Flutter外掛開發---iOS篇

chonglingliu發表於2021-05-10

Flutter的願景是一般的開發者只需要開發Flutter程式碼就能實現跨平臺的應用，官方提供了一些外掛，也有很多可以可以直接拿來使用的第三方外掛。

但是現實是現實，例如當遇到定製化的功能時，編寫外掛是不可避免的。譬如我們有一個自定義協議的藍芽功能，這個功能在Flutter中就不可能直接拿來使用了，需要編寫外掛讓Flutter進行呼叫。本文我們將來看看Flutter外掛是如何實現的。

前言

本文我們用Flutter來仿寫網易雲音樂的播放頁面的功能，其中音樂的播放，音樂的暫停，快進，音樂的時長獲取，音樂播放的進度等功能我們需要用原生程式碼編寫外掛來實現。

效果如圖所示

提示：本文用音樂播放器的外掛只是為了提供一個編寫Flutter外掛的思路和方法，當需要自己編寫外掛的時候可以方便的來實現。播放音視訊的Flutter外掛已經有一些優秀的三方庫已經實現了。

說明：

由於是音訊播放，我製作GIF的時候沒法體現音樂元素，所以音樂只能我自己獨自欣賞了，哈哈~~
本文先只介紹iOS的外掛製作，下篇文章我們再來介紹Android的外掛製作。

架構概覽

架構圖

我們從上面的官方架構圖可以看出，Flutter和Native程式碼是通過MethodChannel進行通訊的。

Flutter端向iOS端傳送訊息

Flutter端的程式碼

建立一個播放器類AudioPlayer, 然後定義為單例模式

class AudioPlayer {
// 單例
  factory AudioPlayer() => _getInstance();
  static AudioPlayer get instance => _getInstance();
  static AudioPlayer _instance;
  AudioPlayer._internal() {}

  static AudioPlayer _getInstance() {
    if (_instance == null) {
      _instance = new AudioPlayer._internal();
    }
    return _instance;
  }
}
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建立播放器的MethodChannel

class AudioPlayer {
    static final channel = const MethodChannel("netmusic.com/audio_player");
}
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MethodChannel名字要有意義，其組成遵循"域名"+"/"+"功能"，隨意寫就顯得不夠專業。

通過MethodChannel的invokeMethod實現播放音樂

/// 播放
  Future<int> play() async {
    final result = await channel.invokeMethod("play", {'url': audioUrl});
    return result ?? 0;
  }
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play就是方法名， {'url': audioUrl}就是引數

invokeMethod是非同步的，所以返回值需要用Future包裹。

通過MethodChannel的invokeMethod實現暫停音樂

/// 暫停
Future<int> pause() async {
    final result = await channel.invokeMethod("pause", {'url': audioUrl});
    return result ?? 0;
}
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通過MethodChannel的invokeMethod實現繼續播放音樂

/// 繼續播放
Future<int> resume() async {
    final result = await channel.invokeMethod("resume", {'url': audioUrl});
    return result ?? 0;
}
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通過MethodChannel的invokeMethod實現拖動播放位置

/// 拖動播放位置
Future<int> seek(int time) async {
    final result = await channel.invokeMethod("seek", {
      'position': time,
    });
    return result ?? 0;
}
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iOS端的程式碼

前提：需要用Xcode開啟iOS專案，這是開始編寫的基礎。

建立一個播放器類PlayerWrapper

class PlayerWrapper: NSObject {
    
    var vc: FlutterViewController
    var channel: FlutterMethodChannel
    var player: AVPlayer?
    
}
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在AppDelegate中初始化PlayerWrapper,並將FlutterViewController作為初始化引數。

@objc class AppDelegate: FlutterAppDelegate {
    
    // 持有播放器
    var playerWrapper: PlayerWrapper?
    
