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Android技术分享|【Android踩坑】怀疑人生,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、Demo 验证


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:


在这里插入图片描述


那么问题就来了,为什么会在友盟中出现这个崩溃呢?


二、再探堆栈


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)
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在之前有详细说过,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、再探Demo


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:


在这里插入图片描述


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:


在这里插入图片描述


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);

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、深入源码


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:


在这里插入图片描述


果然如预期所料,此时在主线程弹出Toast就会崩溃。


五、发现原因


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:


在这里插入图片描述


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。


作者:anyRTC
链接:https://juejin.cn/post/7130824794060587016
来源:稀土掘金
著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。

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