踩坑之路:主线程修改UI也会崩溃?

前言

某天早晨,吃完早餐,坐回工位,打开电脑,开启chrome,进入友盟页面,发现了一个崩溃信息:

java.lang.RuntimeException: Unable to resume activity {com.youdao.youdaomath/com.youdao.youdaomath.view.PayCourseVideoActivity}: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3824)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3856)
    at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51)
    at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:201)
    at android.app.ActivityThread.main(ActivityThread.java:6806)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
    at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:8000)
    at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1292)
    at android.view.View.requestLayout(View.java:23147)
    at android.view.View.requestLayout(View.java:23147)
    at android.widget.TextView.checkForRelayout(TextView.java:8914)
    at android.widget.TextView.setText(TextView.java:5736)
    at android.widget.TextView.setText(TextView.java:5577)
    at android.widget.TextView.setText(TextView.java:5534)
    at android.widget.Toast.setText(Toast.java:332)
    at com.youdao.youdaomath.view.common.CommonToast.showShortToast(CommonToast.java:40)
    at com.youdao.youdaomath.view.PayCourseVideoActivity.checkNetWork(PayCourseVideoActivity.java:137)
    at com.youdao.youdaomath.view.PayCourseVideoActivity.onResume(PayCourseVideoActivity.java:218)
    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1413)
    at android.app.Activity.performResume(Activity.java:7400)
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3816)

一眼看上去似乎是比较常见的子线程修改UI的问题。并且是在Toast上面报出的,常识告诉我Toast在子线程弹出是会报错,但是应该是提示Looper没有生成的错,而不应该是上面所报出的错误。那么会不会是生成Looper以后报的错的?


一、

所以我先做了一个demo,如下:

    @Override
    protected void onResume() {
        super.onResume();
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                Toast.makeText(MainActivity.this,"子线程弹出Toast",Toast.LENGTH_SHORT).show();
            }
        });
        thread.start();
    }

运行一下,果不其然崩溃掉,错误信息就是提示我必须准备好looper才能弹出toast:

    java.lang.RuntimeException: Can't toast on a thread that has not called Looper.prepare()
        at android.widget.Toast$TN.<init>(Toast.java:393)
        at android.widget.Toast.<init>(Toast.java:117)
        at android.widget.Toast.makeText(Toast.java:280)
        at android.widget.Toast.makeText(Toast.java:270)
        at com.netease.photodemo.MainActivity$1.run(MainActivity.java:22)
        at java.lang.Thread.run(Thread.java:764)

接下来就在toast里面准备好looper,再试试吧:

        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                Toast.makeText(MainActivity.this,"子线程弹出Toast",Toast.LENGTH_SHORT).show();
                Looper.loop();
            }
        });
        thread.start();

运行发现是能够正确的弹出Toast的:


子线程弹出Toast.jpg

那么问题就来了,为什么会在友盟中出现这个崩溃呢?

二、

然后仔细看了下报错信息有两行重要信息被我之前略过了:

at com.youdao.youdaomath.view.PayCourseVideoActivity.onResume(PayCourseVideoActivity.java:218)
t android.widget.Toast.setText(Toast.java:332)

发现是在主线程报了Toast设置Text的时候的错误。这就让我很纳闷了,子线程修改UI会报错,主线程也会报错?
感觉这么多年Android白做了。这不是最基本的知识么?
于是我只能硬着头皮往源码深处看了:
先来看看Toast是怎么setText的:

    public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
            @NonNull CharSequence text, @Duration int duration) {
        Toast result = new Toast(context, looper);

        LayoutInflater inflate = (LayoutInflater)
                context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
        TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
        tv.setText(text);

        result.mNextView = v;
        result.mDuration = duration;

        return result;
    }

很常规的一个做法,先是inflate出来一个View对象,再从View对象找出对应的TextView,然后TextView将文本设置进去。至于setText在我之前的文章震惊!Android子线程也能修改UI?有详细说过,是在ViewRootImpl里面进行checkThread是否在主线程上面。所以感觉似乎一点问题都没有。那么既然出现了这个错误,总得有原因吧,或许是自己源码看漏了?
那就重新再看一遍ViewRootImpl#checkThread方法吧:

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

这一看,还真的似乎给我了一点头绪,系统在checkThread的时候并不是将Thread.currentThread和MainThread作比较,而是跟mThread作比较,那么有没有一种可能mThread是子线程?一想到这里,我就兴奋了,全类查看mThread到底是怎么初始化的:

    public ViewRootImpl(Context context, Display display) {
        ...代码省略...
        mThread = Thread.currentThread();
       ...代码省略...
    }

可以发现全类只有这一处对mThread进行了赋值。那么会不会是子线程初始化了ViewRootimpl呢?似乎我之前好像也没有研究过Toast为什么会弹出来,所以顺便就先去了解下Toast是怎么show出来的好了:

    /**
     * Show the view for the specified duration.
     */
    public void show() {
        if (mNextView == null) {
            throw new RuntimeException("setView must have been called");
        }

