blocks|key|271562|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|271563|如何为新的布局清除(或重画)整个画布(=尝试游戏)？|blockquote|271564|271565|271566|只需调用Canvas.drawColor(Color.BLACK)，或者您想用来清除Canvas的任何颜色。|offset|length|style|CODE|271567|271568|和:如何只更新屏幕的一部分？|271569|271570|271571|没有这种方法只更新“屏幕的一部分”，因为Android操作系统在更新屏幕时会重新绘制每个像素。但是，当您不清除Canvas上的旧图形时，旧图形仍然在表面上，这可能是“只更新屏幕的一部分”的一种方法。|271572|所以，如果你想“更新屏幕的一部分”，只要避免调用Canvas.drawColor()方法即可。|271573|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.android.com/reference/android/graphics/Canvas.html#drawColor%2528int%2529^0|0|0|0|0|4|T|16|6|4|T|0|0|0|0|0|0|1J|6|0|O|I|0^^$0|@$1|2|3|-4|4|5|6|12|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|13|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|14|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|15|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|16|7|@$H|17|I|18|J|K]|$H|19|I|1A|J|K]]|8|@$H|1B|I|1C|1|1D]]|9|$]]|$1|L|3|-4|4|5|6|1E|7|@]|8|@]|9|$]]|$1|M|3|N|4|C|6|1F|7|@]|8|@]|9|$]]|$1|O|3|-4|4|5|6|1G|7|@]|8|@]|9|$]]|$1|P|3|-4|4|5|6|1H|7|@]|8|@]|9|$]]|$1|Q|3|R|4|5|6|1I|7|@$H|1J|I|1K|J|K]]|8|@]|9|$]]|$1|S|3|T|4|5|6|1L|7|@$H|1M|I|1N|J|K]]|8|@]|9|$]]|$1|U|3|-4|4|5|6|1O|7|@]|8|@]|9|$]]]|V|$W|$4|X|Y|Z|9|$10|11]]]]

<blockquote>
 How do I clear (or redraw) the WHOLE canvas for a new layout (= try at the game) ?
</blockquote>

Just call <a href="http://developer.android.com/reference/android/graphics/Canvas.html#drawColor%28int%29"><code>Canvas.drawColor(Color.BLACK)</code></a>, or whatever color you want to clear your <code>Canvas</code> with.

<blockquote>
 And: how can I update just a part of the screen ?
</blockquote>

There is no such method that just update a "part of the screen" since Android OS is redrawing every pixel when updating the screen. But, when you're not clearing old drawings on your <code>Canvas</code>, the old drawings are still on the surface and that is probably one way to "update just a part" of the screen.

So, if you want to "update a part of the screen", just avoid calling <code>Canvas.drawColor()</code> method.

blocks|key|275003|text|下面是一个最小示例的代码，它显示您总是必须在每一帧重新绘制画布的每个像素。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275004|此活动在SurfaceView上每秒绘制一个新的位图，而不清除之前的屏幕。如果您测试它，您将看到位图并不总是写入相同的缓冲区，并且屏幕将在两个缓冲区之间交替。|275005|我在我的手机(Nexus+S，Android+2.3.3)和模拟器(Android+2.2)上测试了它。|275006|public+class+TestCanvas+extends+Activity+{
++++@Override
++++public+void+onCreate(Bundle+savedInstanceState)+{
++++++++super.onCreate(savedInstanceState);
++++++++setContentView(new+TestView(this));
++++}
}

class+TestView+extends+SurfaceView+implements+SurfaceHolder.Callback+{

++++private+TestThread+mThread;
++++private+int+mWidth;
++++private+int+mHeight;
++++private+Bitmap+mBitmap;
++++private+SurfaceHolder+mSurfaceHolder;

++++public+TestView(Context+context)+{
++++++++super(context);
++++++++mThread+=+new+TestThread();
++++++++mBitmap+=+BitmapFactory.decodeResource(context.getResources(),+R.drawable.icon);
++++++++mSurfaceHolder+=+getHolder();
++++++++mSurfaceHolder.addCallback(this);
++++}

