ps：flutter版本已經(jīng)更新到1.2.1

引入包

在build.gradle中可以找到這個(gè)

apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

然后我們到該目錄下找到flutter.gradle

flutter.gradle

在該文件中可以找到

Path baseEnginePath = Paths.get(flutterRoot.absolutePath, "bin", "cache", "artifacts", "engine")
String targetArch = 'arm'
if (project.hasProperty('target-platform') &&
     project.property('target-platform') == 'android-arm64') {
     targetArch = 'arm64'
}
debugFlutterJar = baseEnginePath.resolve("android-${targetArch}").resolve("flutter.jar").toFile()
profileFlutterJar = baseEnginePath.resolve("android-${targetArch}-profile").resolve("flutter.jar").toFile()
releaseFlutterJar = baseEnginePath.resolve("android-${targetArch}-release").resolve("flutter.jar").toFile()
dynamicProfileFlutterJar = baseEnginePath.resolve("android-${targetArch}-dynamic-profile").resolve("flutter.jar").toFile()
dynamicReleaseFlutterJar = baseEnginePath.resolve("android-${targetArch}-dynamic-release").resolve("flutter.jar").toFile()

我們繼續(xù)查找目錄

flutter.jar

這就是flutter.jar android部分編譯生成的包

在AS中直接看源碼的話只能找到編譯好的flutter.jar包中的.class文件穴豫，這里可以去github下載源碼
flutter內(nèi)核源碼：
https://github.com/flutter/engine/tree/45f69ac471b47e95dfeef36e5e81597b05ed19f5/shell/platform/android

OnCreate流程

flutter初始化分為4個(gè)部分：
FlutterMain、FlutterNativeView雏逾、FlutterView和Flutter Bundle的初始化
先看下Flutter初始化的時(shí)序圖（圖片來(lái)源于阿里咸魚(yú)，自己就不獻(xiàn)丑了）

時(shí)序圖

分析過(guò)android啟動(dòng)流程的話茬底，知道Application先于Activity執(zhí)行，在AndroidManifest.xml中可以找到FlutterApplication

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

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

先看FlutterMain的初始化：

public class FlutterMain {
    ...
    public static void startInitialization(Context applicationContext, Settings settings) {
        if (Looper.myLooper() != Looper.getMainLooper()) {
          throw new IllegalStateException("startInitialization must be called on the main thread");
        }
        // Do not run startInitialization more than once.
        if (sSettings != null) {
          return;
        }

        sSettings = settings;
        //記錄初始化資源的起始時(shí)間
        long initStartTimestampMillis = SystemClock.uptimeMillis();
        initConfig(applicationContext);
        initAot(applicationContext);
        initResources(applicationContext);
        //加載libflutter.so庫(kù)
        if (sResourceUpdater == null) {
            System.loadLibrary("flutter");
        } else {
            sResourceExtractor.waitForCompletion();
            File lib = new File(PathUtils.getDataDirectory(applicationContext), DEFAULT_LIBRARY);
            if (lib.exists()) {
                System.load(lib.getAbsolutePath());
            } else {
                System.loadLibrary("flutter");
            }
        }
        //初始化完成時(shí)間
        long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;
        nativeRecordStartTimestamp(initTimeMillis);
    }
}

接下來(lái)看一下MainActivity卷中，繼承自FlutterActivity，找到其的onCreate方法

public class FlutterActivity extends Activity implements FlutterView.Provider, PluginRegistry, ViewFactory {
    private final FlutterActivityDelegate delegate = new FlutterActivityDelegate(this, this);
    private final FlutterActivityEvents eventDelegate = delegate;
    ...
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        eventDelegate.onCreate(savedInstanceState);
    }
}

