我们都知道字线程里更新不能更新UI，否则系统会报Only the original thread that created a view hierarchy can touch its views.错误，具体如下：

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
        at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7313)
        at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1161)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at android.view.View.requestLayout(View.java:21995)
        at androidx.recyclerview.widget.RecyclerView.requestLayout(RecyclerView.java:4202)
        at androidx.recyclerview.widget.RecyclerView$RecyclerViewDataObserver.onChanged(RecyclerView.java:5286)
        at androidx.recyclerview.widget.RecyclerView$AdapterDataObservable.notifyChanged(RecyclerView.java:11997)
        at androidx.recyclerview.widget.RecyclerView$Adapter.notifyDataSetChanged(RecyclerView.java:7070)
        at com.github.jokar.wechat_moments.view.adapter.MomentsAdapter.submitList(MomentsAdapter.java:71)
        at com.github.jokar.wechat_moments.view.MainActivity$1.run(MainActivity.java:51)

那么Activity.onCreate可以在字线程里更新UI么？，答案是可以的。但是不是全部可以，如果子线程是立马执行的可以，若休眠了一定时间后就不可以了。 这是为什么呢？

为什么会报`Only the original thread that created a view hierarchy can touch its views.`错误？

首先我们要搞懂一个问题就是为什么会报Only the original thread that created a view hierarchy can touch its views.错误？

从上面错误信息堆栈可以看到是ViewRootImpl.requestLayout()方法里调用的checkThread里爆出了这个错误：

    @Override
    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.");
        }
    }

这里就可以看到具体检查报错的是在ViewRootImpl.requestLayout()方法里，但是这个ViewRootImpl是啥？为什么我们更新view会到这里？这里就要说到了requestLayout()方法了。

`requestLayout()`

（1）如果我们修改了一个 View，如果修改结果影响了它的尺寸,那么就会触发这个方法。（2）从方法名字可以知道，“请求布局”，那就是说，如果调用了这个方法，那么对于一个子View来说，应该会重新进行布局流程。但是，真实情况略有不同，如果子View调用了这个方法，其实会从View树重新进行一次测量、布局、绘制这三个流程，最终就会显示子View的最终情况。

源码分析View#requestLayout：

    public void requestLayout() {
        if (mMeasureCache != null) mMeasureCache.clear();

        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
            // Only trigger request-during-layout logic if this is the view requesting it,
            // not the views in its parent hierarchy
            ViewRootImpl viewRoot = getViewRootImpl();
            if (viewRoot != null && viewRoot.isInLayout()) {
                if (!viewRoot.requestLayoutDuringLayout(this)) {
                    return;
                }
            }
            mAttachInfo.mViewRequestingLayout = this;
        }

        //为当前view设置标记位 PFLAG_FORCE_LAYOUT
        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;

        if (mParent != null && !mParent.isLayoutRequested()) {
            //向父容器请求布局
            mParent.requestLayout();
        }
        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
            mAttachInfo.mViewRequestingLayout = null;
        }
    }

在requestLayout方法中，首先先判断当前View树是否正在布局流程，接着为当前子View设置标记位，该标记位的作用就是标记了当前的View是需要进行重新布局的
接着调用mParent.requestLayout方法，这个十分重要，因为这里是向父容器请求布局，即调用父容器的requestLayout方法，为父容器添加PFLAG_FORCE_LAYOUT标记位，而父容器又会调用它的父容器的requestLayout方法，即requestLayout事件层层向上传递，直到DecorView，即根View
而根View又会传递给ViewRootImpl，也即是说子View的requestLayout事件，最终会被ViewRootImpl接收并得到处理

纵观这个向上传递的流程，其实是采用了责任链模式，即不断向上传递该事件，直到找到能处理该事件的上级，在这里，只有ViewRootImpl能够处理requestLayout事件。到这里我们就明道了为什么当更新View的时候如果触发了requestLayout方法为什么会到ViewRootImpl.requestLayout()处理。