++++@Override
++++public+void+surfaceChanged(SurfaceHolder+holder,+int+format,+int+width,
++++++++++++int+height)+{
++++++++mWidth+=+width;
++++++++mHeight+=+height;
++++++++mThread.start();
++++}

++++@Override
++++public+void+surfaceCreated(SurfaceHolder+holder)+{/*+Do+nothing+*/}

++++@Override
++++public+void+surfaceDestroyed(SurfaceHolder+holder)+{
++++++++if+(mThread+!=+null+&&+mThread.isAlive())
++++++++++++mThread.interrupt();
++++}

++++class+TestThread+extends+Thread+{
++++++++@Override
++++++++public+void+run()+{
++++++++++++while+(!isInterrupted())+{
++++++++++++++++Canvas+c+=+null;
++++++++++++++++try+{
++++++++++++++++++++c+=+mSurfaceHolder.lockCanvas(null);
++++++++++++++++++++synchronized+(mSurfaceHolder)+{
++++++++++++++++++++++++c.drawBitmap(mBitmap,+(int)+(Math.random()+*+mWidth),+(int)+(Math.random()+*+mHeight),+null);
++++++++++++++++++++}
++++++++++++++++}+finally+{
++++++++++++++++++++if+(c+!=+null)
++++++++++++++++++++++++mSurfaceHolder.unlockCanvasAndPost(c);
++++++++++++++++}

++++++++++++++++try+{
++++++++++++++++++++sleep(1000);
++++++++++++++++}+catch+(InterruptedException+e)+{
++++++++++++++++++++interrupt();
++++++++++++++++}
++++++++++++}
++++++++}+++
++++}
}|code-block|syntax|javascript|275007|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Here is the code of a minimal example showing that you always have to redraw every pixel of the Canvas at each frame.

This activity draw a new Bitmap every second on the SurfaceView, without clearing the screen before.
If you test it, you will see that the bitmap is not always written to the same buffer, and the screen will alternate between the two buffers.

I tested it on my phone (Nexus S, Android 2.3.3), and on the emulator (Android 2.2).

<pre><code>public class TestCanvas extends Activity {
 @Override
 public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(new TestView(this));
 }
}

class TestView extends SurfaceView implements SurfaceHolder.Callback {

 private TestThread mThread;
 private int mWidth;
 private int mHeight;
 private Bitmap mBitmap;
 private SurfaceHolder mSurfaceHolder;

 public TestView(Context context) {
 super(context);
 mThread = new TestThread();
 mBitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon);
 mSurfaceHolder = getHolder();
 mSurfaceHolder.addCallback(this);
 }

 @Override
 public void surfaceChanged(SurfaceHolder holder, int format, int width,
 int height) {
 mWidth = width;
 mHeight = height;
 mThread.start();
 }

 @Override
 public void surfaceCreated(SurfaceHolder holder) {/* Do nothing */}

 @Override
 public void surfaceDestroyed(SurfaceHolder holder) {
 if (mThread != null &amp;&amp; mThread.isAlive())
 mThread.interrupt();
 }

 class TestThread extends Thread {
 @Override
 public void run() {
 while (!isInterrupted()) {
 Canvas c = null;
 try {
 c = mSurfaceHolder.lockCanvas(null);
 synchronized (mSurfaceHolder) {
 c.drawBitmap(mBitmap, (int) (Math.random() * mWidth), (int) (Math.random() * mHeight), null);
 }
 } finally {
 if (c != null)
 mSurfaceHolder.unlockCanvasAndPost(c);
 }

 try {
 sleep(1000);
 } catch (InterruptedException e) {
 interrupt();
 }
 }
 } 
 }
}
</code></pre>

blocks|key|3186042|text|只要打个电话|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3186043|canvas.drawColor(Color.TRANSPARENT)|3186044|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Just call 

canvas.drawColor(Color.TRANSPARENT)

blocks|key|2972329|text|使用PorterDuff+clear模式绘制透明颜色可以达到我想要的效果。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2972330|Canvas.drawColor(Color.TRANSPARENT,+PorterDuff.Mode.CLEAR)|code-block|syntax|javascript|2972331|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Draw transparent color with PorterDuff clear mode does the trick for what I wanted.