實(shí)際上調(diào)用的是FlutterActivityDelegate的onCreate方法

public final class FlutterActivityDelegate
        implements FlutterActivityEvents,
                   FlutterView.Provider,
                   PluginRegistry {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        //sdk版本大于21篙耗，即5.0，和沉浸式狀態(tài)欄有關(guān)
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            Window window = activity.getWindow();
            window.addFlags(LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS);
            window.setStatusBarColor(0x40000000);
            window.getDecorView().setSystemUiVisibility(PlatformPlugin.DEFAULT_SYSTEM_UI);
        }

        String[] args = getArgsFromIntent(activity.getIntent());
// 1.        
 FlutterMain.ensureInitializationComplete(activity.getApplicationContext(), args);

        flutterView = viewFactory.createFlutterView(activity);
        //flutterView默認(rèn)的創(chuàng)建結(jié)果是null欲主，必然進(jìn)入
        if (flutterView == null) {
            FlutterNativeView nativeView = viewFactory.createFlutterNativeView();
            // 2. 
            flutterView = new FlutterView(activity, null, nativeView);
            //界面置為全屏
            flutterView.setLayoutParams(matchParent);
            //flutterView為顯示的主界面
            activity.setContentView(flutterView);
            launchView = createLaunchView();
            if (launchView != null) {
                addLaunchView();
            }
        }

        if (loadIntent(activity.getIntent())) {
            return;
        }

        String appBundlePath = FlutterMain.findAppBundlePath(activity.getApplicationContext());
        if (appBundlePath != null) {
            //3. 
            runBundle(appBundlePath);
        }
    }
}

assets資源已經(jīng)初始化完畢，將資源路徑傳遞給native端進(jìn)行處理

public class FlutterMain {
    public static void ensureInitializationComplete(Context applicationContext, String[] args) {
       ...
        String appBundlePath = findAppBundlePath(applicationContext);
        String appStoragePath = PathUtils.getFilesDir(applicationContext);
        String engineCachesPath = PathUtils.getCacheDirectory(applicationContext);
        nativeInit(applicationContext, shellArgs.toArray(new String[0]),
                appBundlePath, appStoragePath, engineCachesPath);
        sInitialized = true;
        ...       
    }
}

然后看下FlutterView是個(gè)什么

public class FlutterView extends SurfaceView implements BinaryMessenger, TextureRegistry {
    ...
    public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {
        super(context, attrs);

        Activity activity = (Activity) getContext();
        if (nativeView == null) {
            mNativeView = new FlutterNativeView(activity.getApplicationContext());
        } else {
            mNativeView = nativeView;
        }

        dartExecutor = mNativeView.getDartExecutor();
        flutterRenderer = new FlutterRenderer(mNativeView.getFlutterJNI());
        mIsSoftwareRenderingEnabled = FlutterJNI.nativeGetIsSoftwareRenderingEnabled();
        mMetrics = new ViewportMetrics();
        mMetrics.devicePixelRatio = context.getResources().getDisplayMetrics().density;
        setFocusable(true);
        setFocusableInTouchMode(true);

        mNativeView.attachViewAndActivity(this, activity);

        mSurfaceCallback = new SurfaceHolder.Callback() {
            @Override
            public void surfaceCreated(SurfaceHolder holder) {
                assertAttached();
                mNativeView.getFlutterJNI().onSurfaceCreated(holder.getSurface());
            }

            @Override
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
                assertAttached();
                mNativeView.getFlutterJNI().onSurfaceChanged(width, height);
            }

            @Override
            public void surfaceDestroyed(SurfaceHolder holder) {
                assertAttached();
                mNativeView.getFlutterJNI().onSurfaceDestroyed();
            }
        };
        getHolder().addCallback(mSurfaceCallback);

        mActivityLifecycleListeners = new ArrayList<>();
        mFirstFrameListeners = new ArrayList<>();

        // Create all platform channels
        navigationChannel = new NavigationChannel(dartExecutor);
        keyEventChannel = new KeyEventChannel(dartExecutor);
        lifecycleChannel = new LifecycleChannel(dartExecutor);
        localizationChannel = new LocalizationChannel(dartExecutor);
        platformChannel = new PlatformChannel(dartExecutor);
        systemChannel = new SystemChannel(dartExecutor);
        settingsChannel = new SettingsChannel(dartExecutor);

