RxSwift是一个正式履行Reactive X (aka Rx)，Rx支持很多语言和平台，Rx可以说无处不在most major languages and platforms.
Add observer with `id` and print each emitted event.
- parameter id: an identifier for the subscription.
Received unhandled error: /var/folders/tr/g4phkfhs50324y0b15wlldhw0000gn/T/./lldb/5118/playground23.swift:69:__lldb_expr_23 -> Sequence contains more than one element.
Dec  1 10:53:11   <Error>: CGContextSetFillColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Dec  1 10:53:11   <Error>: CGContextSetStrokeColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Dec  1 10:53:11   <Error>: CGContextGetCompositeOperation: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Dec  1 10:53:11   <Error>: CGContextSetCompositeOperation: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Dec  1 10:53:11   <Error>: CGContextFillRects: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Run carthage build --configuration Debug.
Build project (Product → Build).