<pre><code>Canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)
</code></pre>

blocks|key|275013|text|请将以下代码粘贴到surfaceview扩展类构造函数中.............|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275014|构造函数编码|275015|++++SurfaceHolder+holder+=+getHolder();
++++holder.addCallback(this);

++++SurfaceView+sur+=+(SurfaceView)findViewById(R.id.surfaceview);
++++sur.setZOrderOnTop(true);++++//+necessary
++++holder+=+sur.getHolder();
++++holder.setFormat(PixelFormat.TRANSPARENT);|code-block|syntax|javascript|275016|xml编码|275017|++++<com.welcome.panelview.PanelViewWelcomeScreen
++++++++android:id="@%2Bid/one"
++++++++android:layout_width="600px"
++++++++android:layout_height="312px"
++++++++android:layout_gravity="center"
++++++++android:layout_marginTop="10px"
++++++++android:background="@drawable/welcome"+/>|275018|尝试上面的代码...|275019|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

please paste below code on surfaceview extend class constructor.............

constructor coding

<pre><code> SurfaceHolder holder = getHolder();
 holder.addCallback(this);

 SurfaceView sur = (SurfaceView)findViewById(R.id.surfaceview);
 sur.setZOrderOnTop(true); // necessary
 holder = sur.getHolder();
 holder.setFormat(PixelFormat.TRANSPARENT);
</code></pre>

xml coding

<pre><code> &lt;com.welcome.panelview.PanelViewWelcomeScreen
 android:id="@+id/one"
 android:layout_width="600px"
 android:layout_height="312px"
 android:layout_gravity="center"
 android:layout_marginTop="10px"
 android:background="@drawable/welcome" /&gt;
</code></pre>

try above code...

blocks|key|271588|text|我在谷歌群组里找到了这个，这对我很有效..|type|unstyled|depth|inlineStyleRanges|entityRanges|data|271589|Paint+clearPaint+=+new+Paint();
clearPaint.setXfermode(new+PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0,+0,+width,+height,+clearPaint);+|code-block|syntax|javascript|271590|这将删除绘图、矩形等，同时保留设置的位图。|271591|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Found this in google groups and this worked for me..

<pre><code>Paint clearPaint = new Paint();
clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, width, height, clearPaint); 
</code></pre>

This removes drawings rectangles etc. while keeping set bitmap..

blocks|key|271593|text|mBitmap.eraseColor(Color.TRANSPARENT);

canvas.drawBitmap(mBitmap,+0,+0,+mBitmapPaint);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|271594|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>mBitmap.eraseColor(Color.TRANSPARENT);

canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
</code></pre>

blocks|key|271604|text|您的第一个需求，如何清除或重绘整个画布-答案-使用canvas.drawColor(color.Black)方法来清除带有黑色或您指定的任何颜色的屏幕。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|271605|你的第二个要求，如何更新屏幕的一部分-答案-例如，如果你想保持屏幕上的所有其他东西不变，但在屏幕的一小块区域显示一个整数(比如计数器)，该整数每隔五秒就会增加。然后使用canvas.drawrect方法通过指定left、top、right、bottom和paint来绘制该小区域。然后计算您的计数器值(使用postdalayed+5秒等，如Handler.postDelayed(Runnable_Object，5000);)，将其转换为文本字符串，计算这个小矩形中的x和y坐标，并使用文本视图显示不断变化的计数器值。|271606|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Your first requirement, how to clear or redraw whole canvas - Answer - use canvas.drawColor(color.Black) method for clearing the screen with a color of black or whatever you specify .

