为什么能在子线程通过setText进行更新UI

void checkThread() {
        if (mThread != Thread.currentThread()) {
            throw new CalledFromWrongThreadException(
                    "Only the original thread that created a view hierarchy can touch its views.");
        }
    }

一般情况下在子线程更新UI是会报错的，因为在ViewRootImpl中会通过checkThread进行检查，在ViewRootImpl还没创建的时候是不会检查的，但这里主要是对已经创建ViewRootImpl还可以更新UI进行分析。

我们直接看下通过子线程setText不会报错的代码和运行结果。

public class MainActivity extends AppCompatActivity implements View.OnClickListener  {

    TextView text;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text= findViewById(R.id.text); 
       }
    @Override
    public void onClick(View v) {
       new Thread((Runnable)()->{
           text.setText("改变后");
       }).start();
    }

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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=".MainActivity"
    android:orientation="vertical">

    <TextView
        android:id="@+id/text"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:background="@color/colorAccent"
        android:text="改变前"
        />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        android:onClick="onClick"
        />

</LinearLayout>

我们能看到在点击后是能顺利进行更新UI的。关键点就在于 android:layout_width=“100dp” android:layout_height=“100dp”
我这里把视图大小进行了固定。出错肯定是触发了checkThread，但是只要不触发就可以顺利更新UI，而setText不是在任何情况下都会触发checkThread的。

在setText方法中会调用 checkForRelayout，而这个方法就展现了不会触发checkThread的原因。checkThread是在requestLayout中才会进行的，所有这里只要不调用requestLayout就不会进行checkThread。

通过下面的源代码就会发现不是所有情况都会调用requestLayout。在两个return那里都是没用调用requestLayout，所以我们可以知道只要View的大小不是wrap_content，而是match_parent或者是固定不变的dp，就都不会触发requestLayout，自然就可以在子线程更新了。

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
                || (mMaxWidthMode == mMinWidthMode && mMaxWidth == mMinWidth))
                && (mHint == null || mHintLayout != null)
                && (mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight() > 0)) {
            // Static width, so try making a new text layout.

            int oldht = mLayout.getHeight();
            int want = mLayout.getWidth();
            int hintWant = mHintLayout == null ? 0 : mHintLayout.getWidth();

            /*
             * No need to bring the text into view, since the size is not
             * changing (unless we do the requestLayout(), in which case it
             * will happen at measure).
             */
            makeNewLayout(want, hintWant, UNKNOWN_BORING, UNKNOWN_BORING,
                          mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight(),
                          false);

            if (mEllipsize != TextUtils.TruncateAt.MARQUEE) {
                // In a fixed-height view, so use our new text layout.
                if (mLayoutParams.height != LayoutParams.WRAP_CONTENT
                        && mLayoutParams.height != LayoutParams.MATCH_PARENT) {
                    autoSizeText();
                    invalidate();
                    return;
                }

                // Dynamic height, but height has stayed the same,
                // so use our new text layout.
                if (mLayout.getHeight() == oldht
                        && (mHintLayout == null || mHintLayout.getHeight() == oldht)) {
                    autoSizeText();
                    invalidate();
                    return;
                }
            }

            // 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();
        }
    }

总结：
1.调用requestLayout是因为内容需要新的视图布局，当不需要的情况下就不会调用。
2.在子线程对UI更新对是通过checkThread来防止在子线程更新。
3.当View更新时调用requestLayout才会触发CheckThred，只要不调用requestLayout就意味着可以在子线程进行更新。