Day16 开发套件 - 实作EventChannel

使用上跟MethodChannel类似，EventChannel即为MethodChannel与Stream的结合

Flutter 端：

首先同样先建立Channel并指定名称（记得所有通道名称都不能重复），然後要对Native 端传递来的讯息进行监听，通过EventChannel调用receiveBroadcastStream()来取得一个Stream物件，後续通过在Stream中新增监听来完成对 Native 端讯息的接收等操作

lib/batterylevel.dart：

这边建立一个EvnetChannel来取得Native 端一个骰子持续摇出来的数字，并建立一个取得骰子讯息的Stream物件方法

  static const EventChannel _eventChannel = const EventChannel('dice_number');
 
	static Stream<int> get getRandomNumberStream {
    return _eventChannel.receiveBroadcastStream().cast();
  }

Native 端（Android）：

android/.../BatterylevelPlugin.kt：

这边一样要设置好EvnetChannel，指定名称（需要与Flutter 端使用的EvnetChannel名称相对应），并通过EvnetChannel呼叫setStreamHandler来注册一个MessageHandler来接收此通道讯息，这边不像预设的范例直接让BatterylevelPlugin实现setStreamHandler的接口，我们将接口的实现拉出来一个类别来实作，这样把实现接口的逻辑拉出去实作，BatterylevelPlugin才不会在功能变多时变得太复杂

所以建立一个RandomNumberStreamHandler.kt 来实现接口，这边每一秒骰一次骰子并将结果回传至Flutter 端

class RandomNumberStreamHandler : EventChannel.StreamHandler {
    private var sink: EventChannel.EventSink? = null
    private var handler: Handler? = null

    private val runnable = Runnable {
        sendNewRandomNumber()
    }

    private fun sendNewRandomNumber() {
        val randomNumber = (1..6).random()
        sink?.success(randomNumber)
        handler?.postDelayed(runnable, 1000)
    }

    override fun onListen(arguments: Any?, events: EventChannel.EventSink?) {
        sink = events
        handler = Handler()
        handler?.post(runnable)
    }

    override fun onCancel(arguments: Any?) {
        sink = null
        handler?.removeCallbacks(runnable)
    }
}

BatterylevelPlugin.kt 加上：

    private lateinit var eventChannel: EventChannel

    override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        //Added
        eventChannel = EventChannel(flutterPluginBinding.binaryMessenger, "dice_number")
        eventChannel.setStreamHandler(RandomNumberStreamHandler())
    }

    override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
        //Added
        eventChannel.setStreamHandler(null)
    }

Native 端（iOS）：

一样可以把setStreamHandler的接口实现的逻辑拉出去实作，同样建立一个RandomNumberStreamHandler.swift 来实现接口，每一秒骰一次骰子并将结果回传至Flutter 端

ios/Classes/RandomNumberStreamHandler.swift

import Foundation
import Flutter

class RandomNumberStreamHandler: NSObject, FlutterStreamHandler{
    var sink: FlutterEventSink?
    var timer: Timer?
    
    @objc func sendNewRandomNumber() {
        guard let sink = sink else { return }
        
        let randomNumber = Int.random(in: 1..<7)
        sink(randomNumber)
    }
    
    func onListen(withArguments arguments: Any?, eventSink events: @escaping FlutterEventSink) -> FlutterError? {
        sink = events
        timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(sendNewRandomNumber), userInfo: nil, repeats: true)
        return nil
    }
    
    func onCancel(withArguments arguments: Any?) -> FlutterError? {
        sink = nil
        timer?.invalidate()
        return nil
    }
}

ios/Classes/SwiftBatterylevelPlugin.swift：

import Flutter
import UIKit

public class SwiftBatterylevelPlugin: NSObject, FlutterPlugin {
    public static func register(with registrar: FlutterPluginRegistrar) {
				//Added
        let eventChannel = FlutterEventChannel(name: "dice_number", binaryMessenger: registrar.messenger())
        let randomNumberStreamHandler = RandomNumberStreamHandler()
        eventChannel.setStreamHandler(randomNumberStreamHandler)
    }
}