记录自己分析RN的过程。
先看入口的MainActivity，它是继承自ReactActivity 的。
ReactActivity的内容

/**
 * 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，这个类吧所有的操作都交给一个ReactActivityDelegate对象来执行。
我想主要原因是，这个类需要经常被继承，如果直接在这个类实现方法，不免会让其子类变得臃肿。使用代理的方法就避免了这个问题。

先看其最核心的入口，onCreate .这个类来创建视图等。
我们在做安卓开发时一般在这类初始化视图。
使用setContentView传入一个layout的id，或者一个view对象。

我们跟进Delegate中，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();
  }

对于androidM，请求了android.settings.action.MANAGE_OVERLAY_PERMISSION权限。

随后判断mMainComponentName 是否为空。这个变量的初始化是在其构造函数中

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

这个构造函数的调用者是ReactActivity的create..方法

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

是通过getMainComponentName获取的。而这个方法子类可以继承和修改。

我们回到onCreate中。
调用 loadApp(mMainComponentName); 接着调用
DoubleTapReloadRecognizer （这个应该是双击R重新加载）

loadApp代码

  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已经有了，就不需要load了。

调用createRootView创建。

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

其实是调用了构造函数。转到ReactRootView看一下。

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

这是一个SizeMonitoringFrameLayout ，它又是一个FrameLayout。

好了。总结来说，在ReactActivity创建的时候使用了一个ReactRootView作为其跟视图。

未完待续。。。