Android端的Flutter应用是如何启动的

flutter create {application}

可以创一个纯Flutter应用，以Android端分析，分析其AndroidManifest.xml文件

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.flutter_template">
    <!-- io.flutter.app.FlutterApplication is an android.app.Application that
         calls FlutterMain.startInitialization(this); in its onCreate method.
         In most cases you can leave this as-is, but you if you want to provide
         additional functionality it is fine to subclass or reimplement
         FlutterApplication and put your custom class here. -->
    <application
        android:name="io.flutter.app.FlutterApplication"
        android:label="flutter_template"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <!-- Don't delete the meta-data below.
             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
        <meta-data
            android:name="flutterEmbedding"
            android:value="2" />
    </application>
</manifest>

可以看到核心就两个，一个是io.flutter.app.FlutterApplication 与MainActivity(extends FlutterActivity)，先看FlutterApplication

public class FlutterApplication extends Application {
    @Override
    @CallSuper
    public void onCreate() {
        super.onCreate();
        FlutterMain.startInitialization(this);
    }

    private Activity mCurrentActivity = null;
    public Activity getCurrentActivity() {
        return mCurrentActivity;
    }
    public void setCurrentActivity(Activity mCurrentActivity) {
        this.mCurrentActivity = mCurrentActivity;
    }
}

初始化全部都在startInitialization()@FlutterMain.java中

//FlutterMain.java
public static void startInitialization(Context applicationContext){
  FlutterLoader.getInstance().startInitialization(applicationContext);
}

//FlutterLoader.java
public void startInitialization(Context ApplicationContext){
  startInitialization(applicationContext, newSettings()); 
}

public void startInitialization(Context applicationContext, Settings settings){
  initConfig(applicationContext);
  initResources(applicationContext);
  //加载libflutter.so，对应到./shell/platform/android/library_loader.cc的JNI_OnLoad()函数,主要是进行Java函数与Native函数的JNI映射
  System.loadLibrary("flutter");  
VsyncWaiter.getInstance((WindowManager)applicationContext.getSystemService(Context.WINDOW_SERVICE)).init();
}

最主要的初始化就是上面initConfig(),initResource(),Vsync相关。
其中Vsync在Flutter与Vsync信号中已经提及，现在来看其他两个。

private void initConfig(Context applicationContext){
  //即在AndroidManifest.xml中配置的meta-data集合
  Bundle metadata = getApplicationInfo(applicationContext).metaData;
  //默认值为"libapp.so"
  String aotSharedLibraryName = meta.getString(PUBLIC_AOT_SHARED_LIBRARY_NAME, DEFAULT_AOT_SHARED_LIBRARY_NAME);
  //默认值为"flutter_assets"
  flutterAssetsDir = metadata.getString(PUBLIC_FLUTTER_ASSETS_DIR_KEY, DEFAULT_FLUTTER_ASSETS_DIR);
  //默认值为"vm_snapshot_data"
  vmSnapshotData = metadata.getString(PUBLIC_VM_SNAPSHOT_DATA_KEY, DEFAULT_VM_SNAPSHOT_DATA);
  //默认值为"isolate_snapshot_data"
  isolateSnapshotData = metadata.getString(PUBLIC_ISOLATE_SNAPSHOT_DATA_KEY, DEFAULT_ISOLATE_SNAPSHOT_DATA);
}

因此，initConfig()只是对一些参数进行了初始化

private void initResource(Context applicationContext){
  //缓存清理工作
  new ResourceCleaner(applicationContext).start();
  //Context.getDir("flutter", Cotext.MODE_PRIVATE).getPath();
  final String dataDirPath = PathUtils.getDataDirectory(applicationContext);
  final String packageName = applicationContext.getPackagetName();
  final PackageManager packageManager = applicationContext.getPackagetManager();
  final AssetManager assetManager = applicationContext.getResources().getAssets();
  resourceExtractor = new ResourceExtractor(dataDirPath, packageName, packagetManager, assetManager);
  resourceExtractor.start 
        .addResource(fullAssetPathFrom(vmSnapshotData))//vm_snapshot_data
        .addResource(fullAssetPathFrom(isolateSnapshotData))//isolate_snapshot_data
        .addResource(fullAssetPathFrom(DEFALUT_KERNEL_BLOB));//kernel_blob.bin
  resourceExtractor.start();
}

initResource()是将apk中assets中的文件进行提取加载进入内存。

下面看FlutterActivity
//todo