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    <p>It's a synchronization problem. Basically, <code>invalidate()</code> is <strong>not</strong> a blocking call: it just tells the system to redraw at some point in the future. So what happens:</p> <ul> <li>You set a value for (x1, y1) and (x2, y2)</li> <li><code>invaldate()</code> schedules a redraw as soon as possible</li> <li>but another touch event overwrites (x1, y1) and (x2, y2) <strong>before the line is drawn</strong>: the old values are lost forever</li> </ul> <p>You can prove it simply by adding two hits counter, one for <code>invalidate()</code> and one for <code>onDraw()</code>. After some time you see that the number of calls to <code>invalidate()</code> is greater than the hits for <code>onDraw()</code>. My suggestion is to keep the points from touch events in a <code>Queue</code>. Also note that every time you allocate a <code>Bitmap</code> you need to call <code>recycle</code> when you have done to avoid memory leaks. In fact, the pixels are stored in native memory and are not garbage collected when the view is destroyed. I usually call <a href="http://developer.android.com/reference/android/graphics/Bitmap.html#recycle%28%29" rel="nofollow"><code>recycle</code></a> when my activity stops. Here is my code:</p> <pre><code>public class MainActivity extends Activity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); } public static class Drawer extends View { private Bitmap cache; private Queue&lt;PointF&gt; points; private PointF from; public Drawer(Context ctx, AttributeSet attrs) { super(ctx, attrs); points = new ConcurrentLinkedQueue&lt;PointF&gt;(); } @Override public boolean onTouchEvent(MotionEvent evt) { switch (evt.getAction()) { case MotionEvent.ACTION_DOWN: from = new PointF(evt.getX(), evt.getY()); break; case MotionEvent.ACTION_MOVE: points.add(new PointF(evt.getX(), evt.getY())); invalidate(); break; case MotionEvent.ACTION_UP: from = null; break; default: from = null; } return true; } @Override public void onSizeChanged(int w, int h, int oldw, int oldh) { if (w == 0 || h == 0) return; cache = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888); } @Override public void onDraw(Canvas systemCanvas) { int w = getWidth(); int h = getHeight(); if (w == 0 || h == 0) return; if (cache == null) cache = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888); // Draw on the cache Canvas canvas = new Canvas(cache); Paint paint = new Paint(); paint.setStrokeWidth(4); paint.setColor(Color.MAGENTA); paint.setFlags(Paint.ANTI_ALIAS_FLAG); drawPoints(points, canvas, paint); // Draw the cache with the system canvas systemCanvas.drawBitmap(cache, 0, 0, paint); } private void drawPoints(Queue&lt;PointF&gt; points, Canvas canvas, Paint paint) { if (from == null) return; PointF to; while ((to = points.poll()) != null) { canvas.drawLine(from.x, from.y, to.x, to.y, paint); from = to; } } } } </code></pre> <p>and this is the layout</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" android:id="@+id/drawView"&gt; &lt;view class="com.zybnet.test.MainActivity$Drawer" android:layout_width="fill_parent" android:layout_height="fill_parent" /&gt; &lt;/LinearLayout&gt; </code></pre> <p>You can see that customs view can be used in XML, too :)</p>
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    1. COthanks for the codes. and I just learned how to refer to inner-classes from XML. Thanks again for this as well
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    2. COMake sure that the inner class is `public` and `static`, and also it must have a constructor like this `View(Context, AttributeSet)` :) I didn't outline this in my post, but may give you some headache
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    3. COI wondered about that when I was going through and understanding your codes, but since i am on a RUN for time, I planned to check with that later .. thanks for telling me that :)
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