Your second requirement, how to update part of the screen - Answer - for example if you want to keep all other things unchanged on the screen but in a small area of screen to show an integer(say counter) which increases after every five seconds. then use canvas.drawrect method to draw that small area by specifying left top right bottom and paint. then compute your counter value(using postdalayed for 5 seconds etc., llike Handler.postDelayed(Runnable_Object, 5000);) , convert it to text string, compute the x and y coordinate in this small rect and use text view to display the changing counter value.

blocks|key|275047|text|使用Path类的reset方法|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275048|Path.reset();|code-block|syntax|javascript|275049|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

use the reset method of Path class 

<pre><code>Path.reset();
</code></pre>

blocks|key|3186169|text|我尝试了@mobistry的答案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3186170|canvas.drawColor(Color.TRANSPARENT,+Mode.CLEAR);|offset|length|style|CODE|3186171|但这对我不起作用。|3186172|对我来说，解决方案是：|3186173|canvas.drawColor(Color.TRANSPARENT,+Mode.MULTIPLY);|3186174|也许有些人也有同样的问题。|3186175|entityMap^0|0|0|1C|0|0|0|0|1F|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@$D|T|E|U|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|V|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|W|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|N|3|O|5|6|7|10|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|Q|$]]

I tried the answer of @mobistry:

<code>canvas.drawColor(Color.TRANSPARENT, Mode.CLEAR);</code>

But it doesn't worked for me. 

The solution, for me, was:

<code>canvas.drawColor(Color.TRANSPARENT, Mode.MULTIPLY);</code>

Maybe some one has the same problem.

blocks|key|2972459|text|对我来说，调用Canvas.drawColor(Color.TRANSPARENT,+PorterDuff.Mode.CLEAR)或类似的东西只有在我触摸屏幕后才会起作用。所以我会调用上面的代码行，但只有在我触摸屏幕之后，屏幕才会变得清晰。因此，对我来说起作用的是先调用invalidate()，然后调用init()，后者在创建时被调用来初始化视图。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2972460|private+void+init()+{
++++setFocusable(true);
++++setFocusableInTouchMode(true);
++++setOnTouchListener(this);

++++mPaint+=+new+Paint();
++++mPaint.setAntiAlias(true);
++++mPaint.setDither(true);
++++mPaint.setColor(Color.BLACK);
++++mPaint.setStyle(Paint.Style.STROKE);
++++mPaint.setStrokeJoin(Paint.Join.ROUND);
++++mPaint.setStrokeCap(Paint.Cap.ROUND);
++++mPaint.setStrokeWidth(6);

++++mCanvas+=+new+Canvas();
++++mPaths+=+new+LinkedList<>();

++++addNewPath();
}|code-block|syntax|javascript|2972461|entityMap^0|7|1M|3R|C|48|6|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

For me calling <code>Canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)</code> or something similar would only work after I touch the screen. SO I would call the above line of code but the screen would only clear after I then touched the screen. So what worked for me was to call <code>invalidate()</code> followed by <code>init()</code> which is called at the time of creation to initialize the view.

<pre><code>private void init() {
 setFocusable(true);
 setFocusableInTouchMode(true);
 setOnTouchListener(this);

 mPaint = new Paint();
 mPaint.setAntiAlias(true);
 mPaint.setDither(true);
 mPaint.setColor(Color.BLACK);
 mPaint.setStyle(Paint.Style.STROKE);
 mPaint.setStrokeJoin(Paint.Join.ROUND);
 mPaint.setStrokeCap(Paint.Cap.ROUND);
 mPaint.setStrokeWidth(6);

 mCanvas = new Canvas();
 mPaths = new LinkedList&lt;&gt;();

 addNewPath();
}
</code></pre>

blocks|key|2972463|text|canvas.drawColor(Color.TRANSPARENT,+Mode.MULTIPLY);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2972464|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>canvas.drawColor(Color.TRANSPARENT, Mode.MULTIPLY);
</code></pre>

blocks|key|2972485|text|我必须使用单独的绘制过程来清除画布(锁定、绘制和解锁)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2972486|Canvas+canvas+=+null;
try+{
++++canvas+=+holder.lockCanvas();
++++if+(canvas+==+null)+{
++++++++//+exit+drawing+thread
++++++++break;
++++}
++++canvas.drawColor(colorToClearFromCanvas,+PorterDuff.Mode.CLEAR);
}+finally+{
++++if+(canvas+!=+null)+{
++++++++holder.unlockCanvasAndPost(canvas);
++++}
}|code-block|syntax|javascript|2972487|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I had to use a separate drawing pass to clear the canvas (lock, draw and unlock):