        // Create and setup plugins
        PlatformPlugin platformPlugin = new PlatformPlugin(activity, platformChannel);
        addActivityLifecycleListener(platformPlugin);
        mImm = (InputMethodManager) getContext().getSystemService(Context.INPUT_METHOD_SERVICE);
        mTextInputPlugin = new TextInputPlugin(this, dartExecutor);
        androidKeyProcessor = new AndroidKeyProcessor(keyEventChannel, mTextInputPlugin);
        androidTouchProcessor = new AndroidTouchProcessor(flutterRenderer);

        sendLocalesToDart(getResources().getConfiguration());
        sendUserPlatformSettingsToDart();
    }
    ...
}

FlutterView是一個(gè)SurfaceView贿讹，那么再看其surfaceCreated方法，調(diào)用的是FlutterJNI中的native方法

  @UiThread
  public void onSurfaceCreated(@NonNull Surface surface) {
    ensureAttachedToNative();
    nativeSurfaceCreated(nativePlatformViewId, surface);
  }

  private native void nativeSurfaceCreated(long nativePlatformViewId, Surface surface);

也即是說(shuō)flutter其實(shí)是自己繪制面殖，然后在surfaceview上顯示的，稍微深入一下來(lái)驗(yàn)證一下：
來(lái)找一下這個(gè)native方法，看一下里面到底做了什么，來(lái)到platform_view_android_jni.cc文件下（路徑：engine/shell/platform/android/io/platform_view_android_jni.cc）

//platform_view_android_jni.cc
static const JNINativeMethod flutter_jni_methods[] = {
  ...
  {
          .name = "nativeSurfaceCreated",
          .signature = "(JLandroid/view/Surface;)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SurfaceCreated),
   },
  ...
}

這是jni中動(dòng)態(tài)注冊(cè)的方式，nativeSurfaceCreated對(duì)應(yīng)了SurfaceCreated方法

//platform_view_android_jni.cc
static void SurfaceCreated(JNIEnv* env,
                           jobject jcaller,
                           jlong shell_holder,
                           jobject jsurface) {
  fml::jni::ScopedJavaLocalFrame scoped_local_reference_frame(env);
  //創(chuàng)建新的窗口用于顯示
  auto window = fml::MakeRefCounted<AndroidNativeWindow>(
      ANativeWindow_fromSurface(env, jsurface));
  ANDROID_SHELL_HOLDER->GetPlatformView()->NotifyCreated(std::move(window));
}

通過(guò)ANativeWindow與surfaceView進(jìn)行綁定荣病，再看NotifyCreated方法

//platform_view_android.cc
void PlatformViewAndroid::NotifyCreated(
    fml::RefPtr<AndroidNativeWindow> native_window) {
  if (android_surface_) {
    InstallFirstFrameCallback();
    android_surface_->SetNativeWindow(native_window);
  }
  PlatformView::NotifyCreated();
}

SetNativeWindow(native_window)與我們的window相關(guān)脖岛，繼續(xù)追蹤，因android_surface_是一個(gè)std::unique_ptr<AndroidSurface>（ptr是動(dòng)態(tài)指針，即AndroidSurface類型）

//android_surface.h
class AndroidSurface {
 public:
  static std::unique_ptr<AndroidSurface> Create(bool use_software_rendering);
  ...
  virtual bool SetNativeWindow(fml::RefPtr<AndroidNativeWindow> window) = 0;
};
} 

我們找其方法實(shí)現(xiàn)的類

//android_surface_gi.cc
bool AndroidSurfaceGL::SetNativeWindow(
    fml::RefPtr<AndroidNativeWindow> window) {
  onscreen_context_ = nullptr;
  if (!offscreen_context_ || !offscreen_context_->IsValid()) {
    return false;
  }

