当新线程立即操作UI时，为什么没有CalledFromWrongThreadException? [英] Why is there no CalledFromWrongThreadException when a new thread operates UI immediately?

问题描述

我试图将一个视图动态添加到MainActivity中的另一个视图中:

I was trying to dynamically add a view in to another view in MainActivity:

activity_main.xml

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout.xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.aronho.testfragment.MainActivity">
<FrameLayout
    android:id="@+id/myView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

</android.support.constraint.ConstraintLayout>

view_test.xml

view_test.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#FFFF00">
<TextView
    android:text="testString"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
</LinearLayout>

我知道当我启动应用程序时，此代码将引发CalledFromWrongThreadException:

I know this code will throw CalledFromWrongThreadException when I start the app:

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        LayoutInflater vi = (LayoutInflater) getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        final View view=vi.inflate(R.layout.view_test,null);
        final FrameLayout parentView=(FrameLayout) findViewById(R.id.myView);
        parentView.addView(view);

        new Thread(new Runnable(){
            @Override
            public void run() {
                try{
                    Thread.sleep(3000);
                }catch (Exception e){
                }
                parentView.removeView(view);
            }
        }).start();
    }
}

但是当我删除Thread.sleep(3000);时，例如:

but when I remove the Thread.sleep(3000);,eg:

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        LayoutInflater vi = (LayoutInflater) getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        final View view=vi.inflate(R.layout.view_test,null);
        final FrameLayout parentView=(FrameLayout) findViewById(R.id.myView);
        parentView.addView(view);

        new Thread(new Runnable(){
            @Override
            public void run() {
                parentView.removeView(view);
            }
        }).start();
    }
}

尽管我尝试使用新线程删除视图，但该应用程序不会引发CalledFromWrongThreadException.为什么会发生这种情况?

the app doesn't throw CalledFromWrongThreadException despite I try to remove a view using a new thread. Why would that happen?

推荐答案

崩溃的根本原因是 ViewRootImpl.checkThread()方法.

The root cause of the crash is the ViewRootImpl.checkThread () method.

void checkThread() {

 if (mThread != Thread.currentThread()) {

  throw new CalledFromWrongThreadException( "Only the original thread that created a view hierarchy can touch its views.");

}

但是，当未测量视图时，不会调用 invalidate()方法， invalidate()方法最终将执行到 checkThread().

However, the invalidate () method is not called when the view is not measured , the invalidate() method will eventually execute to the checkThread().

好，请参阅:

    new Thread(new Runnable() {
        @Override
        public void run() {
            System.err.println("getMeasuredHeight:" + parentView.getMeasuredHeight());
            System.err.println("getMeasuredWidth:" + parentView.getMeasuredWidth());
            parentView.removeView(view);
        }
    }).start();

将会得到:

W/System.err: getMeasuredWidth:0
W/System.err: getMeasuredHeight:0

可以看到，测量过程尚未完成，因此我们可以在不触发 invalidate()且不触发异常的情况下更改UI.

can see, the measurement process has not been completed , so we can change the UI without triggering the invalidate() and not trigger exception.

然后，让它保持睡眠状态50毫秒:

then, keep it sleep 50 ms:

    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.err.println("getMeasuredHeight:" + parentView.getMeasuredHeight());
            System.err.println("getMeasuredWidth:" + parentView.getMeasuredWidth());
            parentView.removeView(view);
        }
    }).start();

将会得到:

W/System.err: getMeasuredHeight:360
W/System.err: getMeasuredWidth:1080

可以看到，测量过程已经完成，然后，当我们更改UI时，将触发 invalidate ，然后调用 checkThread().

can see, the measurement process has been completed, then , when we change the UI the invalidate will be triggering,and then it call the checkThread().

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