    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        
        // 初始化播放器
        let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
        playerWrapper = PlayerWrapper(vc: controller)
        
        GeneratedPluginRegistrant.register(with: self)
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }
}

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FlutterAppDelegate的根檢視就是一個FlutterViewController，這個我們在以前的文章中有介紹；

FlutterViewController中有一個FlutterBinaryMessenger，建立FlutterMethodChannel時需要，所以將其傳入PlayerWrapper。

建立播放器的FlutterMethodChannel

class PlayerWrapper: NSObject {
    init(vc: FlutterViewController) {
        self.vc = vc
        channel = FlutterMethodChannel(name: "netmusic.com/audio_player", binaryMessenger: vc.binaryMessenger)
        super.init()
    }
}
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name的值必須和Flutter中的對應，否則是沒法通訊的；

binaryMessenger就使用FlutterViewController的FlutterBinaryMessenger，前面提到過。

接收Flutter端的呼叫，然後回撥Flutter端播放進度和結果等。

由於是被動接收，所以可以想象的實現是註冊一個回撥函式，接收Flutter端的呼叫方法和引數。

init(vc: FlutterViewController) {
    //...
    channel.setMethodCallHandler(handleFlutterMessage);
}

// 從Flutter傳過來的方法
public func handleFlutterMessage(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    // 1. 獲取方法名和引數
    let method = call.method
    let args = call.arguments as? [String: Any]
        
    if method == "play" {
        // 2.1 確保有url引數
        guard let url = args?["url"] as! String? else {
            result(0)
            return
        }
        player?.pause()
        // 2.2 確保有url引數正確
        guard let audioURL = URL.init(string: url) else {
            result(0)
            return
        }
        // 2.3 根據url初始化播放內容，然後開始進行播放
        let asset = AVAsset.init(url: audioURL)
        let item = AVPlayerItem.init(asset: asset);
        player = AVPlayer(playerItem: item);
        player?.play();
        
        // 2.4 定時檢測播放進度    
        player?.addPeriodicTimeObserver(forInterval: CMTimeMake(value: 1, timescale: 1), queue: nil, using: { [weak self] (time) in
            // *********回撥Flutter當前播放進度*********
            self?.channel.invokeMethod("onPosition", arguments: ["value": time.value / Int64(time.timescale)])
            })
            
        keyVakueObservation?.invalidate()
        // 2.5 監測播放狀態
        keyVakueObservation = item.observe(\AVPlayerItem.status) { [weak self] (playerItem, change) in
            let status = playerItem.status
            if status == .readyToPlay {
                // *********回撥Flutter當前播放內容的總長度*********
                if let time = self?.player?.currentItem?.asset.duration {
                    self?.channel.invokeMethod("onDuration", arguments: ["value": time.value / Int64(time.timescale)])
            }
        } else if status == .failed {
            // *********回撥Flutter當前播放出現錯誤*********
            self?.channel.invokeMethod("onError", arguments: ["value": "pley failed"])
            }
        }
        
        // 2.6 監測播放完成
        notificationObservation = NotificationCenter.default.addObserver(
            forName: NSNotification.Name.AVPlayerItemDidPlayToEndTime,
            object: item,**
            queue: nil
        ) {
            [weak self] (notification) in
            self?.channel.invokeMethod("onComplete", arguments: [])
        }**
            
        result(1)
            
    } else if method == "pause" || method == "stop" {
        // 3 暫停
        player?.pause()
        result(1)
    } else if method == "resume" {
        // 4 繼續播放
        player?.play()
        result(1)
    } else if method == "seek" {
        
        guard let position = args?["position"] as! Int? else {
            result(0)
            return
        }
        // 4 拖動到某處進行播放
        let seekTime: CMTime = CMTimeMake(value: Int64(position), timescale: 1)
        player?.seek(to: seekTime);
    }
}
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handleFlutterMessage這個回撥函式有兩個引數：FlutterMethodCall接收Flutter傳過來的方法名method和引數arguments, FlutterResult可以返回撥用的結果，例如result(1)就給Flutter返回了1這個結果。

獲取到FlutterMethodCall的方法名和引數後就可以進行處理了，我們以play為例：

根據url初始化播放內容，然後開始進行播放；

通過player.addPeriodicTimeObserver方法檢測播放進度，然後通過FlutterMethodChannel的invokeMethod方法傳遞當前的進度給Flutter端，方法名是onPosition,引數是當前進度；

