我们初学Android的时候就知道“子线程里面是不能直接更新UI的”，那么真的是如此吗?我们来看下面这段代码
public class MainActivity extends AppCompatActivity {
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.tv);
new Thread(new Runnable() {
@Override
public void run() {
tv.setText("我是子线程，我更新UI了");
}
}).start();
}
}
布局文件很简单，里面就只有一个TextView。这段代码在子线程里面更新了UI，理论上应该是会报错的。让我们来看一看实际的运行结果：
居然没有报错，TextView也显示出来为我们设置的值。惊不惊喜？意不意外？
分析
子线程为啥不能更新UI?
子线程不能更新UI，这句基本上没错的，但也有例外情况，我们上面这个例子就是。不过我们还得先分析下在这绝大多数情况下，子线程为啥不能更新UI，然后再来分析在这极少数例外的情况下，子线程为啥可以更新UI
1、设计层面上
Android系统为啥不允许子线程中更新UI呢?这是因为Android的UI控件不是线程安全的。如果多线程中并发访问可能会导致UI控件处于不可预期的状态。你可能会问为啥不对UI控件加锁呢?因为加锁会导致UI访问的逻辑变得复杂，同时会降低UI访问的效率，锁机制会阻塞某些线程的执行。鉴于以上问题。最简单高效的办法就是采用单线程来处理UI操作，我们只需要用Handler来切换一下线程就可以了。
2、代码层面上
下面这段代码是ViewRootImpl的一个方法，其实我们在调用UI控件
setText等更新UI的方法时，会调用到ViewRootImpl的这个方法，这个方法就是去检查当前线程是不是主线程(mThread是主线程)，只有那么几行代码而已的，如果当前线程不是主线程，就会抛出异常。这也就是子线程不能更新UI的代码层面上的原因?
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
然而我们在子线程更新UI为什么不报错？
那么出现上面子线程更新UI居然不报错的原因是啥呢?不是说子线程不能更新UI吗?当访问UI时，ViewRootImpl会调用checkThread方法去检查当前访问UI的线程是哪个，如果不是UI线程则会抛出异常，这是没问题的。但是为什么一开始在MainActivity的onCreate方法中创建一个子线程访问UI，程序还是正常能跑起来呢？唯一的解释就是执行onCreate方法的那个时候ViewRootImpl还没创建，无法去检查当前线程。有了这个想法，那么我们就去验证下，这个想法是否正确，怎么验证呢?当然是看ViewRootImpl实在何处创建的。
我们可以找到ActivityThread的handleResumeActivity方法
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// TODO Push resumeArgs into the activity for consideration
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
if (localLOGV) Slog.v(
TAG, "Resume " + r + " started activity: " +
a.mStartedActivity + ", hideForNow: " + r.hideForNow
+ ", finished: " + a.mFinished);
final int forwardBit = isForward ?
WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
// If the window hasn't yet been added to the window manager,
// and this guy didn't finish itself or start another activity,
// then go ahead and add the window.
boolean willBeVisible = !a.mStartedActivity;
if (!willBeVisible) {
try {
willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible(
a.getActivityToken());
} catch (RemoteException e) {
}
}
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);
}
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// Get rid of anything left hanging around.
cleanUpPendingRemoveWindows(r);
// The window is now visible if it has been added, we are not
// simply finishing, and we are not starting another activity.
if (!r.activity.mFinished && willBeVisible
&& r.activity.mDecor != null && !r.hideForNow) {
if (r.newConfig != null) {
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "
+ r.activityInfo.name + " with newConfig " + r.newConfig);
performConfigurationChanged(r.activity, r.newConfig);
freeTextLayoutCachesIfNeeded(r.activity.mCurrentConfig.diff(r.newConfig));
r.newConfig = null;
}
if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="
+ isForward);
WindowManager.LayoutParams l = r.window.getAttributes();
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
l.softInputMode = (l.softInputMode
& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
| forwardBit;
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
r.activity.mVisibleFromServer = true;
mNumVisibleActivities++;
if (r.activity.mVisibleFromClient) {
r.activity.makeVisible();//第一处
}
}
if (!r.onlyLocalRequest) {
r.nextIdle = mNewActivities;
mNewActivities = r;
if (localLOGV) Slog.v(
TAG, "Scheduling idle handler for " + r);
Looper.myQueue().addIdleHandler(new Idler());
}
r.onlyLocalRequest = false;
// Tell the activity manager we have resumed.
if (reallyResume) {
try {
ActivityManagerNative.getDefault().activityResumed(token);
} catch (RemoteException ex) {
}
}
} else {
// If an exception was thrown when trying to resume, then
// just end this activity.
try {
ActivityManagerNative.getDefault()
.finishActivity(token, Activity.RESULT_CANCELED, null, false);
} catch (RemoteException ex) {
}
}
}
其实这段代码中会调Activity的onResume方法，从方法名上也可以看出来。这段代码很长，我们只需要看到我标注为第一处的那段代码。也就是下面这句。
r.activity.makeVisible();
我们跟进去看看
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}
ViewManager中添加DecorView，那现在应该关注的就是ViewManager的addView方法了。而ViewManager是一个接口来的，我们应该找到ViewManager的实现类才行，而ViewManager的实现类是WindowManagerImpl。
找到了WindowManagerImpl的addView方法，如下：
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);//第三行
}
然后第三行又调用了WindowManagerGlobal的addView方法，咱们继续跟进去看看.
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
……省略代码
ViewRootImpl root;//第1处
View panelParentView = null;
……省略代码
root = new ViewRootImpl(view.getContext(), display);//第2处
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
……省略代码
}
看到第1,2处的代码木有，ViewRootImpl在这里创建了。也就是说ViewRootImpl实在Activity的onResume方法之后才调用的。onCreate方法时ViewRootImpl还没有创建，自然没办法检查线程，也就不会报错。
再次尝试
既然知道了结论，那么我们再试一下，这次我们在更新UI之前先sleep500毫秒，让它有时间执行到onResume，创建ViewRootImpl。代码如下：
public class MainActivity extends AppCompatActivity {
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.tv);
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(500);
tv.setText("我是子线程，我更新UI了");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
}
结果如下，正如我们所想报错了
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7357)
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1099)
