knows exactly what it does. This is not achievable using pure delegates, where state is shared between large number of methods and sequences of operations are not clearly visible.

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KacperSoftware Engineer
PrzemekStaff Software Engineer

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