後面還有一列邏輯：例如監聽播放狀態，監聽播放完成等。

目前為止，iOS端的程式碼完成了。接下來就是Flutter端接收iOS端的方法和引數了。

Flutter端接收iOS端傳送的訊息

iOS端向Flutter端傳送了onPosition(當前播放進度),onComplete(播放完成),onDuration(當前歌曲的總長度)和onError(播放出現錯誤)等幾個方法呼叫。

Flutter端註冊回撥

AudioPlayer._internal() {
    channel.setMethodCallHandler(nativePlatformCallHandler);
}

/// Native主動呼叫的方法
Future<void> nativePlatformCallHandler(MethodCall call) async {
    try {
      // 獲取引數
      final callArgs = call.arguments as Map<dynamic, dynamic>;
      print('nativePlatformCallHandler call ${call.method} $callArgs');
      switch (call.method) {
        case 'onPosition':
          final time = callArgs['value'] as int;
          _currentPlayTime = time;
          _currentPlayTimeController.add(_currentPlayTime);
          break;
        case 'onComplete':
          this.updatePlayerState(PlayerState.COMPLETED);
          break;
        case 'onDuration':
          final time = callArgs['value'] as int;
          _totalPlayTime = time;
          _totalPlayTimeController.add(totalPlayTime);
          break;
        case 'onError':
          final error = callArgs['value'] as String;
          this.updatePlayerState(PlayerState.STOPPED);
          _errorController.add(error);
          break;
      }
    } catch (ex) {
      print('Unexpected error: $ex');
    }
}
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註冊回撥也是使用setMethodCallHandler方法,MethodCall對應的也包含方法名和引數；

獲取到對應的資料後Flutter就可進行資料的展示了。

Flutter端對資料的更新

我們以onDuration(當前歌曲的總長度)為例進行介紹。

class AudioPlayer {
    
    // 1. 記錄下總時間
    int _totalPlayTime = 0;
    int get totalPlayTime => _totalPlayTime;
    
    // 2. 代表歌曲時長的流
    final StreamController<int> _totalPlayTimeController =
      StreamController<int>.broadcast();
    Stream<int> get onTotalTimeChanged => _totalPlayTimeController.stream;
    
    Future<void> nativePlatformCallHandler(MethodCall call) async {
        try {
          final callArgs = call.arguments as Map<dynamic, dynamic>;
          print('nativePlatformCallHandler call ${call.method} $callArgs');
          switch (call.method) {
            // 3. 記錄下總時間和推送更新
            case 'onDuration':
              final time = callArgs['value'] as int;
              _totalPlayTime = time;
              _totalPlayTimeController.add(totalPlayTime);
              break;
          }
        } catch (ex) {
          print('Unexpected error: $ex');
        }
    }
}
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_totalPlayTime記錄下總播放時長；

_totalPlayTimeController是總播放時長的流，當呼叫add方法時，onTotalTimeChanged的監聽者就能收到新的值；

StreamBuilder監聽流的資料

StreamBuilder(
    initialData: "00:00",
    stream: AudioPlayer().onTotalTimeChanged,
    builder: (context, snapshot) {
        if (!snapshot.hasData)
            return Text(
                "00:00",
                style: TextStyle(color: Colors.white70),
            );
        return Text(
            AudioPlayer().totalPlayTimeStr,
            style: TextStyle(color: Colors.white70),
            );
        },
),
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監聽AudioPlayer().onTotalTimeChanged的資料變化，然後最新的值展示在Text上。

程式碼

audio_player.dart

import 'dart:async';

import 'package:flutter/services.dart';
import 'package:netmusic_flutter/music_item.dart';

class AudioPlayer {
  // 定義一個MethodChannel
  static final channel = const MethodChannel("netmusic.com/audio_player");

  // 單例
  factory AudioPlayer() => _getInstance();
  static AudioPlayer get instance => _getInstance();
  static AudioPlayer _instance;
  AudioPlayer._internal() {
    // 初始化
    channel.setMethodCallHandler(nativePlatformCallHandler);
  }

  static AudioPlayer _getInstance() {
    if (_instance == null) {
      _instance = new AudioPlayer._internal();
    }
    return _instance;
  }

