为什么是有可能的Andr​​oid 5（棒棒堂）上直​​接从其他线程改变UI的意见？ [英] Why is it possible on Android 5 (Lollipop) to directly change UI views from other threads?

问题描述

考虑以下code：

new Thread() {
    @Override
    public void run() {
        myTextView.setText("Some text");
    }
}.start();

在pre-棒棒糖机器人，即code使 CalledFromWrongThreadException 例外，但为什么它的工作原理罚款棒棒堂机器人？

On pre-Lollipop androids, that code causes the CalledFromWrongThreadException exception, but why does it work fine on Lollipop androids?

测试环境：Genymotion模拟器运行Android 5.1

Testing environment: Genymotion emulator running Android 5.1

在code是片段类的<​​code> onCreateView（）方法内。

The code was inside the onCreateView() method of the Fragment class.

推荐答案

这是个时机问题，例如中插入的onCreate您的code（）会的没有的崩溃对三星Galaxy应用程序S3也没有的Nexus 7 2013年的Andr​​oid 5.1。但是，如果修改了code，使得它不断更新的TextView的：

It's a matter of timing, for example inserting your code in onCreate() would not crash the app on Samsung Galaxy S3 nor Nexus 7 2013 on Android 5.1. However, if you modify the code such that it's constantly updating the TextView:

    new Thread() {
        @Override
        public void run() {
            int count = 0;
            while (true) {
                SystemClock.sleep(16);
                ((TextView) findViewById(R.id.test)).setText(count++ + "");
            }
        }
    }.start();

然后，它会崩溃在18〜呼叫，当 TextView.setText（字符串）无意中调用 View.requestLayout（）; 最终调用 ViewRootImpl.requestLayout（），实际上并检查正确的线程。 这可能是做是为了减少线程的检查，以最小的开销

Then it'll crash on ~18th call, when TextView.setText(String) inadvertently calls View.requestLayout(); which eventually calls ViewRootImpl.requestLayout() that actually does check for correct thread. This is probably done to reduce the overhead of thread checking to a minimum.

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