Android 子线程 UI 操作真的不可以？

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

ViewRootImpl.checkThread()  (android.view)
  ViewRootImpl.invalidateChildInParent(int[], Rect)  (android.view)
  ViewGroup.invalidateChild(View, Rect)  (android.view)
    ViewRootImpl.invalidateChild(View, Rect)  (android.view)
    View.invalidateInternal(int, int, int, int, boolean, boolean)  (android.view)
      View.invalidate(boolean)  (android.view)
      View.invalidate()  (android.view)
        TextView.checkForRelayout()(2 usages)  (android.widget)
        TextView.setText(CharSequence, BufferType, boolean, int)  (android.widget)

父View不为空：该条件显而易见，父view为空时，是无法调用ParentView#invalidateChild方法的。  
   Dirty区域坐标合法：同样显而易见。  
   AttachInfo不为空：目前唯一的变量，该方法为空时，不会真正执行invalidate。

void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
      boolean fullInvalidate) {
  // ...
 
  // Propagate the damage rectangle to the parent view.
  final AttachInfo ai = mAttachInfo; // 此处何时赋值
  final ViewParent p = mParent;
  if (p != null && ai != null && l < r && t < b) { // 此处逻辑若不通过，实际也不会触发invalidate
    final Rect damage = ai.mTmpInvalRect;
    damage.set(l, t, r, b);
    p.invalidateChild(this, damage);
  }
 
  // ...
 
}

窗口（Window）相关的类、信息及IPC类。  
   ViewRootImpl对象：这个类就是会触发CalledFromWrongThreadException的来源。  
   其他信息。

// android.view.view
 
/**
 * A set of information given to a view when it is attached to its parent
 * window.
 */
final static class AttachInfo {
 
  // ...
 
  final IBinder mWindowToken;
 
  /**
   * The view root impl.
   */
  final ViewRootImpl mViewRootImpl;
 
  // ...
 
  AttachInfo(IWindowSession session, IWindow window, Display display,
      ViewRootImpl viewRootImpl, Handler handler, Callbacks effectPlayer,
      Context context) {
 
    // ...
 
    mViewRootImpl = viewRootImpl;
 
    // ...
  }
}

// android.view.ViewGroup
 
private void addViewInner(View child, int index, LayoutParams params,
    boolean preventRequestLayout) {
  // ...                                    
  AttachInfo ai = mAttachInfo;
  if (ai != null && (mGroupFlags & FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW) == 0) {
 
    // ...
    child.dispatchAttachedToWindow(mAttachInfo, (mViewFlags&VISIBILITY_MASK));
    // ...
  }
  // ...
}

private View tryInflatePrecompiled(@LayoutRes int resource, Resources res, @Nullable ViewGroup root,
  boolean attachToRoot) {
  // ...
  if (attachToRoot) {
    root.addView(view, params);
  } else {
    view.setLayoutParams(params);
  }
  // ...
}

// android.view.WindowManagerGlobal
 
public void addView(View view, ViewGroup.LayoutParams params,
    Display display, Window parentWindow) {
  // ...
 
  root = new ViewRootImpl(view.getContext(), display);
 
  // ...
 
  try {
    root.setView(view, wparams, panelParentView);
  } catch (RuntimeException e) {
    // ...
  }
}
 
 
// android.view.RootViewImpl
 
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
  // ...
  mView = view;
  // ...
  mAttachInfo.mRootView = view;
  // ...
  view.assignParent(this);
  // ...
}

WindowManagerGlobal.addView(View, LayoutParams, Display, Window)  (android.view)
  WindowManagerImpl.addView(View, LayoutParams)  (android.view)
    Dialog.show()  (android.app) // Dialog的显示方法
    PopupWindow.invokePopup(LayoutParams)  (android.widget)
      PopupWindow.showAtLocation(IBinder, int, int, int)  (android.widget) // PopupWindow的显示方法
    TN in Toast.handleShow(IBinder)  (android.widget) // Toast的展示方法

// com.google.android.material.snackbar.BaseTransientBottomBar
 
final void showView() {
 
  // ...
 
  if (this.view.getParent() == null) {
    ViewGroup.LayoutParams lp = this.view.getLayoutParams();
   
    if (lp instanceof CoordinatorLayout.LayoutParams) {
    setUpBehavior((CoordinatorLayout.LayoutParams) lp);
    }
   
    extraBottomMarginAnchorView = calculateBottomMarginForAnchorView();
    updateMargins();
   
    // Set view to INVISIBLE so it doesn't flash on the screen before the inset adjustment is
    // handled and the enter animation is started
    view.setVisibility(View.INVISIBLE);
    targetParent.addView(this.view);
  }
 
  // ...
 