<pre><code>Canvas canvas = null;
try {
 canvas = holder.lockCanvas();
 if (canvas == null) {
 // exit drawing thread
 break;
 }
 canvas.drawColor(colorToClearFromCanvas, PorterDuff.Mode.CLEAR);
} finally {
 if (canvas != null) {
 holder.unlockCanvasAndPost(canvas);
 }
}
</code></pre>

blocks|key|269605|text|别忘了调用invalidate()；|type|unstyled|depth|inlineStyleRanges|entityRanges|data|269606|canvas.drawColor(backgroundColor);
invalidate();
path.reset();|code-block|syntax|javascript|269607|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Don't forget to call invalidate();

<pre><code>canvas.drawColor(backgroundColor);
invalidate();
path.reset();
</code></pre>

blocks|key|273197|text|使用以下方法，您可以清除整个画布或只清除其中的一部分。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|273198|请不要忘记禁用硬件加速，因为PorterDuff.Mode.CLEAR不能使用硬件加速，最后调用setWillNotDraw(false)是因为我们覆盖了onDraw方法。|offset|length|style|BOLD|CODE|273199|//view's+constructor
setWillNotDraw(false);
setLayerType(LAYER_TYPE_SOFTWARE,+null);

//view's+onDraw
Paint+TransparentPaint+=+new+Paint();
TransparentPaint.setXfermode(new+PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0,+0,+width,+height,+TransparentPaint);+|code-block|syntax|javascript|273200|entityMap^0|0|E|L|1C|L|25|6|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@$D|R|E|S|F|G]|$D|T|E|U|F|H]|$D|V|E|W|F|H]]|9|@]|A|$]]|$1|I|3|J|5|K|7|X|8|@]|9|@]|A|$L|M]]|$1|N|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|O|$]]

With the following approach, you can clear the whole canvas or just a part of it. 
Please do not forget to disable Hardware acceleration since PorterDuff.Mode.CLEAR doesn’t work with hardware acceleration and finally call <code>setWillNotDraw(false)</code> because we override the <code>onDraw</code> method.
<pre><code>//view's constructor
setWillNotDraw(false);
setLayerType(LAYER_TYPE_SOFTWARE, null);

//view's onDraw
Paint TransparentPaint = new Paint();
TransparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, width, height, TransparentPaint); 
</code></pre>

blocks|key|271677|text|尝试删除活动的onPause()处的视图并添加onRestart()|type|unstyled|depth|inlineStyleRanges|entityRanges|data|271678|LayoutYouAddedYourView.addView(YourCustomView)；LayoutYouAddedYourView.removeView(YourCustomView);|offset|length|style|BOLD|271679|一旦添加了视图，就会调用onDraw()方法。|271680|YourCustomView是一个扩展了View类的类。|271681|entityMap^0|0|0|2P|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@$D|P|E|Q|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|R|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|S|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

Try to remove the view at onPause() of an activity and add onRestart()

LayoutYouAddedYourView.addView(YourCustomView);
LayoutYouAddedYourView.removeView(YourCustomView);

The moment you add your view, onDraw() method would get called.

YourCustomView, is a class which extends the View class.

blocks|key|275119|text|在java+android中擦除画布类似于用globalCompositeOperation通过javascript擦除HTML+Canvas。逻辑是相似的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275120|您将选择DST_OUT+(目标输出)逻辑。|275121|paint.setXfermode(new+PorterDuffXfermode(PorterDuff.Mode.DST_OUT));|code-block|syntax|javascript|275122|注意:+DST_OUT更有用，因为如果绘制颜色具有50%25的alpha，它可以擦除50%25。因此，要完全透明到透明，颜色的alpha必须是100%25。建议使用Apply+paint.setColor(Color.WHITE)。并确保画布图像格式为RGBA_8888。|275123|擦除后，使用SRC_OVER返回正常绘图(源结束)。|275124|paint.setXfermode(new+PorterDuffXfermode(PorterDuff.Mode.SRC_OVER));|275125|从字面上看，更新小区域显示需要访问图形硬件，并且可能不受支持。|275126|最接近最高性能的是使用多图像层。|275127|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|Y|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|Z|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

Erasing on Canvas in java android is similar erasing HTML Canvas via javascript with globalCompositeOperation. The logic was similar.