  // 播放狀態
  PlayerState _playerState = PlayerState.STOPPED;
  PlayerState get playerState => _playerState;

  // 時間
  int _totalPlayTime = 0;
  int _currentPlayTime = 0;
  int get totalPlayTime => _totalPlayTime;
  int get currentPlayTime => _currentPlayTime;
  String get totalPlayTimeStr => formatTime(_totalPlayTime);
  String get currentPlayTimeStr => formatTime(_currentPlayTime);

  // 歌曲
  MusicItem _item;
  set item(MusicItem item) {
    _item = item;
  }

  String get audioUrl {
    return _item != null
        ? "https://music.163.com/song/media/outer/url?id=${_item.id}.mp3"
        : "";
  }

  Future<int> togglePlay() async {
    if (_playerState == PlayerState.PLAYING) {
      return pause();
    } else {
      return play();
    }
  }

  /// 播放
  Future<int> play() async {
    if (_item == null) return 0;
    // 如果是停止狀態
    if (_playerState == PlayerState.STOPPED ||
        _playerState == PlayerState.COMPLETED) {
      // 更新狀態
      this.updatePlayerState(PlayerState.PLAYING);
      final result = await channel.invokeMethod("play", {'url': audioUrl});
      return result ?? 0;
    } else if (_playerState == PlayerState.PAUSED) {
      return resume();
    }
    return 0;
  }

  /// 繼續播放
  Future<int> resume() async {
    // 更新狀態
    this.updatePlayerState(PlayerState.PLAYING);
    final result = await channel.invokeMethod("resume", {'url': audioUrl});
    return result ?? 0;
  }

  /// 暫停
  Future<int> pause() async {
    // 更新狀態
    this.updatePlayerState(PlayerState.PAUSED);
    final result = await channel.invokeMethod("pause", {'url': audioUrl});
    return result ?? 0;
  }

  /// 停止
  Future<int> stop() async {
    // 更新狀態
    this.updatePlayerState(PlayerState.STOPPED);
    final result = await channel.invokeMethod("stop");
    return result ?? 0;
  }

  /// 播放
  Future<int> seek(int time) async {
    // 更新狀態
    this.updatePlayerState(PlayerState.PLAYING);
    final result = await channel.invokeMethod("seek", {
      'position': time,
    });
    return result ?? 0;
  }

  /// Native主動呼叫的方法
  Future<void> nativePlatformCallHandler(MethodCall call) async {
    try {
      // 獲取引數
      final callArgs = call.arguments as Map<dynamic, dynamic>;
      print('nativePlatformCallHandler call ${call.method} $callArgs');
      switch (call.method) {
        case 'onPosition':
          final time = callArgs['value'] as int;
          _currentPlayTime = time;
          _currentPlayTimeController.add(_currentPlayTime);
          break;
        case 'onComplete':
          this.updatePlayerState(PlayerState.COMPLETED);
          break;
        case 'onDuration':
          final time = callArgs['value'] as int;
          _totalPlayTime = time;
          _totalPlayTimeController.add(totalPlayTime);
          break;
        case 'onError':
          final error = callArgs['value'] as String;
          this.updatePlayerState(PlayerState.STOPPED);
          _errorController.add(error);
          break;
      }
    } catch (ex) {
      print('Unexpected error: $ex');
    }
  }

  // 播放狀態
  final StreamController<PlayerState> _stateController =
      StreamController<PlayerState>.broadcast();
  Stream<PlayerState> get onPlayerStateChanged => _stateController.stream;

  // Video的時長和當前位置時間變化
  final StreamController<int> _totalPlayTimeController =
      StreamController<int>.broadcast();
  Stream<int> get onTotalTimeChanged => _totalPlayTimeController.stream;

  final StreamController<int> _currentPlayTimeController =
      StreamController<int>.broadcast();
  Stream<int> get onCurrentTimeChanged => _currentPlayTimeController.stream;

