Android技术分享|Android踩坑怀疑人生，主线程修改UI也会崩溃?
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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：
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：
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：
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"
/>
</LinearLa以上是关于Android技术分享|Android踩坑怀疑人生，主线程修改UI也会崩溃?的主要内容，如果未能解决你的问题，请参考以下文章
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