記錄自己分析RN的過程。
先看入口的MainActivity，它是繼承自ReactActivity 的刊头。
ReactActivity的內(nèi)容

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */

package com.facebook.react;

import javax.annotation.Nullable;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.KeyEvent;

import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler;
import com.facebook.react.modules.core.PermissionAwareActivity;
import com.facebook.react.modules.core.PermissionListener;

/**
 * Base Activity for React Native applications.
 */
public abstract class ReactActivity extends Activity
    implements DefaultHardwareBackBtnHandler, PermissionAwareActivity {

  private final ReactActivityDelegate mDelegate;

  protected ReactActivity() {
    mDelegate = createReactActivityDelegate();
  }

  /**
   * Returns the name of the main component registered from JavaScript.
   * This is used to schedule rendering of the component.
   * e.g. "MoviesApp"
   */
  protected @Nullable String getMainComponentName() {
    return null;
  }

  /**
   * Called at construction time, override if you have a custom delegate implementation.
   */
  protected ReactActivityDelegate createReactActivityDelegate() {
    return new ReactActivityDelegate(this, getMainComponentName());
  }

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mDelegate.onCreate(savedInstanceState);
  }

  @Override
  protected void onPause() {
    super.onPause();
    mDelegate.onPause();
  }

  @Override
  protected void onResume() {
    super.onResume();
    mDelegate.onResume();
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
    mDelegate.onDestroy();
  }

  @Override
  public void onActivityResult(int requestCode, int resultCode, Intent data) {
    mDelegate.onActivityResult(requestCode, resultCode, data);
  }

  @Override
  public boolean onKeyUp(int keyCode, KeyEvent event) {
    return mDelegate.onKeyUp(keyCode, event) || super.onKeyUp(keyCode, event);
  }

  @Override
  public void onBackPressed() {
    if (!mDelegate.onBackPressed()) {
      super.onBackPressed();
    }
  }

  @Override
  public void invokeDefaultOnBackPressed() {
    super.onBackPressed();
  }

  @Override
  public void onNewIntent(Intent intent) {
    if (!mDelegate.onNewIntent(intent)) {
      super.onNewIntent(intent);
    }
  }

  @Override
  public void requestPermissions(
    String[] permissions,
    int requestCode,
    PermissionListener listener) {
    mDelegate.requestPermissions(permissions, requestCode, listener);
  }

  @Override
  public void onRequestPermissionsResult(
    int requestCode,
    String[] permissions,
    int[] grantResults) {
    mDelegate.onRequestPermissionsResult(requestCode, permissions, grantResults);
  }

  protected final ReactNativeHost getReactNativeHost() {
    return mDelegate.getReactNativeHost();
  }

  protected final ReactInstanceManager getReactInstanceManager() {
    return mDelegate.getReactInstanceManager();
  }

  protected final void loadApp(String appKey) {
    mDelegate.loadApp(appKey);
  }
}

其中仗颈，這個類吧所有的操作都交給一個ReactActivityDelegate對象來執(zhí)行佛舱。
我想主要原因是，這個類需要經(jīng)常被繼承挨决，如果直接在這個類實現(xiàn)方法请祖，不免會讓其子類變得臃腫。使用代理的方法就避免了這個問題脖祈。

先看其最核心的入口肆捕，onCreate .這個類來創(chuàng)建視圖等。
我們在做安卓開發(fā)時一般在這類初始化視圖盖高。
使用setContentView傳入一個layout的id慎陵，或者一個view對象。

我們跟進Delegate中或舞，看到

  protected void onCreate(Bundle savedInstanceState) {
    boolean needsOverlayPermission = false;
    if (getReactNativeHost().getUseDeveloperSupport() && Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
      // Get permission to show redbox in dev builds.
      if (!Settings.canDrawOverlays(getContext())) {
        needsOverlayPermission = true;
        Intent serviceIntent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION, Uri.parse("package:" + getContext().getPackageName()));
        FLog.w(ReactConstants.TAG, REDBOX_PERMISSION_MESSAGE);
        Toast.makeText(getContext(), REDBOX_PERMISSION_MESSAGE, Toast.LENGTH_LONG).show();
        ((Activity) getContext()).startActivityForResult(serviceIntent, REQUEST_OVERLAY_PERMISSION_CODE);
      }
    }

    if (mMainComponentName != null && !needsOverlayPermission) {
      loadApp(mMainComponentName);
    }
    mDoubleTapReloadRecognizer = new DoubleTapReloadRecognizer();
  }

對于androidM荆姆，請求了android.settings.action.MANAGE_OVERLAY_PERMISSION權(quán)限。

隨后判斷mMainComponentName 是否為空映凳。這個變量的初始化是在其構(gòu)造函數(shù)中

  public ReactActivityDelegate(Activity activity, @Nullable String mainComponentName) {
    mActivity = activity;
    mMainComponentName = mainComponentName;
    mFragmentActivity = null;
  }

這個構(gòu)造函數(shù)的調(diào)用者是ReactActivity的create..方法

  protected ReactActivityDelegate createReactActivityDelegate() {
    return new ReactActivityDelegate(this, getMainComponentName());
  }

是通過getMainComponentName獲取的胆筒。而這個方法子類可以繼承和修改。

我們回到onCreate中诈豌。
調(diào)用 loadApp(mMainComponentName); 接著調(diào)用
DoubleTapReloadRecognizer （這個應該是雙擊R重新加載）

loadApp代碼

  protected void loadApp(String appKey) {
    if (mReactRootView != null) {
      throw new IllegalStateException("Cannot loadApp while app is already running.");
    }
    mReactRootView = createRootView();
    mReactRootView.startReactApplication(
      getReactNativeHost().getReactInstanceManager(),
      appKey,
      getLaunchOptions());
    getPlainActivity().setContentView(mReactRootView);
  }

如果mReactRootView已經(jīng)有了仆救，就不需要load了。

調(diào)用createRootView創(chuàng)建矫渔。

  protected ReactRootView createRootView() {
    return new ReactRootView(getContext());
  }

其實是調(diào)用了構(gòu)造函數(shù)彤蔽。轉(zhuǎn)到ReactRootView看一下。

public class ReactRootView extends SizeMonitoringFrameLayout implements RootView {
...
}

這是一個SizeMonitoringFrameLayout 庙洼，它又是一個FrameLayout顿痪。

好了∮凸唬總結(jié)來說蚁袭，在ReactActivity創(chuàng)建的時候使用了一個ReactRootView作為其跟視圖。

未完待續(xù)石咬。揩悄。。