}

由一个已attachWindow的父View调用其addView方法，将子view也attach至同一个window，从而拥有viewRootImpl。（material Snackbar方式）  
   通过WindowManager#addView，自建一个Window及ViewRootImpl，完成view与window的attach操作。（PopupWindow方式）

Window是一个抽象的概念，每一个Window都对应着一个View和一个ViewRootImpl，Window和View通过ViewRootImpl来建立联系。——《Android开发艺术探索》 
 // 理解：每个Window对应一个ViewTree，其根节点是ViewRootImpl，ViewRootImpl自上而下地控制着ViewTree的一切（事件 & 绘制 & 更新）

/**
 * Invalidate the whole view. If the view is visible,
 * {@link #onDraw(android.graphics.Canvas)} will be called at some point in
 * the future.
 * <p>
 * This must be called from a UI thread. To call from a non-UI thread, call
 * {@link #postInvalidate()}.
 */
// 咬文嚼字：「from a UI thread」，不是「from the UI thread」
public void invalidate() {
  invalidate(true);
}

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

// ViewRootImpl.java
 
public ViewRootImpl(Context context, Display display) {
  // ...
  mThread = Thread.currentThread();
  // ...
  mChoreographer = Choreographer.getInstance();
}

The top of a view hierarchy, implementing the needed protocol between View and the WindowManager. This is for the most part an internal implementation detail of WindowManagerGlobal.

// ViewRootImpl.java
 
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
 
  // ...
                                    
  invalidateRectOnScreen(dirty);
                                    
  return null;
}
 
private void invalidateRectOnScreen(Rect dirty) {
 
  // ...
   
  if (!mWillDrawSoon && (intersected || mIsAnimating)) {
    scheduleTraversals();
  }
}

首先会在UI线程的消息队列中添加同步消息屏障，保障后续的绘制异步消息的优先执行；  
   之后会向Choreographer注册一个Runnable对象，由前者决定何时调用Runnable的run方法；  
   而该Runnable对象就是doTraversal方法，即真正执行视图树遍历绘制的方法。

// ViewRootImpl.java
final class TraversalRunnable implements Runnable {
  @Override
  public void run() {
    doTraversal();
  }
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
 
void scheduleTraversals() {
  // ...
  mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
  mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
  // ...
}

Choreographer#postCallback →postCallbackDelayed → 
 postCallbackDelayedInternal→mHandler#sendMessage →MSG_DO_SCHEDULE_CALLBACK

MessageQueue#next→ mHandler#handleMessage →MSG_DO_SCHEDULE_CALLBACK→ doScheduleCallback→scheduleFrameLocked → scheduleVsyncLocked→DisplayEventReceiver#scheduleVsync

// android.view.DisplayEventReceiver
 
/**
 * Schedules a single vertical sync pulse to be delivered when the next
 * display frame begins.
 */
@UnsupportedAppUsage
public void scheduleVsync() {
  if (mReceiverPtr == 0) {
    Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
        + "receiver has already been disposed.");
  } else {
    nativeScheduleVsync(mReceiverPtr);
  }
}

// android.view.Choreographer
 
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
    implements Runnable {
 
  // ...
 
  @Override
  public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
    // ...
    // Post the vsync event to the Handler.
    Message msg = Message.obtain(mHandler, this);
    msg.setAsynchronous(true);
    mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
  }
 
  @Override
  public void run() {
    mHavePendingVsync = false;
    doFrame(mTimestampNanos, mFrame);
  }
 
}

// android.view.Choreographer
 
void doFrame(long frameTimeNanos, int frame) {
  // ...
  try {
    // ...
    doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
    // ...
  } finally {
    // ...
  }
}

// ViewRootImpl.java
 
void doTraversal() {
  // ...
  mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
  // ...                              
  performTraversals();                              
  // ...
}
 
private void performTraversals() {
  // ...
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
  // ...
  performLayout(lp, desiredWindowWidth, desiredWindowHeight);
  // ...
  performDraw();
}

// ViewRootImpl.java
 
final class ViewRootHandler extends Handler {
  @Override
  public String getMessageName(Message message) {
    // ...
  }
                                                 
  @Override
  public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    // ...
  }
                                                 
  @Override
  public void handleMessage(Message msg) {
    // ...
  }
}
                                                 
final ViewRootHandler mHandler = new ViewRootHandler();

// andriod.os.Handler
public Handler(@Nullable Callback callback, boolean async) {
  // ...
  mLooper = Looper.myLooper();
  // ...
  mQueue = mLooper.mQueue;
  // ...
}
 
// andriod.os.Looper
/**
 * Return the Looper object associated with the current thread.  Returns
 * null if the calling thread is not associated with a Looper.
 */
public static @Nullable Looper myLooper() {
  return sThreadLocal.get();
}

// ViewRootImpl.java
 
public ViewRootImpl(Context context, Display display) {
  // ...
  mThread = Thread.currentThread();
  // ...
  mChoreographer = Choreographer.getInstance();
}

自View#invalidate方法触发，至注册垂直同步信号监听（DisplayEventReceiver#nativeScheduleVsync），以及垂直同步信号回调（DisplayEventReceiver#onVsync）至View的measue/layout/draw方法调用，均在同一个线程（UI线程），而系统并未限制该现场必须为主线程。

