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如題，Android能否在子線程中更新UI呢？這是一道面試題。那么這道題應該怎么去回答呢？在此我給出個人答案：“Android是不允許在子線程中更新UI的，但是在某種特殊情況下子線程是可以更新UI的”。為什么這么說呢？下面我們來看一個例子：

MainActivity

package example.lc.com.uicheckdemo;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private TextView text;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text= (TextView) findViewById(R.id.text);
        new Thread(new Runnable() {
            @Override
            public void run() {
                text.setText("在子線程中更新的UI");
            }
        }).start();

    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="example.lc.com.uicheckdemo.MainActivity">

    <TextView
        android:layout_centerInParent="true"
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />
</RelativeLayout>

例子很簡單就是一個Activity和一個布局，在onCreate中寫了一個Thread并且在子線程中更新了UI，但是程序不會報錯，不信大家可以去試一下。但是，將代碼做如下修改：
MainActivity

package example.lc.com.uicheckdemo;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private TextView text;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text= (TextView) findViewById(R.id.text);
        new Thread(new Runnable() {
            @Override
            public void run() {
               try { 
                   Thread.sleep(200); 
              } catch (InterruptedException e) 
              { 
               e.printStackTrace(); 
                  }
                text.setText("在子線程中更新的UI");
            }
        }).start();

    }
}

再次運行程序就會崩潰，logcat報錯如下：

01-07 18:21:11.207 1476-2957/example.lc.com.uicheckdemo E/AndroidRuntime: FATAL EXCEPTION: Thread-96 Process: example.lc.com.uicheckdemo, PID: 1476 android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6118) 
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:848) 
at android.view.View.requestLayout(View.java:16431) 
at android.view.View.requestLayout(View.java:16431) 
at android.view.View.requestLayout(View.java:16431) 
at android.view.View.requestLayout(View.java:16431)
at android.view.View.requestLayout(View.java:16431)
at android.view.View.requestLayout(View.java:16431) 
at android.widget.RelativeLayout.requestLayout(RelativeLayout.java:352)
at android.view.View.requestLayout(View.java:16431) 
at android.widget.TextView.checkForRelayout(TextView.java:6600) 
at android.widget.TextView.setText(TextView.java:3813) 
at android.widget.TextView.setText(TextView.java:3671) 
at android.widget.TextView.setText(TextView.java:3646)
at example.lc.com.uicheckdemo.MainActivity$1$override.run(MainActivity.java:25) 
at example.lc.com.uicheckdemo.MainActivity$1$override.access$dispatch(MainActivity.java) 
at example.lc.com.uicheckdemo.MainActivity$1.run(MainActivity.java:0) 
at java.lang.Thread.run(Thread.java:841)

那么這到底是為什么呢？這里我們提出問題，本文下面所有的內容都是要去解答這個問題的，接下來就正式開始我們今天的探索。

**
從錯誤信息入手尋找問題的根源
**
從logcat顯示的信息中我們發現了很有用的幾行：

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. 
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6118) 
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:848)

這幾行信息直接告訴了我們錯誤是從ViewRootImpl的checkThread方法中報出來的，那么我們就去證實一下，打開checkThread方法：

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

是的錯誤就是這個方法報出的，mThread指的是UI線程而Thread.currentThread()則是當前線程。由此，我們知道了Android中更新UI的時候檢查線程的操作是在ViewRootImpl中進行的。到這里我們還是沒有辦法去解答我們提出的問題，我們接著分析。

ViewRootImpl是何時創建的？

想要知道ViewRootImpl是在哪里創建的，我們要先要找到handleResumeActivity()方法，這個方法在ActivityThread中，代碼如下：

 final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume) {

            //代碼省略
            if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (a.mVisibleFromClient) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }
           //代碼省略
    }

