很多初學(xué)者肯定有這樣一個經(jīng)驗肩刃，在Activity的一個子線程中更新UI精绎，發(fā)現(xiàn)會報錯速缨。很多人知道這個錯誤，但卻不知道是什么原因引起的代乃。今天我們來分析一下是什么原因引起的旬牲。我今天以textview更新ui為例。
首先看代碼

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.generalwei.mytest.ThreadActivity">

    <TextView
        android:id="@+id/tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="hello"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:id="@+id/btn_change"
        android:text="更換UI"/>

</RelativeLayout>

public class ThreadActivity extends AppCompatActivity {

    private android.widget.TextView tv;
    private android.widget.Button btnchange;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_thread);
        this.btnchange = (Button) findViewById(R.id.btn_change);
        this.tv = (TextView) findViewById(R.id.tv);
        btnchange.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        tv.setText(new Date().getTime()+"");
                    }
                }).start();

            }
        });
    }
}

運行app搁吓，點擊btnchange按鈕原茅，發(fā)現(xiàn)app崩潰。查看日志,發(fā)現(xiàn)這樣一個錯誤：

我們來查看一下tv.setText()方法的源碼堕仔。

public final void setText(CharSequence text) {
        setText(text, mBufferType);
}

 public void setText(CharSequence text, BufferType type) {
        setText(text, type, true, 0);
        if (mCharWrapper != null) {
            mCharWrapper.mChars = null;
        }
    }


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

這里我們還是沒有看見什么原因引起的擂橘，繼續(xù)追查checkForRelayout()方法。

 private void checkForRelayout() {
       ... 
       invalidate();
       ...
}

查看invalidate()方法,我們會看見這樣一個注釋:

  /**
     * Invalidate the whole view. If the view is visible,
     * {@link #onDraw(android.graphics.Canvas)} will be called at some point in
     * the future.
     * <p>
     * This must be called from a UI thread. To call from a non-UI thread, call
     * {@link #postInvalidate()}.
     */
  public void invalidate() {
        invalidate(true);
    }

意思是說如果view是可見的摩骨，這個方法會刷新view通贞。但是必須發(fā)生在ui線程上∧瘴澹看到這邊我們發(fā)現(xiàn)我們的追蹤是正確昌罩。那么接著看，為什么一定要在ui線程上更新ui唤冈。

  void invalidate(boolean invalidateCache) {
        invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
    }


void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
            boolean fullInvalidate) {
            ...     
            final ViewParent p = mParent;
            if (p != null && ai != null && l < r && t < b) {
                final Rect damage = ai.mTmpInvalRect;
                damage.set(l, t, r, b);
                p.invalidateChild(this, damage);
            }  
        ...
    }

ViewParent 是一個接口,ViewRootImpl是它的實現(xiàn)類,那么我們繼續(xù)追查，代碼如下:

@Override
    public void invalidateChild(View child, Rect dirty) {
        invalidateChildInParent(null, dirty);
    }
    @Override
    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
        checkThread();
      ...
    }

你會發(fā)現(xiàn)一個檢查線程的方法银伟，那么查看方法checkThread()你虹。

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

查看代碼后發(fā)現(xiàn)有一個mThread線程，它是會是UI線程嗎彤避，我們就查看Thread的來源傅物。

    public ViewRootImpl(Context context, Display display) {
      ...
        mThread = Thread.currentThread();
     ...