为什么 `Activity.onCreate`可以在字线程里更新UI？

上面介绍到最终报错是由ViewRootImpl处理的，那么这里就涉及到了Activity的创建过程了。这里贴一个网上大佬画的startActivity流程图

Activity的启动过程，我们可以从Context的startActivity说起，其实现是ContextImpl的startActivity，然后内部会通过Instrumentation来尝试启动Activity，这是一个跨进程过程，它会调用ams的startActivity方法，当ams校验完activity的合法性后，会通过ApplicationThread回调到我们的进程，这也是一次跨进程过程，而applicationThread就是一个binder，回调逻辑是在binder线程池中完成的，所以需要通过Handler H将其切换到ui线程，第一个消息是LAUNCH_ACTIVITY，它对应handleLaunchActivity，在这个方法里完成了Activity的创建和启动。我们在这里主要分析ActivityThread.handleLaunchActiivty

`ActivityThread.handleLaunchActiivty`

    private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        WindowManagerGlobal.initialize();
        //创建Activity类实例
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            Bundle oldState = r.state;
            //
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

            if (!r.activity.mFinished && r.startsNotResumed) {
                if (r.isPreHoneycomb()) {
                    r.state = oldState;
                }
            }
        } else {
            try {
                ActivityManager.getService()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

可以看到Activity类实例是在performLaunchActivity创建的，然后又调用了handleResumeActivity方法

`ActivityThread.handleResumeActivity`

    final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
        ActivityClientRecord r = mActivities.get(token);

        ...
        //调用Activity.onResume
        r = performResumeActivity(token, clearHide, reason);

        if (r != null) {
            final Activity a = r.activity;
            
            ....

            boolean willBeVisible = !a.mStartedActivity;
            if (!willBeVisible) {
                try {
                    willBeVisible = ActivityManager.getService().willActivityBeVisible(
                            a.getActivityToken());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            if (r.window == null && !a.mFinished && willBeVisible) {
                
                ....
                //创建添加ViewRootImpl
                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);
                    }
                }

          
            } 

            .....
        } 
    }

这里主要关注两个方法performResumeActivity和 wm.addView(decor, l);

`performResumeActivity`

    public final ActivityClientRecord performResumeActivity(IBinder token,
            boolean clearHide, String reason) {
        ActivityClientRecord r = mActivities.get(token);
       
        if (r != null && !r.activity.mFinished) {
            if (clearHide) {
                r.hideForNow = false;
                r.activity.mStartedActivity = false;
            }
            try {
               ...

                r.activity.performResume();

                ....
            
            } catch (Exception e) {
              if (!mInstrumentation.onException(r.activity, e)) {
                    throw new RuntimeException(
                        "Unable to resume activity "
                        + r.intent.getComponent().toShortString()
                        + ": " + e.toString(), e);
                }
            }
        }
        return r;
    }

performResumeActivity里调用了Activity的performResume()方法，这里操作了mInstrumentation的callActivityOnResume()方法里调用了Activity生命周期的onResume方法

#Activity.performResume

    final void performResume() {
        performRestart();

        mFragments.execPendingActions();

        mLastNonConfigurationInstances = null;

        mCalled = false;
        // mResumed is set by the instrumentation
        mInstrumentation.callActivityOnResume(this);
        ...

        onPostResume();
        
    }

#Instrumentation.callActivityOnResume

    public void callActivityOnResume(Activity activity) {
        activity.mResumed = true;
        activity.onResume();
        
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    am.match(activity, activity, activity.getIntent());
                }
            }
        }
    }

`wm.addView(decor, l)`

wm.addView(decor, l)最终调用了WindowManagerImpl.addView

#WindowManagerImpl.addView


 private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

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

#WindowManagerGlobal.addView


    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        ...

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;

        ...

        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
            // Start watching for system property changes.
            ....

            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

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

        }
    }

到这里我们终于看到了ViewRootImpl的创建,从上面过程可以看到ViewRootImpl的创建是在Activity.onResume之后的，这也解释了为什么我们可以在Activity.onCreate甚至Activity.onResume里实现子线程里操作UI，因为此时ViewRootImpl并为创建不会进行线程检查。