U will choose DST_OUT (Destination Out) logic.

<pre><code>paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
</code></pre>

Note: DST_OUT is more useful because it can erase 50% if the paint color have 50% alpha. So, to clear completely to transparent, the alpha of color must be 100%. Apply paint.setColor(Color.WHITE) is recommended. And make sure that the canvas image format was RGBA_8888.

After erased, go back to normal drawing with SRC_OVER (Source Over).

<pre><code>paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OVER));
</code></pre>

Update small area display literally will need to access graphic hardware, and it maybe not supported.

The most close for highest performance is using multi image layer.

blocks|key|2972582|text|在我的例子中，我将画布绘制成线性布局。要清除并重绘，请执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2972583|++++LinearLayout+linearLayout+=+findViewById(R.id.myCanvas);
++++linearLayout.removeAllViews();|code-block|syntax|javascript|2972584|然后，我用新值调用这个类：|2972585|++++Lienzo+fondo+=+new+Lienzo(this,items);
++++linearLayout.addView(fondo);|2972586|这是Lienzo类：|2972587|class+Lienzo+extends+View+{
++++Paint+paint;
++++RectF+contenedor;
++++Path+path;
++++ArrayList<Items>elementos;

++++public+Lienzo(Context+context,ArrayList<Items>+elementos)+{
++++++++super(context);
++++++++this.elementos=elementos;
++++++++init();
++++}

++++private+void+init()+{
++++++++path=new+Path();
++++++++paint+=+new+Paint();
++++++++contenedor+=+new+RectF();
++++++++paint.setStyle(Paint.Style.FILL);
++++}


++++@Override
++++protected+void+onDraw(Canvas+canvas)+{
++++++++super.onDraw(canvas);

++++++++contenedor.left+=+oneValue;
++++++++contenedor.top+=+anotherValue;
++++++++contenedor.right+=+anotherValue;
++++++++contenedor.bottom+=+anotherValue;

++++++++float+angulo+=+-90;+//starts+drawing+at+12+o'clock
++++++++//total=+sum+of+all+element+values
++++++++for+(int+i=0;i<elementos.size();i%2B%2B){
++++++++++++if+(elementos.get(i).angulo!=0+&&+elementos.get(i).visible){
++++++++++++++++paint.setColor(elementos.get(i).backColor);
++++++++++++++++canvas.drawArc(contenedor,angulo,(float)(elementos.get(i).value*360)/total,true,paint);

++++++++++++++++angulo%2B=(float)(elementos.get(i).value*360)/total;
++++++++++++}
++++++++}+//for+example
++++}
}|2972588|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

In my case, I draw my canvas into linearlayout.
To clean and redraw again:
<pre><code> LinearLayout linearLayout = findViewById(R.id.myCanvas);
 linearLayout.removeAllViews();
</code></pre>
and then, I call the class with the new values:
<pre><code> Lienzo fondo = new Lienzo(this,items);
 linearLayout.addView(fondo);
</code></pre>
This is the class Lienzo:
<pre><code>class Lienzo extends View {
 Paint paint;
 RectF contenedor;
 Path path;
 ArrayList&lt;Items&gt;elementos;

 public Lienzo(Context context,ArrayList&lt;Items&gt; elementos) {
 super(context);
 this.elementos=elementos;
 init();
 }

 private void init() {
 path=new Path();
 paint = new Paint();
 contenedor = new RectF();
 paint.setStyle(Paint.Style.FILL);
 }