發(fā)現(xiàn)mThread是在ViewRootImpl創(chuàng)建的時候賦值，這個的線程一定是UI線程琉预。所以當(dāng)前線程不是UI線程的時候會拋異常董饰。

為什么在onResume之前非UI線程也能更新UI

發(fā)現(xiàn)mThread是在ViewRootImpl創(chuàng)建的時候賦值，這個的線程一定是UI線程圆米。所以當(dāng)前線程不是UI線程的時候會拋異常卒暂。
但是有時候在也能在非UI線程中更新，后來我們發(fā)現(xiàn)在onResume之前用非UI線程更新能UI娄帖，而onResume之后就不行了也祠。這是因為onResume之前還沒有創(chuàng)建ViewRootImpl這個類，ActivityThread類中有一個handleResumeActivity方法近速，這個方法是用來回調(diào)Activity的onResume方法诈嘿，具體的看如下代碼:

 final void handleResumeActivity(IBinder token,boolean clearHide, boolean   isForward, boolean reallyResume, int seq, String reason) {
    ActivityClientRecord r = mActivities.get(token);
    if (!checkAndUpdateLifecycleSeq(seq, r, "resumeActivity")) {
        return;
    }

    // If we are getting ready to gc after going to the background, well
    // we are back active so skip it.
    unscheduleGcIdler();
    mSomeActivitiesChanged = true;

    // TODO Push resumeArgs into the activity for consideration
    r = performResumeActivity(token, clearHide, reason);
    if (r != null) {
            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 (r.mPreserveWindow) {
                    a.mWindowAdded = true;
                    r.mPreserveWindow = false;
                    // Normally the ViewRoot sets up callbacks with the Activity
                    // in addView->ViewRootImpl#setView. If we are instead reusing
                    // the decor view we have to notify the view root that the
                    // callbacks may have changed.
                    ViewRootImpl impl = decor.getViewRootImpl();
                    if (impl != null) {
                        impl.notifyChildRebuilt();
                    }
                }
                if (a.mVisibleFromClient && !a.mWindowAdded) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }

            // If the window has already been added, but during resume
            // we started another activity, then don't yet make the
            // window visible.
            } else if (!willBeVisible) {
                if (localLOGV) Slog.v(
                    TAG, "Launch " + r + " mStartedActivity set");
                r.hideForNow = true;
            }
            ...
            // Tell the activity manager we have resumed.
           if (reallyResume) {
                try {
                    ActivityManagerNative.getDefault().activityResumed(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }

        } else {
            // If an exception was thrown when trying to resume, then
            // just end this activity.
            try {
                ActivityManagerNative.getDefault()
                    .finishActivity(token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

我們可以看見這樣一個方法performResumeActivity()堪旧，它的源碼如下:

public final ActivityClientRecord performResumeActivity(IBinder token,boolean clearHide, String reason) {
    ...
    if (r != null && !r.activity.mFinished) {
        ...
        r.activity.performResume();
        ...
    }
}

這里可以看見它調(diào)用了activity.performResume(),那么再繼續(xù)查看下面的源碼:

final void performResume() {
    performRestart();
    ...
    mInstrumentation.callActivityOnResume(this);
    ...
    onPostResume();
    ...   
}

performRestart()方法主要是為了執(zhí)行回調(diào)onRestart方法，具體內(nèi)容就不做分析了奖亚。mInstrumentation.callActivityOnResume()方法則是為了回調(diào)Activity的OnResume()方法淳梦。onPostResume()方法這是為了激活Window。
在handleResumeActivity()方法中我們可以看見一個WindowManager類昔字，這個類是用來控制窗口顯示的爆袍，而它的addView是用來添加視圖。WindowManagerImpl是WindowManager的實現(xiàn)類李滴，WindowManagerImpl的addView方法代碼如下:

public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
    }

mGlobal是WindowManagerGlobal的對象螃宙，繼續(xù)看mGlobal.addView的代碼:

 public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        ...
        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);
        }
       ...
    }

你可以看做ViewRootImpl的初始化時在這里進行的，這就是為什么在onResume之前可以更新UI了所坯。

為什么要這么設(shè)計呢谆扎？

因為所有的UI控件都是非線程安全的，如果在非UI線程更新UI會造成UI混亂芹助。所以一般我們會在Handler中更新UI堂湖。
如有寫的不當(dāng)之處，請多指教状土。