  // Create the onscreen context.
  onscreen_context_ = fml::MakeRefCounted<AndroidContextGL>(
      offscreen_context_->Environment(),
      offscreen_context_.get() );

  if (!onscreen_context_->IsValid()) {
    onscreen_context_ = nullptr;
    return false;
  }

  if (!onscreen_context_->CreateWindowSurface(std::move(window))) {
    onscreen_context_ = nullptr;
    return false;
  }
  return true;
}

一切相關(guān)的信息都在CreateWindowSurface中

//android_context_gi.cc
bool AndroidContextGL::CreateWindowSurface(
    fml::RefPtr<AndroidNativeWindow> window) {
  window_ = std::move(window);
  EGLDisplay display = environment_->Display();
  const EGLint attribs[] = {EGL_NONE};
  //創(chuàng)建一個(gè)通用的surface
  surface_ = eglCreateWindowSurface(
      display, config_,
      reinterpret_cast<EGLNativeWindowType>(window_->handle()), attribs);
  return surface_ != EGL_NO_SURFACE;
}

到這一步基本追蹤完了霸奕，通過(guò)egl創(chuàng)建了一個(gè)surface_（EGLSurface）供c端使用

繪制

還記得之前在自定義控件是我們繪制圓形時(shí)使用的方法嗎适揉？

canvas.drawCircle(Offset(0, 0), 100, _paint);

深入源碼一下

void drawCircle(Offset c, double radius, Paint paint) {
    assert(_offsetIsValid(c));
    assert(paint != null);
    _drawCircle(c.dx, c.dy, radius, paint._objects, paint._data);
  }
  void _drawCircle(double x,
                   double y,
                   double radius,
                   List<dynamic> paintObjects,
                   ByteData paintData) native 'Canvas_drawCircle';

這里也使用了native方法，即dart調(diào)用c語(yǔ)言汤善，我們也來(lái)找一下它的源碼簡(jiǎn)單分析一下
Canvas_drawCircle降铸，和jni的命名不同桶蝎，這個(gè)是表示在Canvas類中的drawCircle方法
其實(shí)dart.ui包就在engine中，painting.dart文件也在其中
在canvas.cc（路徑：engine/lib/ui/painting/canvas.cc）文件中找到drawCircle方法

//canvas.cc
void Canvas::drawCircle(double x,
                        double y,
                        double radius,
                        const Paint& paint,
                        const PaintData& paint_data) {
  if (!canvas_)
    return;
  canvas_->drawCircle(x, y, radius, *paint.paint());
}

canvas_是一個(gè)SkCanvas*

//canvas.h
#include "third_party/skia/include/core/SkCanvas.h"
#include "third_party/skia/include/utils/SkShadowUtils.h"
class Canvas : public RefCountedDartWrappable<Canvas> {
 ...
 private:
  explicit Canvas(SkCanvas* canvas);
  SkPictureRecorder::beginRecording,
  SkCanvas* canvas_;
};

SkCanvas.h在engine工程中無(wú)法找到仇味，不過(guò)我們能在github的skia工程中找到

void SkCanvas::drawCircle(SkScalar cx, SkScalar cy, SkScalar radius,
                          const SkPaint& paint) {
    if (radius < 0) {
        radius = 0;
    }

    SkRect  r;
    r.set(cx - radius, cy - radius, cx + radius, cy + radius);
    this->drawOval(r, paint);
}

到這就不再分析下去了贩挣，flutter是通過(guò)skia引擎直接渲染的，所以在繪制速度上要比原生還快些

互調(diào)接口

1. AndroidView 來(lái)調(diào)用android原生的控件(Ios使用 UiKitView)
   目前還存在一些bug疯溺，感興趣的可以查看一下flutter_webview_plugin開(kāi)源庫(kù)的實(shí)現(xiàn)源碼
2. EventChannel 傳遞和接收事件流
   MethodChannel 傳遞和接收方法
   BasicMessageChannel 傳遞和接收字符串

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