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本文原作者: BennuC，原文發布于: BennuCTech

在移動端中啟動 Flutter 頁面會有短暫空白，雖然官方提供了引擎預熱機制，但是需要提前將所有頁面都進行預熱，這樣開發成本較高，在研究了閑魚的 FlutterBoost 插件后，看看能不能自己實現一個簡單的快速啟動框架。

開發啟動框架 plugin

創建一個 Flutter Plugin 項目，并添加 git，然后編寫三端代碼:

Flutter 代碼首先是 Flutter 端的代碼

1. RouteManager

import'package:flutter/cupertino.dart';import 'package:flutter/material.dart';import 'package:flutter_boot/BasePage.dart';
class RouteManager{  factory RouteManager() => _getInstance();
  static RouteManager get instance => _getInstance();
  static RouteManager _instance;
  RouteManager._internal(){
  }
  static RouteManager _getInstance(){    if(_instance == null){      _instance = new RouteManager._internal();    }    return _instance;  }
  Map routes = Map();
  void registerRoute(String route, BasePage page){    routes[route] = page;  }
  RouteFactory getRouteFactory(){    return getRoute;  }
  MaterialPageRoute getRoute(RouteSettings settings){    if(routes.containsKey(settings.name)){      return MaterialPageRoute(builder: (BuildContext context) {        return routes[settings.name];      }, settings: settings);    }    else{      return MaterialPageRoute(builder: (BuildContext context) {        return PageNotFount();      });    }  }
  BasePage getPage(String name){    if(routes.containsKey(name)) {      return routes[name];    }    else{      return PageNotFount();    }  }}
class PageNotFount extends BasePage{
  @override  State createState() {    return _PageNotFount();  }
}
class _PageNotFount extends BaseState<PageNotFount>{
  @override  Widget buildImpl(BuildContext context) {    return Scaffold(      body: Center(        child: Text("page not found"),      ),    );  }}

它的作用就是管理路由，是一個單例，用一個 map 來維護路由映射。其中三個函數比較重要:

registerRoute: 注冊路由，一般在啟動時調用；
getRouteFactory: 返回 RouteFactory，將它賦值給 MaterialApp 的 onGenerateRoute 字段；
getPage: 通過 route 名稱返回頁面 widget。

這里 getRouteFactory 和 getPage 共用一個路由 map，所以不論是頁面內切換還是頁面切換都保持統一。

2.BaseApp

import'dart:convert';
import 'package:flutter/cupertino.dart';import 'package:flutter/services.dart';import 'package:flutter_boot/RouteManager.dart';
abstract class BaseApp extends StatefulWidget{
  @override  State createState() {    registerRoutes();    return _BaseApp(build);  }
  Widget build(BuildContext context, Widget page);
  void registerRoutes();
}
class _BaseApp extends State<BaseApp>{
  Function buildImpl;  static const bootChannel = const BasicMessageChannel("startPage", StringCodec());  Widget curPage = RouteManager.instance.getPage("");
  _BaseApp(this.buildImpl){    bootChannel.setMessageHandler((message) async {      setState(() {        var json = jsonDecode(message);        var route = json["route"];        var page = RouteManager.instance.getPage(route);        page.args = json["params"];        curPage = page;      });      return "";    });  }
  @override  Widget build(BuildContext context) {    return buildImpl.call(context, curPage);  }
}

是一個抽象類，真正的 Flutter app 需要繼承它。主要是封裝了一個 BasicMessageChannel 用來與 Android/iOS 交互，并根據收到的消息處理頁面內的切換，實現快速啟動。

繼承它的子類需要實現 registerRoutes 函數，在這里使用 RouteManager 的 registerRoute 將每個頁面注冊一下即可。

3.BasePage

import'package:flutter/material.dart';
abstract class BasePage extends StatefulWidget{  dynamic args;}
abstract class BaseState<T extends BasePage> extends State<T>{  dynamic args;
  @override  Widget build(BuildContext context) {    if(ModalRoute.of(context).settings.arguments == null){      args = widget.args;    }    else{      args = ModalRoute.of(context).settings.arguments;    }    return buildImpl(context);  }
  Widget buildImpl(BuildContext context);}

