如题，Android能否在子线程中更新UI呢？这是一道面试题。那么这道题应该怎么去回答呢？在此我给出个人答案：“Android是不允许在子线程中更新UI的，但是在某种特殊情况下子线程是可以更新UI的”。为什么这么说呢？下面我们来看一个例子：
MainActivity
package example.lc.com.uicheckdemo;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private TextView text;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
text= (TextView) findViewById(R.id.text);
new Thread(new Runnable() {
@Override
public void run() {
text.setText("在子线程中更新的UI");
}
}).start();
}
}activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="example.lc.com.uicheckdemo.MainActivity">
<TextView
android:layout_centerInParent="true"
android:id="@+id/text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!" />
</RelativeLayout>例子很简单就是一个Activity和一个布局，在onCreate中写了一个Thread并且在子线程中更新了UI，但是程序不会报错，不信大家可以去试一下。但是，将代码做如下修改：
MainActivity
package example.lc.com.uicheckdemo;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
import org.w3c.dom.Text;
public class MainActivity extends AppCompatActivity {
private TextView text;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
text= (TextView) findViewById(R.id.text);
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
text.setText("在子线程中更新的UI");
}
}).start();
}
}再次运行程序就会崩溃，logcat报错如下：
01-07 18:21:11.207 1476-2957/example.lc.com.uicheckdemo E/AndroidRuntime: FATAL EXCEPTION: Thread-96 Process: example.lc.com.uicheckdemo, PID: 1476 android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6118) at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:848) at android.view.View.requestLayout(View.java:16431) at android.view.View.requestLayout(View.java:16431) at android.view.View.requestLayout(View.java:16431) at android.view.View.requestLayout(View.java:16431) at android.view.View.requestLayout(View.java:16431) at android.view.View.requestLayout(View.java:16431) at android.widget.RelativeLayout.requestLayout(RelativeLayout.java:352) at android.view.View.requestLayout(View.java:16431) at android.widget.TextView.checkForRelayout(TextView.java:6600) at android.widget.TextView.setText(TextView.java:3813) at android.widget.TextView.setText(TextView.java:3671) at android.widget.TextView.setText(TextView.java:3646) at example.lc.com.uicheckdemo.MainActivity$1$override.run(MainActivity.java:25) at example.lc.com.uicheckdemo.MainActivity$1$override.access$dispatch(MainActivity.java) at example.lc.com.uicheckdemo.MainActivity$1.run(MainActivity.java:0) at java.lang.Thread.run(Thread.java:841)那么这到底是为什么呢？这里我们提出问题，本文下面所有的内容都是要去解答这个问题的，接下来就正式开始我们今天的探索。
**
从错误信息入手寻找问题的根源
**
从logcat显示的信息中我们发现了很有用的几行：
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6118) at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:848)这几行信息直接告诉了我们错误是从ViewRootImpl的checkThread方法中报出来的，那么我们就去证实一下，打开checkThread方法：
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}是的错误就是这个方法报出的，mThread指的是UI线程而Thread.currentThread()则是当前线程。由此，我们知道了Android中更新UI的时候检查线程的操作是在ViewRootImpl中进行的。到这里我们还是没有办法去解答我们提出的问题，我们接着分析。
ViewRootImpl是何时创建的？
想要知道ViewRootImpl是在哪里创建的，我们要先要找到handleResumeActivity()方法，这个方法在ActivityThread中，代码如下：
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume) {
//代码省略
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
wm.addView(decor, l);
}
//代码省略
}在上面代码中，有两个需要我们注意的地方。第一个就是 View decor = r.window.getDecorView();，这个decor 就是我们熟知的DecorView，但是这不是我们今天研究的重点。第二个是 ViewManager wm = a.getWindowManager();，这个才是今天真正的重点。ViewManager是一个接口其定义了addView、updateViewLayout、removeView三个方法，a.getWindowManager是一个WindowManager，WindowManager也是一个接口并且继承了ViewManager，也就是说wm 是一个WindowManager。WindowManager是一个借口它的实现类是WindowManagerImpl， 也就是说wm.addView(decor, l);这句话调用的是WindowManagerImpl中的addView方法，我们看下WindowManagerImpl代码：
package android.view;
import android.annotation.NonNull;
import android.os.IBinder;
public final class WindowManagerImpl implements WindowManager {
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
private final Display mDisplay;
private final Window mParentWindow;
private IBinder mDefaultToken;
public WindowManagerImpl(Display display) {
this(display, null);
}
//代码省略
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);
}
//代码省略
}看到在WindowManagerImpl 的addView方法中，又调用了WindowManagerGlobal的addView方法，接着打开WindowManagerGlobal类，代码如下：
package android.view;
//代码省略
public final class WindowManagerGlobal {
private static final String TAG = "WindowManager";
//代码省略
private Runnable mSystemPropertyUpdater;
private WindowManagerGlobal() {
}
public static void initialize() {
getWindowManagerService();
}
public static WindowManagerGlobal getInstance() {
synchronized (WindowManagerGlobal.class) {
if (sDefaultWindowManager == null) {
sDefaultWindowManager = new WindowManagerGlobal();
}
return sDefaultWindowManager;
}
}
//代码省略
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
} else {
// If there's no parent, then hardware acceleration for this view is
// set from the application's hardware acceleration setting.
final Context context = view.getContext();
if (context != null
&& (context.getApplicationInfo().flags
& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// Start watching for system property changes.
if (mSystemPropertyUpdater == null) {
mSystemPropertyUpdater = new Runnable() {
@Override public void run() {
synchronized (mLock) {
for (int i = mRoots.size() - 1; i >= 0; --i) {
mRoots.get(i).loadSystemProperties();
}
}
}
};
SystemProperties.addChangeCallback(mSystemPropertyUpdater);
}
int index = findViewLocked(view, false);
if (index >= 0) {
if (mDyingViews.contains(view)) {
// Don't wait for MSG_DIE to make it's way through root's queue.
mRoots.get(index).doDie();
} else {
throw new IllegalStateException("View " + view
+ " has already been added to the window manager.");
}
// The previous removeView() had not completed executing. Now it has.
}
// If this is a panel window, then find the window it is being
// attached to for future reference.
if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
final int count = mViews.size();
for (int i = 0; i < count; i++) {
if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
panelParentView = mViews.get(i);
}
}
}
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
// do this last because it fires off messages to start doing things
try {
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
//代码省略
}以上是WindowManagerGlobal类，只给出了addView方法其他代码省略了。我们仔细的来看下addView方法，再把无用的代码省去，结果如下：
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
//代码省略
ViewRootImpl root;
//代码省略
int index = findViewLocked(view, false);
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
//代码省略
}好了代码省去的比较多，不过我们终于找到了我们想要的代码了。
root = new ViewRootImpl(view.getContext(), display);
到这ViewRootImpl就创建完了！也就是说ViewRootImpl是在WindowManagerGlobal中的addView方法中创建的！
那么现在我们来尝试回答一下我们提出的问题。合理的说法是让线程睡眠200ms以后再次醒来此时onResume方法已经被调用ViewRootImpl已经创建完成，此时可以检查线程了。
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