View的绘制流程

Activity所有的生命周期方法调用都是在ActivityThread类中执行的。

比如Activity的onCreate生命周期：

 @Override
    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;
  //通过反射得到activity对象。
  java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
  activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback,
                        r.assistToken);
  if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
  return activity;
    }

最后在Instrumentation类中执行callActivityOnCreate方法，callActivityOnCreate方法中，执行Activity中的performCreate方法，最后执行onCreate方法。
可以在ActivityThread中找到Activity其他生命周期方法：

• handleStartActivity
• handleResumeActivity
• handlePauseActivity
  等等。

Activity的onCreate

启动Activity首先执行onCreate方法，也就是刚刚performLaunchActivity方法，在performLaunchActivity方法中，在执行callActivityOnCreate之前也就是执行Activity的onCreate方法之前先执行了Activity的attach（）方法。

final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback, IBinder assistToken) {
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        mWindow.setWindowControllerCallback(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
        mWindow.setColorMode(info.colorMode);
        setAutofillOptions(application.getAutofillOptions());
    }

在attach方法中创建了PhoneWindow，并执行了setWindowManager方法，

public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
            boolean hardwareAccelerated) {
        mAppToken = appToken;
        mAppName = appName;
        mHardwareAccelerated = hardwareAccelerated;
        if (wm == null) {
            wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        }
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

mWindow 是PhoneWindow。
这个方法设置的是WindowManagerImpl类。

记住这一点
-- 海绵宝宝

setContentView()

在Activity方法中执行setContentView方法

public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

海绵宝宝上面说getWindow就是PhoneWindow,
所以就是调用PhoneWindow的setContentView方法：

public void setContentView(int layoutResID) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }

最后一行，是将传入的布局id加载到mContentParent中，在第一行中，首先会判断mContentParent是否是空，如果是null进入到installDecor方法中，先看一下这个方法都在做什么操作：

private void installDecor() {
  if (mDecor == null) {
            mDecor = generateDecor(-1);
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        } else {
            mDecor.setWindow(this);
        }
   if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);
  }
}

上面方法会创建一个DecorView和mContentParent，
进入到generateLayout方法中：

protected ViewGroup generateLayout(DecorView decor) {
  int layoutResource;
  layoutResource = R.layout.screen_simple;
  mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
  ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
  return contentParent;
}

layoutResource 布局文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

执行DecorView的onResourcesLoaded方法：

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
  final View root = inflater.inflate(layoutResource, null);
  if (mDecorCaptionView != null) {
            if (mDecorCaptionView.getParent() == null) {
                addView(mDecorCaptionView,
                        new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
            }
            mDecorCaptionView.addView(root,
                    new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
        } else {

            // Put it below the color views.
            addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
}

根据布局id，将View添加到DecorView中。

 ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

这个就是我们布局当中的FrameLayout。
这样我们就把我们的布局View添加到了系统的FrameLayout中。

总结：

在ActivityThread的handleLaunchAcitivity方法进入到performLaunchActivity方法，performLaunchActivity方法进行实例化Activity，并执行Activity的attach方法和onCreate方法，其中attach方法实例化了PhoneWindow，和设置了WindowManagerImpl。
执行完attach方法之后执行onCreate方法中的setContentView方法，调用PhoneWindow的setContentView方法，在setContentView方法中，实例化了DecorView和然后将我们的系统布局添加到了DecorView中，然后根据id找到FrameLayout，最后将用户的布局文件放到FrameLayout中。

流程图

结构图

等等好像没有ViewRootImpl什么关系：

在ActivityThread的handleResumeActivity方法:

public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason) {
  final Activity a = r.activity;
  r.window = r.activity.getWindow();
  View decor = r.window.getDecorView();
  decor.setVisibility(View.INVISIBLE);
  ViewManager wm = a.getWindowManager();
  WindowManager.LayoutParams l = r.window.getAttributes();
  a.mDecor = decor;
  l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
  l.softInputMode |= forwardBit;
  if (a.mVisibleFromClient) {
      if (!a.mWindowAdded) {
           a.mWindowAdded = true;
           wm.addView(decor, l);
       } else {
             // The activity will get a callback for this {@link LayoutParams} change
             // earlier. However, at that time the decor will not be set (this is set
              // in this method), so no action will be taken. This call ensures the
            // callback occurs with the decor set.
           a.onWindowAttributesChanged(l);
        }
     }
}

最后执行wm.addView(decor, l),海绵宝宝说这个是wm是WindowManagerImpl类，decor就是DecorView，进入WindowManagerImpl的addView方法：

public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
    }

从WindowMangerImpl调用WindowManagerGlobal的addView方法：

public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
  ViewRootImpl root;
  synchronized (mLock) {
  root = new ViewRootImpl(view.getContext(), display);

  view.setLayoutParams(wparams);

  mViews.add(view);
  mRoots.add(root);
  mParams.add(wparams);

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

在此方法中会实例化ViewRootImpl，并将decorView传入到ViewRootIpml的setView方法中。

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
  //view就是decorview
  mView = view;
  requestLayout();
  //设置decorview的父布局为viewrootImpl
  view.assignParent(this);
}

在设置decorview的父布局为viewrootImpl之前执行requestLayout方法:

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

在此方法里面会做检查线程的操作然后执行scheduleTraversals():

void scheduleTraversals() {
  mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
}
final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }
    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

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

            performTraversals();

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

最后在performTraversals方法里面会执行DecorView的测量布局和绘制。