  // 發生錯誤
  final StreamController<String> _errorController = StreamController<String>();
  Stream<String> get onError => _errorController.stream;

  // 更新播放狀態
  void updatePlayerState(PlayerState state, {bool stream = true}) {
    _playerState = state;
    if (stream) {
      _stateController.add(state);
    }
  }

  // 這裡需要關閉流
  void dispose() {
    _stateController.close();
    _currentPlayTimeController.close();
    _totalPlayTimeController.close();
    _errorController.close();
  }

  // 格式化時間
  String formatTime(int time) {
    int min = (time ~/ 60);
    int sec = time % 60;
    String minStr = min < 10 ? "0$min" : "$min";
    String secStr = sec < 10 ? "0$sec" : "$sec";
    return "$minStr:$secStr";
  }
}

/// 播放狀態
enum PlayerState {
  STOPPED,
  PLAYING,
  PAUSED,
  COMPLETED,
}

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AppDelegate.swift

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
    
    var playerWrapper: PlayerWrapper?
    
    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        
        // 播放器
        let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
        playerWrapper = PlayerWrapper(vc: controller)
        
        GeneratedPluginRegistrant.register(with: self)
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }
}
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PlayerWrapper.swift

import Foundation
import Flutter
import AVKit
import CoreMedia

class PlayerWrapper: NSObject {
    
    var vc: FlutterViewController
    var channel: FlutterMethodChannel
    var player: AVPlayer?
    var keyVakueObservation: NSKeyValueObservation?
    var notificationObservation: NSObjectProtocol?
    
    init(vc: FlutterViewController) {
        self.vc = vc
        channel = FlutterMethodChannel(name: "netmusic.com/audio_player", binaryMessenger: vc.binaryMessenger)
        super.init()
        channel.setMethodCallHandler(handleFlutterMessage);
    }
    
    // 從Flutter傳過來的方法
    public func handleFlutterMessage(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
        let method = call.method
        let args = call.arguments as? [String: Any]
        
        if method == "play" {
            guard let url = args?["url"] as! String? else {
                NSLog("無播放地址")
                result(0)
                return
            }
            player?.pause()
            guard let audioURL = URL.init(string: url) else {
                NSLog("播放地址錯誤")
                result(0)
                return
            }
            let asset = AVAsset.init(url: audioURL)
            let item = AVPlayerItem.init(asset: asset);
            player = AVPlayer(playerItem: item);
            player?.play();
            
            player?.addPeriodicTimeObserver(forInterval: CMTimeMake(value: 1, timescale: 1), queue: nil, using: { [weak self] (time) in
                self?.channel.invokeMethod("onPosition", arguments: ["value": time.value / Int64(time.timescale)])
            })
            
            keyVakueObservation?.invalidate()
            keyVakueObservation = item.observe(\AVPlayerItem.status) { [weak self] (playerItem, change) in
                let status = playerItem.status
                if status == .readyToPlay {
                    if let time = self?.player?.currentItem?.asset.duration {
                        self?.channel.invokeMethod("onDuration", arguments: ["value": time.value / Int64(time.timescale)])
                    }
                } else if status == .failed {
                    self?.channel.invokeMethod("onError", arguments: ["value": "pley failed"])
                }
            }
            
            notificationObservation = NotificationCenter.default.addObserver(
                forName: NSNotification.Name.AVPlayerItemDidPlayToEndTime,
                object: item,
                queue: nil
            ) {
                [weak self] (notification) in
                self?.channel.invokeMethod("onComplete", arguments: [])
            }
            
            result(1)
            
        } else if method == "pause" || method == "stop" {
            player?.pause()
            result(1)
        } else if method == "resume" {
            player?.play()
            result(1)
        } else if method == "seek" {
            guard let position = args?["position"] as! Int? else {
                NSLog("無播放時間")
                result(0)
                return
            }
            let seekTime: CMTime = CMTimeMake(value: Int64(position), timescale: 1)
            player?.seek(to: seekTime);
        }
    }
}
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有沒有感覺編寫外掛其實也很簡單，附上所有Flutter程式碼，下篇介紹Android的外掛編寫。

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