Android: 如何利用Handler、Thread更新視圖

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總是感覺android中UI更新很讓人糾結！自己小結一下，算是拋磚引玉。讀這篇文章之前，假設你已經明白線程、handler的使用。

1. 在onCreate()方法中開啓線程更新UI

public class MasterActivity extends Activity {
    TextView tv = null;
    Button btn = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
        tv = (TextView)findViewById(R.id.text);
        btn = (Button)findViewById(R.id.btn);

        /*onCreate中開啓新線程，更新UI。沒有報錯或者異常信息！*/
       Thread thread = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
                tv.setText("update UI is success!");
                btn.setText("update UI is success!");
            }});
        thread.start();
    }

 

隨便折騰，不會報錯或者異常！以爲開啓的線程和UI線程（主線程）是同一個線程，但是很不幸，他們的線程id根本是風牛馬不相及！

不知道爲什麼在這裏開啓子線程更新UI就沒有問題！真的想不明白？？？？

2. 在activity如onResume、onStart、反正是以on開頭的回調方法

@Override
    protected void onRestart() {
        super.onRestart();
         /*onRestart中開啓新線程，更新UI*/
        Thread thread = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
                tv.setText("update UI is success!");
                btn.setText("update UI is success!");
            }});
        thread.start();
    }

  

不好意思，按下返回按鈕在啓動程序，或者按Home健再啓動程序，就這麼折騰幾下，就會包異常！信息如下：

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

意思是：只有主線程纔可以更新UI。

解決辦法：加上postInvalidate()方法。

@Override
    protected void onRestart() {
        super.onRestart();
         /*onRestart中開啓新線程，更新UI*/
        Thread thread = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
                tv.postInvalidate();
                btn.postInvalidate();
                tv.setText("update UI is success!");
                btn.setText("update UI is success!");
            }});
        thread.start();
    }

 

postInvalidate()方法，源碼：

public void postInvalidate() {
        postInvalidateDelayed(0);
    }
public void postInvalidateDelayed(long delayMilliseconds) {
        // We try only with the AttachInfo because there's no point in invalidating
        // if we are not attached to our window
        if (mAttachInfo != null) {
            Message msg = Message.obtain();
            msg.what = AttachInfo.INVALIDATE_MSG;
            msg.obj = this;
            mAttachInfo.mHandler.sendMessageDelayed(msg, delayMilliseconds);
        }
    }

 

其實，是調用了Handler的處理消息的機制！該方法可以在子線程中直接用來更新UI。還有一個方法invalidate ()，稍候再說！

 3.  在Button的事件中開啓線程，更新UI

public class MasterActivity extends Activity {
    TextView tv = null;
    Button btn = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
        tv = (TextView)findViewById(R.id.text);
        btn = (Button)findViewById(R.id.btn);
        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Thread thread = new Thread(new Runnable() {

                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
                        tv.setText("update UI is success!");
                        btn.setText("update UI is success!");
                    }});
                thread.start();
            }
        });
}

 

 Sorry，報錯！即使你加上postInvalidate()方法，也會報這個錯誤。

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

 

4. 使用Handler結合多線程更新UI

a. 開啓一個線程,在run方法中通知Handler

b. Handler中使用handleMessage方法更新UI。

public class MasterActivity extends Activity {
    TextView tv = null;
    Button btn = null;

    Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            if(msg.what == 1) {
                tv.setText("update UI is success!");
                btn.setText("update UI is success!");
            }
            super.handleMessage(msg);
        }
    };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
        tv = (TextView)findViewById(R.id.text);
        btn = (Button)findViewById(R.id.btn);
        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Thread thread = new Thread(new Runnable() {

                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + ": " + Thread.currentThread().getId());
                        Message msg = mHandler.obtainMessage();
                        msg.what = 1;
                        msg.sendToTarget();
                    }});
                thread.start();
            }
        });
}

 

5. Handler和invalidate 方法結合多線程更新UI

方法invalidate主要用在主線程中（即UI線程中），不可以用於子線程。如果在子線程中需要使用postInvalidate方法。

sdk的api有說明：

public void invalidate ()
Since: API Level 1
Invalidate the whole view. If the view is visible, onDraw(Canvas) will be called at some point in the future. This must be called from a UI thread. To call from a non-UI thread, call postInvalidate().

 

看看該方法源碼：

public void invalidate() {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
        }
        if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS)) {
            mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;
            final ViewParent p = mParent;
            final AttachInfo ai = mAttachInfo;
            if (p != null && ai != null) {
                final Rect r = ai.mTmpInvalRect;
                r.set(0, 0, mRight - mLeft, mBottom - mTop);
                // Don't call invalidate -- we don't want to internally scroll
                // our own bounds
                p.invalidateChild(this, r);
            }
        }
    }

 

invalidate 方法如果你直接在主線程中調用，是看不到任何更新的。需要與Handler結合！

感謝這位“雷鋒”，一個不錯的例子：http://disanji.net/2010/12/12/android-invalidate-ondraw/

只是被我修改了一點，加入times，看看onDraw到底運行多少次。

Android在 onDraw 事件處理繪圖,

而 invalidate() 函數可以再一次觸發onDraw事件,然後再一次進行繪圖動作。

public class MasterActivity extends Activity {
    static int times = 1;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView( new View(null){

            Paint vPaint = new Paint();  //繪製樣式物件
            private int i = 0;           //弧形角度

            @Override
            protected void onDraw (Canvas canvas) {
                super.onDraw(canvas);
                System.out.println("this run " + (times++) +" times!");

                // 設定繪圖樣式
                vPaint.setColor( 0xff00ffff ); //畫筆顏色
                vPaint.setAntiAlias( true );   //反鋸齒
                vPaint.setStyle( Paint.Style.STROKE );

                // 繪製一個弧形
                canvas.drawArc(new RectF(60, 120, 260, 320), 0, i, true, vPaint );

                // 弧形角度
                if( (i+=10) > 360 )
                    i = 0;

                // 重繪, 再一次執行onDraw 程序
                invalidate();
            }
        });
    }
}

 

經過測試，發現times一直在++，說明onDraw被多次調用，並且一致在畫圖！

sdk的api有時候讓人很鬱悶，無語.....關於invalidate的使用，還待探索。革命尚未成功，同志仍需努力！