 @Override
 protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);

 contenedor.left = oneValue;
 contenedor.top = anotherValue;
 contenedor.right = anotherValue;
 contenedor.bottom = anotherValue;

 float angulo = -90; //starts drawing at 12 o'clock
 //total= sum of all element values
 for (int i=0;i&lt;elementos.size();i++){
 if (elementos.get(i).angulo!=0 &amp;&amp; elementos.get(i).visible){
 paint.setColor(elementos.get(i).backColor);
 canvas.drawArc(contenedor,angulo,(float)(elementos.get(i).value*360)/total,true,paint);

 angulo+=(float)(elementos.get(i).value*360)/total;
 }
 } //for example
 }
}
</code></pre>

blocks|key|275136|text|在我的例子中，每次创建画布对我来说都是有效的，即使它不是内存友好的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275137|Bitmap+bm+=+BitmapFactory.decodeResource(getResources(),+R.drawable.image);
imageBitmap+=+Bitmap.createBitmap(bm.getWidth(),+bm.getHeight(),+bm.getConfig());
canvas+=+new+Canvas(imageBitmap);
canvas.drawBitmap(bm,+0,+0,+null);|code-block|syntax|javascript|275138|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

In my case, creating canvas every time worked for me, even though it's not memory-friendly
<pre><code>Bitmap bm = BitmapFactory.decodeResource(getResources(), R.drawable.image);
imageBitmap = Bitmap.createBitmap(bm.getWidth(), bm.getHeight(), bm.getConfig());
canvas = new Canvas(imageBitmap);
canvas.drawBitmap(bm, 0, 0, null);
</code></pre>

blocks|key|273239|text|下面的方法对我很有效：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|273240|canvas.drawColor(Color.TRANSPARENT,+PorterDuff.Mode.SCREEN);|code-block|syntax|javascript|273241|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The following worked for me:
<pre class="lang-java prettyprint-override"><code>canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.SCREEN);
</code></pre>

blocks|key|275171|text|我找到了我的解决方案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|275172|PaintView类：|275173|public+void+clear()+{
++++mPath.reset();
++++mCanvas.drawColor(Color.TRANSPARENT,+PorterDuff.Mode.CLEAR);
++++paths.clear();
}|code-block|syntax|javascript|275174|和MainActivity：|275175|++clear_canvas_.setOnClickListener(new+View.OnClickListener()+{
++++++++@Override
++++++++public+void+onClick(View+view)+{
++++++++++++paintView.clear();
++++++++}
++++});|275176|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I found my solution.
PaintView class:
<pre><code>public void clear() {
 mPath.reset();
 mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
 paths.clear();
}
</code></pre>
And MainActivity:
<pre><code> clear_canvas_.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View view) {
 paintView.clear();
 }
 });
</code></pre>

In order to make a simple game, I used a template that draws a canvas with bitmaps like this:

<pre><code>private void doDraw(Canvas canvas) {
 for (int i=0;i&lt;8;i++)
 for (int j=0;j&lt;9;j++)
 for (int k=0;k&lt;7;k++) {
 canvas.drawBitmap(mBits[allBits[i][j][k]], i*50 -k*7, j*50 -k*7, null); } }
</code></pre>

(The canvas is defined in "run()" / the SurfaceView lives in a GameThread.)

My first question is how do I clear (or redraw) the whole canvas for a new layout? 
Second, how can I update just a part of the screen? 

<pre><code>// This is the routine that calls "doDraw":
public void run() {
 while (mRun) {
 Canvas c = null;
 try {
 c = mSurfaceHolder.lockCanvas(null);
 synchronized (mSurfaceHolder) {
 if (mMode == STATE_RUNNING) 
 updateGame();
 doDraw(c); }
 } finally {
 if (c != null) {
 mSurfaceHolder.unlockCanvasAndPost(c); } } } }
</code></pre>

Android, canvas: How do I clear (delete contents of) a canvas (= bitmaps), living in a surfaceView?

Android

为了制作一个简单的游戏，我使用了一个模板来绘制一个带有位图的画布，如下所示：private void doDraw(Canvas canvas) { for (int i=0;i<8;i++) for (int j=0;j<9;j++) for (int k=0;k<7;k++...

问如何清除(删除)画布(=位图)，生活在surfaceView中？
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问如何清除(删除)画布(=位图)，生活在surfaceView中？
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