同樣是抽象類，每個 Flutter 頁面都需要繼承它，它主要是處理兩種啟動方式傳過來的參數，統一到 args 中，這樣子類就可以直接使用而不需要考慮是如何啟動的。

Android 代碼接下來是 plugin 中的 Android 的代碼

1.BootEngine

package com.bennu.flutter_boot
import android.app.Applicationimport io.flutter.embedding.engine.FlutterEngineimport io.flutter.embedding.engine.FlutterEngineCacheimport io.flutter.embedding.engine.dart.DartExecutorimport io.flutter.plugin.common.BasicMessageChannelimport io.flutter.plugin.common.StringCodec
object BootEngine {    public var flutterBoot : BasicMessageChannel? = null
    fun init(context: Application){        var flutterEngine = FlutterEngine(context)        flutterEngine.dartExecutor.executeDartEntrypoint(            DartExecutor.DartEntrypoint.createDefault()        )        FlutterEngineCache.getInstance().put("main", flutterEngine)
        flutterBoot = BasicMessageChannel(flutterEngine.dartExecutor.binaryMessenger, "startPage", StringCodec.INSTANCE)    }}

這個是單例，初始化并預熱 FlutterEngine，同時創建 BasicMessageChannel 用于后續交互。需要在 Application 的 onCreate 中調用它的 init 函數來初始化。 2. FlutterBootActivity

package com.bennu.flutter_boot
import android.content.ComponentNameimport android.content.Contextimport android.content.Intentimport android.os.Bundleimport android.os.PersistableBundleimport io.flutter.embedding.android.FlutterActivityimport org.json.JSONObject
class FlutterBootActivity : FlutterActivity() {
    companion object{        const val ROUTE_KEY = "flutter.route.key"
        fun build(context: Context, routeName : String, params : Map<String, String>?) : Intent{            var intent = withCachedEngine("main").build(context)            intent.component = ComponentName(context, FlutterBootActivity::class.java)            var json = JSONObject()            json.put("route", routeName)
            var paramsObj = JSONObject()            params?.let {                for(entry in it){                    paramsObj.put(entry.key, entry.value)                }            }            json.put("params", paramsObj)            intent.putExtra(ROUTE_KEY, json.toString())            return intent        }    }
    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)    }
    override fun onCreate(savedInstanceState: Bundle?, persistentState: PersistableBundle?) {        super.onCreate(savedInstanceState, persistentState)    }
    override fun onResume() {        super.onResume()        var route = intent.getStringExtra(ROUTE_KEY)        BootEngine.flutterBoot?.send(route)    }
    override fun onDestroy() {        super.onDestroy()    }}

繼承 FlutterActivity，提供一個 build (context: Context, routeName: String, params: Map?) 函數來啟動，傳遞路由名稱和參數。在 onResume 的時候通過 BasicMessageChannel 將這兩個數據 send 給 Flutter 處理。

iOS

iOS 與 Android 類似

1.FlutterBootEngine FlutterBootEngine.h

#ifndef FlutterBootEngine_h#define FlutterBootEngine_h
#import #import 
@interface FlutterBootEngine : NSObject
+ (nonnull instancetype)sharedInstance;
- (FlutterBasicMessageChannel *)channel;- (FlutterEngine *)engine;- (void)initEngine;@end
#endif /* FlutterBootEngine_h */FlutterBootEngine.m#import "FlutterBootEngine.h"#import 
@implementation FlutterBootEngine
static FlutterBootEngine * instance = nil;
FlutterEngine * engine = nil;FlutterBasicMessageChannel * channel = nil;
+(nonnull FlutterBootEngine *)sharedInstance{    if(instance == nil){        instance = [self.class new];    }    return instance;}
+(id)allocWithZone:(struct _NSZone *)zone{    if(instance == nil){        instance = [[super allocWithZone:zone]init];    }    return instance;}
- (id)copyWithZone:(NSZone *)zone{    return instance;}
- (FlutterEngine *)engine{    return engine;}
- (FlutterBasicMessageChannel *)channel{    return channel;}
- (void)initEngine{    engine = [[FlutterEngine alloc]initWithName:@"flutter engine"];    channel = [FlutterBasicMessageChannel messageChannelWithName:@"startPage" binaryMessenger:engine.binaryMessenger codec:[FlutterStringCodec sharedInstance]];    [engine run];}
@end