        INotificationManager service = getService();
        String pkg = mContext.getOpPackageName();
        TN tn = mTN;
        tn.mNextView = mNextView;

        try {
            service.enqueueToast(pkg, tn, mDuration);
        } catch (RemoteException e) {
            // Empty
        }
    }

调用Toast的show方法时,会通过Binder获取Service即NotificationManagerService,然后执行enqueueToast方法(NotificationManagerService的源码就不做分析),然后会执行Toast里面如下方法:

        @Override
        public void show(IBinder windowToken) {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.obtainMessage(SHOW, windowToken).sendToTarget();
        }

发送一个Message,通知进行show的操作:

        @Override
        public void show(IBinder windowToken) {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.obtainMessage(SHOW, windowToken).sendToTarget();
        }

在Handler的handleMessage方法中找到了SHOW的case,接下来就要进行真正show的操作了:

        public void handleShow(IBinder windowToken) {
            if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
                    + " mNextView=" + mNextView);
            // If a cancel/hide is pending - no need to show - at this point
            // the window token is already invalid and no need to do any work.
            if (mHandler.hasMessages(CANCEL) || mHandler.hasMessages(HIDE)) {
                return;
            }
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                Context context = mView.getContext().getApplicationContext();
                String packageName = mView.getContext().getOpPackageName();
                if (context == null) {
                    context = mView.getContext();
                }
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                // We can resolve the Gravity here by using the Locale for getting
                // the layout direction
                final Configuration config = mView.getContext().getResources().getConfiguration();
                final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection());
                mParams.gravity = gravity;
                if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
                    mParams.horizontalWeight = 1.0f;
                }
                if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
                    mParams.verticalWeight = 1.0f;
                }
                mParams.x = mX;
                mParams.y = mY;
                mParams.verticalMargin = mVerticalMargin;
                mParams.horizontalMargin = mHorizontalMargin;
                mParams.packageName = packageName;
                mParams.hideTimeoutMilliseconds = mDuration ==
                    Toast.LENGTH_LONG ? LONG_DURATION_TIMEOUT : SHORT_DURATION_TIMEOUT;
                mParams.token = windowToken;
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }
                if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
                // Since the notification manager service cancels the token right
                // after it notifies us to cancel the toast there is an inherent
                // race and we may attempt to add a window after the token has been
                // invalidated. Let us hedge against that.
                try {
                    mWM.addView(mView, mParams);
                    trySendAccessibilityEvent();
                } catch (WindowManager.BadTokenException e) {
                    /* ignore */
                }
            }
        }

代码有点长,我们最需要关心的就是mWm.addView方法。相信看过ActivityThread的同学应该知道mWm.addView方法是在ActivityThread的handleResumeActivity里面也有调用过,意思就是进行ViewRootImpl的初始化,然后通过ViewRootImp进行View的测量,布局,以及绘制。
看到这里,我想到了一个可能的原因:那就是我的Toast是一个全局静态的Toast对象,然后第一次是在子线程的时候show出来,这个时候ViewRootImpl在初始化的时候就会将子线程的对象作为mThread,然后下一次在主线程弹出来就出错了吧?想想应该是这样的。

三、

所以继续做我的demo来印证我的想法:

    @Override
    protected void onResume() {
        super.onResume();
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                sToast = Toast.makeText(MainActivity.this,"子线程弹出Toast",Toast.LENGTH_SHORT);
                sToast.show();
                Looper.loop();
            }
        });
        thread.start();
    }

    public void click(View view) {
        sToast.setText("主线程弹出Toast");
        sToast.show();
    }

做了个静态的toast,然后点击按钮的时候弹出toast,运行一下:


主线程弹出toast.gif

发现竟然没问题,这时候又开始怀疑人生了,这到底怎么回事。ViewRootImpl此时的mThread应该是子线程啊,没道理还能正常运行,怎么办呢?debug一步一步调试吧,一步一步调试下来,发现在View的requestLayout里面parent竟然为空了:


image.png

然后在仔细看了下当前View是一个LinearLayout,然后这个View的子View是TextView,文本内容是"主线程弹出toast",所以应该就是Toast在new的时候inflate的布局
View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);

Android源码社区中搜索"transient_notification"找到了对应的toast布局文件,打开一看,果然如此:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="?android:attr/toastFrameBackground">

    <TextView
        android:id="@android:id/message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:layout_marginHorizontal="24dp"
        android:layout_marginVertical="15dp"
        android:layout_gravity="center_horizontal"
        android:textAppearance="@style/TextAppearance.Toast"
        android:textColor="@color/primary_text_default_material_light"
        />

</LinearLayout>

也就是说此时的View已经是顶级View了,它的parent应该就是ViewRootImpl,那么为什么ViewRootImpl是null呢,明明之前已经show过了。看来只能往Toast的hide方法找原因了

四、

所以重新回到Toast的类中,查看下Toast的hide方法(此处直接看Handler的hide处理,之前的操作与show类似):

public void handleHide() {
            if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
            if (mView != null) {
                // note: checking parent() just to make sure the view has
                // been added...  i have seen cases where we get here when
                // the view isn't yet added, so let's try not to crash.
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeViewImmediate(mView);
                }


