Android里子线程真的不能刷新UI吗？实例证明

原文链接：Android里子线程真的不能刷新UI吗？
原文写的很优秀，格式不敢恭维，示例代码也有bug，博主也一直没有更新，所以我改了下格式，并且修改了子线程刷新UI的示例代码！

如果你在网上搜索：

CalledFromWrongThreadException：
Only the original thread that created a view hierarchy can touch its views. 

那么你肯定能看到很多文章说android里子线程不能刷新UI。这句话不能说错，只是有些不太严谨。其实线程能否刷新UI的关键在于ViewRoot是否属于该线程。

让我们一起看看代码吧！
首先，CalledFromWrongThreadException这个异常是有下面的代码抛出的：

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

该段代码出自 framework/base/core/java/android/view/ViewRoot.java
其次，看看RootView的构造函数：

    public ViewRoot(Context context) {
        super();
        if (MEASURE_LATENCY && lt == null) {
            lt = new LatencyTimer(100, 1000);
        }
        // For debug only
        //++sInstanceCount;
        // Initialize the statics when this class is first instantiated. This is
        // done here instead of in the static block because Zygote does not
        // allow the spawning of threads.
        getWindowSession(context.getMainLooper());
        mThread = Thread.currentThread();
        mLocation = new WindowLeaked(null);
        mLocation.fillInStackTrace();
        mWidth = -1;
        mHeight = -1;
        mDirty = new Rect();
        mTempRect = new Rect();
        mVisRect = new Rect();
        mWinFrame = new Rect();
        mWindow = new W(this, context);
        mInputMethodCallback = new InputMethodCallback(this);
        mViewVisibility = View.GONE;
        mTransparentRegion = new Region();
        mPreviousTransparentRegion = new Region();
        mFirst = true; // true for the first time the view is added
        mAdded = false;
        mAttachInfo = new View.AttachInfo(sWindowSession, mWindow, this, this);
        mViewConfiguration = ViewConfiguration.get(context);
        mDensity = context.getResources().getDisplayMetrics().densityDpi;
    }

最后，我们看看ViewRoot.checkThread的调用顺序：

com.david.test.helloworld.MainActivity$TestThread2.run

  -> android.widget.TextView.setText

    -> android.widget.TextView.checkForRelayout

      -> android.view.View.invalidate

        -> android.view.ViewGroup.invalidateChild

          -> android.view.ViewRoot.invalidateChildInParent

            -> android.view.ViewRoot.invalidateChild

              -> android.view.ViewRoot.checkThread

到这里相信网友已经明白CalledFromWrongThreadException为什么出现了。那到底非主线程以外的线程能否刷新UI呢？呵呵，答案当然是能，前提条件是它要拥有自己的ViewRoot。如果你要直接创建ViewRoot的实例的话，你会失望的发现不能找到这个类。那么我们要如何做呢？让我们用实例来说说吧，代码如下：（运行环境：Android studio 3.4.0 sdk 28 测试机：vivo 6.0.1，不需要在AndroidManifest.xml加任何权限）

 public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        new TestThread1().start();
    }
    
    class TestThread1 extends Thread {
        @Override
        public void run() {
            Looper.prepare();
            TextView tx = new TextView(MainActivity.this);
            tx.setText("test11111111111111111");
            WindowManager wm = MainActivity.this.getWindowManager();
            WindowManager.LayoutParams params = new WindowManager.LayoutParams(
                    250, 250, 200, 200,
                    WindowManager.LayoutParams.FIRST_SUB_WINDOW,
                    WindowManager.LayoutParams.TYPE_TOAST, PixelFormat.OPAQUE
            );

            Dialog dialog = new Dialog(MainActivity.this);
            dialog.addContentView(tx, params);
            dialog.show();
            Looper.loop();
        }
    }
}

MainActivity是建立android工程时生成的入口类，TestThread1是MainActivity的内部类。感兴趣的话，试试吧！看看是不是在屏幕上看到了"test11111111111111111"？

最后，说说那里创建了ViewRoot，这里：wm.addView(tx, params)。还是看看具体流程吧：

WindowManagerImpl.addView(View view, ViewGroup.LayoutParams params)

-> WindowManagerImpl.addView(View view, ViewGroup.LayoutParams params, boolean nest)，奥妙就在这里，具体看看代码吧！

    private void addView(View view, ViewGroup.LayoutParams params, boolean nest){
        if (Config.LOGV) Log.v("WindowManager", "addView view=" + view);
        if (!(params instanceof WindowManager.LayoutParams)) {
            throw new IllegalArgumentException(
                    "Params must be WindowManager.LayoutParams");
        }
        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;  
        ViewRoot root;
        View panelParentView = null;    
        synchronized (this) {
            // Here's an odd/questionable case: if someone tries to add a
            // view multiple times, then we simply bump up a nesting count
            // and they need to remove the view the corresponding number of
            // times to have it actually removed from the window manager.
            // This is useful specifically for the notification manager,
            // which can continually add/remove the same view as a
            // notification gets updated.
            int index = findViewLocked(view, false);
            if (index >= 0) {
                if (!nest) {
                    throw new IllegalStateException("View " + view
                            + " has already been added to the window manager.");
                }
                root = mRoots[index];
               root.mAddNesting++;
                // Update layout parameters.
                view.setLayoutParams(wparams);
                root.setLayoutParams(wparams, true);
                return;
            }           
            // If this is a panel window, then find the window it is being
            // attached to for future reference.
            if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
                    wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
                final int count = mViews != null ? mViews.length : 0;
                for (int i=0; i<count; i++) {
                    if (mRoots[i].mWindow.asBinder() == wparams.token) {
                        panelParentView = mViews[i];
                    }
                }
            }     
               
            root = new ViewRoot(view.getContext());
            root.mAddNesting = 1;
            view.setLayoutParams(wparams);
          
            if (mViews == null) {
                index = 1;
                mViews = new View[1];
                mRoots = new ViewRoot[1];
                mParams = new WindowManager.LayoutParams[1];
            } else {
                index = mViews.length + 1;
                Object[] old = mViews;
                mViews = new View[index];
                System.arraycopy(old, 0, mViews, 0, index-1);
                old = mRoots;
                mRoots = new ViewRoot[index];
                System.arraycopy(old, 0, mRoots, 0, index-1);
                old = mParams;
                mParams = new WindowManager.LayoutParams[index];
                System.arraycopy(old, 0, mParams, 0, index-1);
            }
            index--;
            mViews[index] = view;
            mRoots[index] = root;
           mParams[index] = wparams;
        }
        // do this last because it fires off messages to start doing things
        root.setView(view, wparams, panelParentView);
}

出自：frameworks/base/core/java/android/view/WindowManagerImpl.java

Ok，相信到了这里，大家都已经明白了：子线程是能够刷新UI的！！！

本文地址：https://blog.csdn.net/zhangjin1120/article/details/110213639