面试？看完这篇就够了-深入分析从点击应用图标到应用界面展示

// 已精简非核心代码  
public static void main(String[] args) {
      Looper.prepareMainLooper(); // 初始化主线程 Looper

      ActivityThread thread = new ActivityThread();
      thread.attach(false, startSeq);

      Looper.loop(); // 开启主线程消息循环
}

  final ApplicationThread mAppThread = new ApplicationThread();

  // 已精简非核心代码
  private void attach(boolean system, long startSeq) {
      // 1. 这里传入的是 false, 所以一定会进入这个分支
      if (!system) {
          // 2. 这里做的事情就是将 ApplicationThread 传到 AMS 中，往后 AMS 和 应用就可以通过 Binder 直接通信了
          RuntimeInit.setApplicationObject(mAppThread.asBinder());
          final IActivityManager mgr = ActivityManager.getService();
          try {
              mgr.attachApplication(mAppThread, startSeq);
          } catch (RemoteException ex) {
              throw ex.rethrowFromSystemServer();
          }
         // ...
      } else {
          // ...
      }
  }

private class ApplicationThread extends IApplicationThread.Stub {

      @Override
      public final void bindApplication(...) {
          //...
          sendMessage(H.BIND_APPLICATION, data);
      }

      //...
}

  private void handleBindApplication(AppBindData data) {
      // ...
      try {
          app = data.info.makeApplication(data.restrictedBackupMode, null); // 1. 实例化 Application, 内部调用了 Application.attachBaseContext()

          if (!data.restrictedBackupMode) {
              if (!ArrayUtils.isEmpty(data.providers)) {
                  installContentProviders(app, data.providers); // 2. 实例化 ContentProvider，调用 ContentProvider.onCreate()
              }
          }

          //...
          try {
              mInstrumentation.callApplicationOnCreate(app); // 3. 这里面调用 Application.onCreate()
          } catch (Exception e) {
              // ...
          }
      } finally {
          // ...
      }

      // ...
  }

  public Activity handleLaunchActivity(ActivityClientRecord r,
          PendingTransactionActions pendingActions, Intent customIntent) {
      final Activity a = performLaunchActivity(r, customIntent);
      return a;
  }

  private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
      //...
      Activity activity = null;
      try {
          // 实例化 Activity
          java.lang.ClassLoader cl = appContext.getClassLoader();
          activity = mInstrumentation.newActivity(
                  cl, component.getClassName(), r.intent);
          // ...
      } catch (Exception e) {
          // ...
      }

      try {
          // 调用 Activity.onCreate()
              if (r.isPersistable()) {
                  mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
              } else {
                  mInstrumentation.callActivityOnCreate(activity, r.state);
              }
          // ...
      } catch (SuperNotCalledException e) {
         // ...
      }

      return activity;
  }

  @Override
  public void handleResumeActivity(ActivityClientRecord r, boolean finalStateRequest,
          boolean isForward, String reason) {
          // 1. 这里面会执行 onResume()
          if (!performResumeActivity(r, finalStateRequest, reason)) {
              return;
          }
          // 2. 获取 decorView
          View decor = r.window.getDecorView();
          decor.setVisibility(View.INVISIBLE);

          ViewManager wm = a.getWindowManager();
          // 3. 将 decorView 添加到 WindowManager 中
          wm.addView(decor, l);
  }

  public void addView(View view, ViewGroup.LayoutParams params,
          Display display, Window parentWindow, int userId) {
          // 实例化 ViewRootImpl
          root = new ViewRootImpl(view.getContext(), display);

          // do this last because it fires off messages to start doing things
          try {
              // 核心代码，调用 setView 传入 view
              root.setView(view, wparams, panelParentView, userId);
          } catch (RuntimeException e) {
              // BadTokenException or InvalidDisplayException, clean up.
              if (index >= 0) {
                  removeViewLocked(index, true);
              }
              throw e;
          }
      }
  }

  public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
          int userId) {
      // ...
      requestLayout();
      //...
  }

