引子：
情形1
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView textView = findViewById(R.id.home_tv);
ImageView imageView = findViewById(R.id.home_img);
new Thread(new Runnable() {
@Override
public void run() {
textView.setText("更新TextView");
imageView.setImageResource(R.drawable.img);
}
}).start();
}
复制代码运行结果：正常运行！！！
情形二
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView textView = findViewById(R.id.home_tv);
ImageView imageView = findViewById(R.id.home_img);
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
textView.setText("更新TextView");
imageView.setImageResource(R.drawable.img);
}
}).start();
}
复制代码运行结果：异常
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6357)
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:874)
at android.view.View.requestLayout(View.java:17476)
at android.view.View.requestLayout(View.java:17476)
at android.view.View.requestLayout(View.java:17476)
at android.view.View.requestLayout(View.java:17476)
at android.view.View.requestLayout(View.java:17476)
at android.view.View.requestLayout(View.java:17476)
at android.widget.RelativeLayout.requestLayout(RelativeLayout.java:360)
at android.view.View.requestLayout(View.java:17476)
at android.widget.TextView.checkForRelayout(TextView.java:6871)
at android.widget.TextView.setText(TextView.java:4057)
at android.widget.TextView.setText(TextView.java:3915)
at android.widget.TextView.setText(TextView.java:3890)
at com.dong.demo.MainActivity$1.run(MainActivity.java:44)
at java.lang.Thread.run(Thread.java:818)
复制代码不是说，子线程不能更新UI吗，为什么情形一可以正常运行，情形二不能正常运行呢；
1.子线程修改UI出现异常，与什么方法有关
1.子线程什么时候可以更新UI
2.Android为什么要规定主线程更新UI
一、子线程修改UI出现异常，与什么方法有关
首先从出现异常的log日志入手，发现出现异常的方法调用顺序如下：
TextView.setText(TextView.java:4057)
TextView.checkForRelayout(TextView.java:6871)
View.requestLayout(View.java:17476)
RelativeLayout.requestLayout(RelativeLayout.java:360)
View.requestLayout(View.java:17476)
ViewRootImpl.requestLayout(ViewRootImpl.java:874)
ViewRootImpl.checkThread(ViewRootImpl.java:6357)
更改ImageView时，出现的异常类似；
首先看TextView.setText()方法的源码
private void setText(CharSequence text, BufferType type,
boolean notifyBefore, int oldlen) {
//省略其他代码
if (mLayout != null) {
checkForRelayout();
}
sendOnTextChanged(text, 0, oldlen, textLength);
onTextChanged(text, 0, oldlen, textLength);
//省略其他代码
复制代码然后，查看以下checkForRelayout()方法的与源码。
private void checkForRelayout() {
// If we have a fixed width, we can just swap in a new text layout
// if the text height stays the same or if the view height is fixed.
if ((mLayoutParams.width != LayoutParams.WRAP_CONTENT
//省略代码
// We lose: the height has changed and we have a dynamic height.
// Request a new view layout using our new text layout.
requestLayout();
invalidate();
} else {
// Dynamic width, so we have no choice but to request a new
// view layout with a new text layout.
nullLayouts();
requestLayout();
invalidate();
}
}
复制代码checkForReLayout方法，首先会调用需要改变的View的requestLayout方法，然后执行invalidate()重绘操作；
TextView没有重写requestLayout方法，requestLayout方法由View实现；
查看RequestLayout方法的源码：
public void requestLayout() {
//省略其他代码
if (mParent != null && !mParent.isLayoutRequested()) {
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}
复制代码View获取到父View（类型是ViewParent，ViewPaerent是个接口，requestLayout由子类来具体实现），mParent，然后调用父View的requestLayout方法，比如示例中的父View就是xml文件的根布局就是RelativeLayout。 RelativeLayout的requestLayout()方法源码：
@Override
public void requestLayout() {
super.requestLayout();
mDirtyHierarchy = true;
}
复制代码继续跟踪super.requestLayout()方法，即ViewGroup没有重新，即调用的是View的requestLayout方法。
经过一系列的调用ViewParent的requestLayout方法，最终调用到ViewRootImp的requestLayout方法。ViewRootImp实现了ViewParent接口，继续查看ViewRootImp的requestLayout方法源码。
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
复制代码ViewRootImp的requestLayout方法中有两个方法：
一、checkThread，检查线程，源码如下
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
复制代码判断当前线程，是否是创建ViewRootImp的线程，而创建ViewRootImp的线程就是主线程，当前线程不是主线程的时候，就抛出异常。 二、scheduleTraversals(),查看源码：
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
复制代码查看mTraversalRunnable中run()方法的具体操作
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
复制代码继续追踪doTraversal()方法
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
复制代码查看到performTraversals()方法，熟悉了吧，这是View绘制的起点。
总结一下：
1.Android更新UI会调用View的requestLayout()方法，在requestLayout方法中，获取ViewParent，然后调用ViewParent的requestLayout()方法，一直调用下去，直到调用到ViewRootImp的requestLayout方法；
2.ViewRootImp的requetLayout方法，主要有两部操作一个是checkThread()方法，检测线程，一个是scheduleTraversals，执行绘制相关工作；