這也是一個單例，初始化并啟動 FlutterEngine，并創建一個 FlutterBasicMessageChannel 與 Flutter 交互。

需要在 iOS 項目的 AppDelegate 初始化時調用它的 initEngine 函數。

2. FlutterBootViewController

FlutterBootViewController.h

#ifndef FlutterBootViewController_h#define FlutterBootViewController_h
#import 
@interface FlutterBootViewController : FlutterViewController
- (nonnull instancetype)initWithRoute:(nonnull NSString*)route                       params:(nullable NSDictionary*)params;
@end
#endif /* FlutterBootViewController_h */FlutterBootViewController.m#import "FlutterBootViewController.h"#import "FlutterBootEngine.h"
@implementation FlutterBootViewController
NSString * mRoute = nil;NSDictionary * mParams = nil;
- (nonnull instancetype)initWithRoute:(nonnull NSString *)route params:(nullable NSDictionary *)params{    self = [super initWithEngine:FlutterBootEngine.sharedInstance.engine nibName:nil bundle:nil];    mRoute = route;    mParams = params;    return self;}
//viewDidAppear時機有點晚，會先顯示一下上一個頁面才更新到新頁面，所以換成viewWillAppear- (void)viewWillAppear:(BOOL)animated{    [super viewWillAppear:animated];    if(mParams == nil){        mParams = [[NSDictionary alloc]init];    }    NSDictionary * dict = @{@"route" : mRoute, @"params" : mParams};    NSData * jsonData = [NSJSONSerialization dataWithJSONObject:dict options:0 error:NULL];    NSString * str = [[NSString alloc]initWithData:jsonData encoding:NSUTF8StringEncoding];    NSLog(@"%@", str);    [FlutterBootEngine.sharedInstance.channel sendMessage:str];}
@end

同樣新增一個使用路由名和參數的構造函數，然后在 viewWillAppear 時通知 Flutter。

注意這里如果改成 viewDidAppear 時機有點晚，會先顯示一下上一個頁面才更新到新頁面，所以換成 viewWillAppear。

3. FlutterBoot.h

#ifndef FlutterBoot_h#define FlutterBoot_h
#import "FlutterBootEngine.h"#import "FlutterBootViewController.h"
#endif /* FlutterBoot_h */

這個是 swift 的橋接文件，通過它 swift 就可以使用我們上面定義的類。這樣我們的 plugin 就開發完成了，可以發布到 pub 上。我這里是 push 到 git 倉庫中，通過 git 的方式依賴使用。

開發 Flutter module

創建一個 Flutter module，然后引入我們的 plugin，在 pubspec.yaml 中:

dependencies:  flutter:    sdk: flutter  ...  flutter_boot:    git: https://gitee.com/chzphoenix/flutter-boot.git

然后我們開發兩個頁面用于測試。

1. FirstPage.dart

import 'package:flutter/material.dart';import 'package:flutter_boot/BasePage.dart';
class FirstPage extends BasePage{
  @override  State createState() {    return _FirstPage();  }}
class _FirstPage extends BaseState<FirstPage>{
  void _goClick() {    Navigator.of(context).pushNamed("second", arguments: {"key":"123"});  }
  @override  Widget buildImpl(BuildContext context) {    return Scaffold(      appBar: AppBar(        title: Text("Flutter Demo Home Page"),      ),      body: Center(        child: ...,      ),      floatingActionButton: FloatingActionButton(        onPressed: _goClick,        tooltip: 'Increment',        child: Icon(Icons.add),      ), // This trailing comma makes auto-formatting nicer for build methods.    );  }}

繼承 BasePage 和 BaseState 即可，點擊按鈕可以跳轉到頁面 2。

2. SecondPage.dart

import 'package:flutter/cupertino.dart';import 'package:flutter/material.dart';import 'package:flutter_boot/BasePage.dart';
class SecondPage extends BasePage{
  @override  State createState() {    return _SecondPage();  }
}
class _SecondPage extends BaseState<SecondPage>{
  @override  Widget buildImpl(BuildContext context) {    return Scaffold(        appBar: AppBar(          title: Text("test"),        ),        body:Text("test:${args["key"]}")    );  }}