  // AppCompatActivity
  @Override
  public void setContentView(@LayoutRes int layoutResID) {
      initViewTreeOwners();
      getDelegate().setContentView(layoutResID);
  }

  // AppCompatDelegateImpl
  @Override
  public void setContentView(int resId) {
      // 1. 实例化 DecorView
      ensureSubDecor(); 
      // 2. R.id.content 就是承载我们自定义 View 的容器
      ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content); 
      contentParent.removeAllViews();
      // 3. 将我们自定义 View 添加到 R.id.content 中
      LayoutInflater.from(mContext).inflate(resId, contentParent); 
      mAppCompatWindowCallback.getWrapped().onContentChanged();
  }

public final class ViewRootImpl implements ViewParent,
      View.AttachInfo.Callbacks, ThreadedRenderer.DrawCallbacks,
      AttachedSurfaceControl { }

public ViewRootImpl(@UiContext Context context, Display display, IWindowSession session,
          boolean useSfChoreographer) {
      // 保存当前线程，对于这里分析的启动 Activity 的场景，这里是主线程
      mThread = Thread.currentThread();
      // Choreographer 是屏幕刷新机制的关键，后续会单独讲
      mChoreographer = useSfChoreographer
              ? Choreographer.getSfInstance() : Choreographer.getInstance();
  }

  // ViewRootImpl
  public void requestLayout() {
      if (!mHandlingLayoutInLayoutRequest) {
          checkThread();
          mLayoutRequested = true;
          scheduleTraversals();
      }
  }

  void checkThread() {
      if (mThread != Thread.currentThread()) {
          throw new CalledFromWrongThreadException(
                  "Only the original thread that created a view hierarchy can touch its views.");
      }
  }

  void scheduleTraversals() {
      if (!mTraversalScheduled) {
          mTraversalScheduled = true;
          // 1. 发送一个同步屏障
          mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
          // 2. 向 Choreographer post 一个 callback
          mChoreographer.postCallback(
                  Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
          notifyRendererOfFramePending();
          pokeDrawLockIfNeeded();
      }
  }

  final class TraversalRunnable implements Runnable {
      @Override
      public void run() {
          // 3. 执行 doTraversal
          doTraversal();
      }
  }


  void doTraversal() {
      if (mTraversalScheduled) {
          mTraversalScheduled = false;
          // 4. 移除同步屏障
          mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

          if (mProfile) {
              Debug.startMethodTracing("ViewAncestor");
          }

          // 5. onMeasure、onLayout、onDraw 
          performTraversals();

          if (mProfile) {
              Debug.stopMethodTracing();
              mProfile = false;
          }
      }
  }

#include <stdio.h>
#include <unistd.h>

int main() {
  pid_t pid = fork();

  if (pid < 0) {
      // fork failed
      perror("fork");
      return 1;
  } else if (pid == 0) {
      // In child process
      printf("I am the child process, my PID is %d\\n", getpid());
  } else {
      // In parent process
      printf("I am the parent process, my PID is %d and my child's PID is %d\\n", getpid(), pid);
  }

  return 0;
}

@Override
public void layout( int l , int t, int r , int b) {
	// 改变传入的顶点位置参数
	super.layout(l，t，r+100，b+100)；
}

public void postInvalidate() {
    postInvalidateDelayed(0);
}
public void postInvalidateDelayed(long delayMilliseconds) {
    // We try only with the AttachInfo because there's no point in invalidating
    // if we are not attached to our window
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);
    }
}
public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {
    Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);
    mHandler.sendMessageDelayed(msg, delayMilliseconds);
}
private void handleMessageImpl(Message msg) {
        switch (msg.what) {
            case MSG_INVALIDATE:
                ((View) msg.obj).invalidate();
                break;
      // ...
}

//调用requestLayout()方法，打印如下
MyView onMeasure: 
MyView onMeasure: 
MyView onLayout: 
MyView onDraw: 
//调用invalidate()方法，打印如下
MyView onDraw:

void checkThread() {
      if (mThread != Thread.currentThread()) {
          throw new CalledFromWrongThreadException(
                  "Only the original thread that created a view hierarchy can touch its views.");
      }
  }