在上面代碼中，有兩個需要我們注意的地方。第一個就是 View decor = r.window.getDecorView();，這個decor 就是我們熟知的DecorView，但是這不是我們今天研究的重點。第二個是 ViewManager wm = a.getWindowManager();，這個才是今天真正的重點。ViewManager是一個接口其定義了addView、updateViewLayout、removeView三個方法，a.getWindowManager是一個WindowManager，WindowManager也是一個接口并且繼承了ViewManager，也就是說wm 是一個WindowManager。WindowManager是一個借口它的實現類是WindowManagerImpl，也就是說wm.addView(decor, l);這句話調用的是WindowManagerImpl中的addView方法，我們看下WindowManagerImpl代碼：

package android.view;

import android.annotation.NonNull;
import android.os.IBinder;

public final class WindowManagerImpl implements WindowManager {
    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
    private final Display mDisplay;
    private final Window mParentWindow;

    private IBinder mDefaultToken;

    public WindowManagerImpl(Display display) {
        this(display, null);
    }
 //代碼省略
    @Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mDisplay, mParentWindow);
    }
 //代碼省略
}

看到在WindowManagerImpl 的addView方法中，又調用了WindowManagerGlobal的addView方法，接著打開WindowManagerGlobal類，代碼如下：

package android.view;
 //代碼省略
public final class WindowManagerGlobal {
    private static final String TAG = "WindowManager";

 //代碼省略

    private Runnable mSystemPropertyUpdater;

    private WindowManagerGlobal() {
    }

    public static void initialize() {
        getWindowManagerService();
    }

    public static WindowManagerGlobal getInstance() {
        synchronized (WindowManagerGlobal.class) {
            if (sDefaultWindowManager == null) {
                sDefaultWindowManager = new WindowManagerGlobal();
            }
            return sDefaultWindowManager;
        }
    }


 //代碼省略

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }
        if (display == null) {
            throw new IllegalArgumentException("display must not be null");
        }
        if (!(params instanceof WindowManager.LayoutParams)) {
            throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
        }

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
        if (parentWindow != null) {
            parentWindow.adjustLayoutParamsForSubWindow(wparams);
        } else {
            // If there's no parent, then hardware acceleration for this view is
            // set from the application's hardware acceleration setting.
            final Context context = view.getContext();
            if (context != null
                    && (context.getApplicationInfo().flags
                            & ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
                wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
            }
        }

        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
            // Start watching for system property changes.
            if (mSystemPropertyUpdater == null) {
                mSystemPropertyUpdater = new Runnable() {
                    @Override public void run() {
                        synchronized (mLock) {
                            for (int i = mRoots.size() - 1; i >= 0; --i) {
                                mRoots.get(i).loadSystemProperties();
                            }
                        }
                    }
                };
                SystemProperties.addChangeCallback(mSystemPropertyUpdater);
            }

            int index = findViewLocked(view, false);
            if (index >= 0) {
                if (mDyingViews.contains(view)) {
                    // Don't wait for MSG_DIE to make it's way through root's queue.
                    mRoots.get(index).doDie();
                } else {
                    throw new IllegalStateException("View " + view
                            + " has already been added to the window manager.");
                }
                // The previous removeView() had not completed executing. Now it has.
            }

            // 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.size();
                for (int i = 0; i < count; i++) {
                    if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
                        panelParentView = mViews.get(i);
                    }
                }
            }

            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);
        }

        // do this last because it fires off messages to start doing things
        try {
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            synchronized (mLock) {
                final int index = findViewLocked(view, false);
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
            }
            throw e;
        }
    }

    //代碼省略


}

以上是WindowManagerGlobal類，只給出了addView方法其他代碼省略了。我們仔細的來看下addView方法，再把無用的代碼省去，結果如下：

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        //代碼省略
        ViewRootImpl root;
        //代碼省略

            int index = findViewLocked(view, false);
            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);
       //代碼省略
    }

好了代碼省去的比較多，不過我們終于找到了我們想要的代碼了。 root = new ViewRootImpl(view.getContext(), display); 到這ViewRootImpl就創建完了！也就是說ViewRootImpl是在WindowManagerGlobal中的addView方法中創建的！
那么現在我們來嘗試回答一下我們提出的問題。合理的說法是讓線程睡眠200ms以后再次醒來此時onResume方法已經被調用ViewRootImpl已經創建完成，此時可以檢查線程了。

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Android能否在子線程中更新UI呢？

Android能否在子線程中更新UI呢？

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Android能否在子線程中更新UI呢？

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三个男躁一个女,国精产品一区一手机的秘密,麦子交换系列最经典十句话,欧美国产综合欧美视频