                // Now that we've removed the view it's safe for the server to release
                // the resources.
                try {
                    getService().finishToken(mPackageName, this);
                } catch (RemoteException e) {
                }

                mView = null;
            }
        }

此处调用了mWm的removeViewImmediate,即WindowManagerImpl里面的removeViewImmediate方法:

    @Override
    public void removeViewImmediate(View view) {
        mGlobal.removeView(view, true);
    }

会调用WindowManagerGlobal的removeView方法:

public void removeView(View view, boolean immediate) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }

        synchronized (mLock) {
            int index = findViewLocked(view, true);
            View curView = mRoots.get(index).getView();
            removeViewLocked(index, immediate);
            if (curView == view) {
                return;
            }

            throw new IllegalStateException("Calling with view " + view
                    + " but the ViewAncestor is attached to " + curView);
        }
    }

然后调用removeViewLocked方法:

private void removeViewLocked(int index, boolean immediate) {
        ViewRootImpl root = mRoots.get(index);
        View view = root.getView();

        if (view != null) {
            InputMethodManager imm = InputMethodManager.getInstance();
            if (imm != null) {
                imm.windowDismissed(mViews.get(index).getWindowToken());
            }
        }
        boolean deferred = root.die(immediate);
        if (view != null) {
            //此处调用View的assignParent方法将viewParent置空
            view.assignParent(null);
            if (deferred) {
                mDyingViews.add(view);
            }
        }
    }

所以也就是说在Toast时间到了以后,会调用hide方法,此时会将parent置成空,所以我刚才试的时候才没有问题。那么按道理说只要在Toast没有关闭的时候点击再次弹出toast应该就会报错。
所以还是原来的代码,再来一次,这次不等Toast关闭,再次点击:


主线程弹出Toast崩溃.gif

果然如预期所料,此时在主线程弹出Toast就会崩溃。

五、

那么问题原因找到了:是在项目子线程中有弹出过Toast,然后Toast并没有关闭,又在主线程弹出了同一个对象的toast,会造成崩溃。此时内心有个困惑:如果是子线程弹出Toast,那我就需要写Looper.prepare方法和Looper.loop方法,为什么我自己一点印象都没有。于是我全局搜索了Looper.prepare,发现并没有找到对应的代码。所以我就全局搜索了Toast调用的地方,发现在JavaBridge的回调当中找到了:

    class JSInterface {
        @JavascriptInterface
        public void handleMessage(String msg) throws JSONException {
            LogHelper.e(TAG, "msg::" + msg);
            JSONObject jsonObject = new JSONObject(msg);
            String callType = jsonObject.optString(JS_CALL_TYPE);
            switch (callType) {
                ...代码省略..
                case JSCallType.SHOW_TOAST:
                    showToast(jsonObject);
                    break;
                default:
                    break;
            }
        }
    }

    /**
     * 弹出吐司
     * @param jsonObject
     * @throws JSONException
     */
    public void showToast(JSONObject jsonObject) throws JSONException {
        JSONObject payDataObj = jsonObject.getJSONObject("data");
        String message = payDataObj.optString("data");
        CommonToast.showShortToast(message);
    }

但是看到这段代码,又有疑问了,我并没有在Javabridge的回调中看到有任何准备Looper的地方,那么为什么Toast没有崩溃掉?
所以在此处加了一段代码:

    class JSInterface {
        @JavascriptInterface
        public void handleMessage(String msg) throws JSONException {
            LogHelper.e(TAG, "msg::" + msg);
            JSONObject jsonObject = new JSONObject(msg);
            String callType = jsonObject.optString(JS_CALL_TYPE);
            Thread currentThread = Thread.currentThread();
            Looper looper = Looper.myLooper();
            switch (callType) {
                ...代码省略..
                case JSCallType.SHOW_TOAST:
                    showToast(jsonObject);
                    break;
                default:
                    break;
            }
        }
    }

并且加了一个断点,来查看下此时的情况:


Thread和Looper.jpg

确实当前线程是JavaBridge线程,另外JavaBridge线程中已经提前给开发者准备好了Looper。所以也难怪一方面奇怪自己怎么没有写Looper的印象,一方面又很好奇为什么这个线程在开发者没有准备Looper的情况下也能正常弹出Toast。


总结

至此,真相终于找出来了。相比较发生这个bug 的原因,解决方案就显得非常简单了。只需要在CommonToast的showShortToast方法内部判断是否为主线程调用,如果不是的话,new一个主线程的Handler,将Toast扔到主线程弹出来。这样就会避免了子线程弹出。
PS:本人还得吐槽一下Android,Android官方一方面明明宣称不能在主线程以外的线程进行UI的更新,另一方面在初始化ViewRootImpl的时候又不把主线程作为成员变量保存起来,而是直接获取当前所处的线程作为mThread保存起来,这样做就有可能会出现子线程更新UI的操作。从而引起类似我今天的这个bug。

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