// andriod.view.Choreographer
 
// Thread local storage for the choreographer.
private static final ThreadLocal<Choreographer> sThreadInstance =
    new ThreadLocal<Choreographer>() {
  @Override
  protected Choreographer initialValue() {
    Looper looper = Looper.myLooper();
    // ...
    Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
    if (looper == Looper.getMainLooper()) {
      mMainInstance = choreographer;
    }
    return choreographer;
  }
};
 
/**
 * Gets the choreographer for the calling thread.  Must be called from
 * a thread that already has a {@link android.os.Looper} associated with it.
 *
 * @return The choreographer for this thread.
 * @throws IllegalStateException if the thread does not have a looper.
 */
public static Choreographer getInstance() {
  return sThreadInstance.get();
}

视频播放时，视频画面的绘制并不是App的主线程及UI线程。  
   系统Toast的弹出等绘制，是由系统层面统一控制，也并非App自身的主线程或UI线程绘制。

// Snackbar弹窗管理类
public class SnackBarPopWinManager {
 
  private static SnackBarPopWinManager instance;
 
  private final Handler h; // 弹窗的UI线程Handler
 
  // ...
 
  private SnackBarPopWinManager() {
    // 弹窗的UI线程
    HandlerThread ht = new HandlerThread("snackbar-ui-thread");
    ht.start();
    h = new Handler(ht.getLooper());
  }
 
  public Handler getSnackbarWorkHandler() {
    return h;
  }
 
  public void presentPopWin(final SnackBarPopWin snackBarPopWin) {
    // UI操作抛至自定义的UI线程
    h.postDelayed(new SafeRunnable() {
      @Override
      public void safeRun() {
        // ..
        // 展示弹窗
        snackBarPopWin.getPopWin().showAtLocation(dependentView, Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, y);
        // 定时自动关闭
        snackBarPopWin.dismissAfter(5000);
        // ...
    });
  }
 
  public void dismissPopWin(final SnackBarPopWin snackBarPopWin) {
    // UI操作抛至自定义的UI线程
    h.postDelayed(new SafeRunnable() {
      @Override
      public void safeRun() {
        // ...
        // dismiss弹窗
        snackBarPopWin.getPopWin().dismiss();
        // ...
    });
  }
 
  // ...
}

// Snackbar弹窗本身（通过PopupWindow实现）
public class SnackBarPopWin extends PointSnackBar implements View.OnClickListener {
 
  private PopupWindow mPopWin;
 
  public static SnackBarPopWin make(String alertText, long points, String actionId) {
    SnackBarPopWin instance = new SnackBarPopWin();
    init(instance, alertText, actionId, points);
    return instance;
  }
 
  private SnackBarPopWin() {
    // infalte等耗时操作
    // ...
    View popView = LayoutInflater.from(context).inflate(R.layout.popwin_layout, null);
    // ...
    mPopWin = new PopupWindow(popView, ...);
    // ...
  }
 
  // 用户的UI操作，回调应该也在UI线程
  public void onClick(View v) {
    int id = v.getId();
    if (id == R.id.tv_popwin_action_btn) {
      onAction();
    } else if (id == R.id.btn_popwin_cross) {
      onClose();
    }
  }
 
  public void show(int delay) {
    // ...
    SnackBarPopWinManager.getInstance().presentPopWin(SnackBarPopWin.this);
  }
 
  public void dismissAfter(long delay) {
    // ...
    SnackBarPopWinManager.getInstance().dismissPopWin(SnackBarPopWin.this);
  }
 
  // ...
 
}

// MainActivity.java
 
public void snackBarSubShowSubMod(View view) {
 
  WorkThreadHandler.getInstance().post(new SafeRunnable() {
    @Override
    public void safeRun() {
      String htmlMsg = "已读新闻<font color=#ff1e02>5</font>篇，剩余<font color=#00af57>10</font>次，延迟0.3s";
      final PointSnackBar snackbar = PointSnackBar.make(htmlMsg, 20, "");
      if (null != snackbar) {
        snackbar.snackBarBackgroundColor(mToastColor)
            .buttonBackgroundColor(mButtonColor)
            .callback(new PointSnackBar.Callback() {
          @Override
          public void onActionClick() {
            snackbar.onCollectSuccess();
          }
        }).show();
      }
 
      // 在自定义UI线程中更新视图
      SnackBarPopWinManager.getInstance().getSnackbarWorkHandler().postDelayed(new SafeRunnable() {
        @Override
        public void safeRun() {
          try {
            snackbar.alertText("恭喜完成<font color='#ff00ff'>“UI更新”</font>任务，请领取积分");
          } catch (Exception e) {
            DemoLogUtils.e(TAG, "error: ", e);
          }
        }
      }, 2000);
    }
  });
}

Webview的所有方法调用必须在主线程，因为其代码中强制做了主线程校验，如PopupWindow中内置Webview，则不适用多UI线程。  
   Activity的使用必须在主线程，因为其创建等操作中使用的Handler也被强制指定为mainThreadHandler。

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