這個頁面獲取傳遞過來的參數 key，并展示。

3. main.dart

import 'package:flutter/material.dart';import 'package:flutter_boot/BaseApp.dart';import 'package:flutter_boot/RouteManager.dart';
import 'FirstPage.dart';import 'SecondPage.dart';
void main() => runApp(MyApp());
class MyApp extends BaseApp {  @override  Widget build(BuildContext context, Widget page) {    return MaterialApp(      title: 'Flutter Demo',      theme: ThemeData(        primarySwatch: Colors.blue,      ),      home: page,      onGenerateRoute: RouteManager.instance.getRouteFactory(),    );  }
  @override  void registerRoutes() {    RouteManager.instance.registerRoute("main", FirstPage());    RouteManager.instance.registerRoute("second", SecondPage());  }}

入口繼承 BaseApp，并實現 registerRoutes，注冊這兩個頁面。

注意這里的 onGenerateRoute 使用 RouteManager.instance.getRouteFactory ()，這樣一次注冊就可以了，不必自己去實現。

引入移動端

Module 開發完后，就可以在 Android/iOS 上使用了。

Android 端

在 Android 上比較簡單，在 Android 項目中引入剛才的 module 即可，然后需要在 Android 的主 module (一般是 app) 的 build.gradle 中引入 module 和 plugin，如下:

dependencies{    implementation fileTree(dir: "libs", include: ["*.jar"])    ...    implementation project(path: ':flutter')  //module    provided rootProject.findProject(":flutter_boot") //plugin}

注意 plugin 的名稱是之前在 module 中的 pubspec.yaml 定義的。

然后就可以在 Android 中使用了，首先要初始化，如下:

importandroid.app.Applicationimport com.bennu.flutter_boot.BootEngine
public class App : Application() {
    override fun onCreate() {        super.onCreate()        BootEngine.init(this)        ...    }}

然后合適的時候啟動 Flutter 頁面即可，啟動代碼如下:

button.setOnClickListener {    startActivity(FlutterBootActivity.build(this, "main", null))}button2.setOnClickListener {    var params = HashMap()    params.put("key", "123")    startActivity(FlutterBootActivity.build(this, "second", params))}

一個啟動無參的頁面 1，一個啟動有參的頁面 2。測試可以發現無論打開哪個頁面都非常快，幾乎沒有加載時間。這樣就實現了快速啟動。

iOS 端

iOS 端稍微復雜一些，需要先了解一下 iOS 如何加入 Flutter。

我選用的是 framework 的方式引入，所以在 Flutter module 項目下通過命令編譯打包 framework。

flutterbuildios-framework--xcframework--no-universal--output=./Flutter/

然后引入到 iOS 項目中，與上一篇文章不同的是，因為這個 module 中加入了 plugin，所以 framework 產物是四個:

App.xcframework
flutter_boot.xcframework (這個就是我們的 plugin 中的 iOS 部分)
Flutter.xcframework
FlutterPluginRegistrant.xcframework

這四個都需要引入到 iOS 項目中。

然后 AppDelegate 需要繼承 FlutterAppDelegate (如果無法繼承，則需要處理每個生命周期，您可以查看: https://flutter.cn/docs/development/add-to-app/ios/add-flutter-screen?tab=engine-swift-tab)。

然后在 AppDelegate 中初始化，如下:

importUIKitimport Flutterimport flutter_boot
@UIApplicationMainclass AppDelegate: FlutterAppDelegate {
    override func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {        FlutterBootEngine.sharedInstance().initEngine()        return true    }
    override func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)    }}

然后在合適的地方啟動 Flutter 頁面即可，如下:

@objcfuncshowMain(){    let flutterViewController =        FlutterBootViewController(route: "main", params: nil)    present(flutterViewController, animated: true, completion: nil)  }
@objc func showSecond() {    let params : Dictionary<String, String> = ["key" : "123"]    let flutterViewController =        FlutterBootViewController(route: "second", params: params)    present(flutterViewController, animated: true, completion: nil)  }