【问题探究】setText在子线程更新UI未报错

前言

在使用okhttp3请求接口数据时，发现在call.enqueue回调的onResponse设置TextView.setText()正常，但使用Toast时报错：java.lang.RuntimeException: Can't toast on a thread that has not called Looper.prepare()

 public void testOkHttp3(String url) {
        OkHttpClient okHttpClient = new OkHttpClient();
        Request request = new Request.Builder().url(url).build();
        Call call = okHttpClient.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
               e.printStackTrace();
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                String s = response.body().string();
                //tv.setText(s);
                Toast.makeText(MainActivity.this, "onFailure", Toast.LENGTH_SHORT).show();
            }
        });
    }

onResponse和onFailure处于子线程？

OkHttp3有execute和enqueue两种请求

execute是同步请求，会直接在主线程中进行请求，因为android中主线程不能进行耗时操作，所以此时应该手动开启子线程进行请求，所以很少使用同步请求，不开启线程会发生ANR。
enqueue是异步请求，此时OkHttp3会创建子线程进行请求，并且将请求结果以Callback回调，然而此时并没有切换到主线程，所以onResponse和onFailure都还处于子线程中。

在onResponse中添加测试打印log：

if (Thread.currentThread() == Looper.getMainLooper().getThread()) {
      Log.e("onResponse", "在主线程:" + Thread.currentThread().getName());
 } else {
      Log.e("onResponse", "在子线程：" + Thread.currentThread().getName());
}
// 执行testOkHttp3("http://www.baidu.com");测试，打印信息如下
// E/onResponse: 在子线程：OkHttp http://www.baidu.com/...

上述代码说明了onResponse确实处于子线程（worker Thread）。

setText为什么不报异常？

没有搞明白onResponse处于子线程时，我直接写了setText来更新数据，没有报错，一度让我以为onResponse转到了主线程。

来看一下setText源码，其中更新UI是因为执行了

 private void setText(CharSequence text, BufferType type,
                         boolean notifyBefore, int oldlen) {
        mTextSetFromXmlOrResourceId = false;
        if (text == null) {
            text = "";
        }
        ...
        if (mLayout != null) {
            checkForRelayout();
        }
        ...
}

重点在于checkForRelayout()，接着看一下源码：

    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
                || (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();
        }
    }

主要是讲是否需要去获取新的View来更新布局。request a new view layout with a new text layout.，这时，会执行requestLayout()。
其中有两个return，直接使用了之前的View，之后讲text layout一下。不会执行requestLayout()。

固定的宽高；
固定的宽，动态计算后高度没变化。

View的requestLayout()

了解WindowManagerService后知道，View的添加，更新，删除操作是由WindowManager管理着，ViewRootImpl是View中的最高层级，属于所有View的根，实现了View和WindowManager之间的通信协议，实现的具体细节在WindowManagerGlobal这个类当中。

简单理解一下View和ViewRootImple关系，那就是，View触发布局请求，在此过程中会调用ViewRootImpl的requestLayout重新进行测量、布局、绘制。

    @Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            scheduleTraversals();
        }
    }

其中checkThread()用来检查是否为主线程。

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

总结

setText可以在子线程中更新，但必须布局宽高没发生变化。
UI更新只能在主线程中执行，是因为ViewRootImpl会执行checkThread()来检查线程

参考：
https://blog.csdn.net/m0_46496806/article/details/105356056
https://blog.csdn.net/stven_king/article/details/78775166