Scrolling a Canvas smoothly in Android

Question

I'm new to Android.

I am drawing bitmaps, lines and shapes onto a Canvas inside the OnDraw(Canvas canvas) method of my view. I am looking for help on how to implement smooth scrolling in response to a drag by the user. I have searched but not found any tutorials to help me with this.

The reference for Canvas seems to say that if a Canvas is constructed from a Bitmap (called bmpBuffer, say) then anything drawn on the Canvas is also drawn on bmpBuffer. Would it be possible to use bmpBuffer to implement a scroll ... perhaps copy it back to the Canvas shifted by a few pixels at a time? But if I use Canvas.drawBitmap to draw bmpBuffer back to Canvas shifted by a few pixels, won't bmpBuffer be corrupted? Perhaps, therefore, I should copy bmpBuffer to bmpBuffer2 then draw bmpBuffer2 back to the Canvas.

A more straightforward approach would be to draw the lines, shapes, etc. straight into a buffer Bitmap then draw that buffer (with a shift) onto the Canvas but so far as I can see the various methods: drawLine(), drawShape() and so on are not available for drawing to a Bitmap ... only to a Canvas.

Could I have 2 Canvases? One of which would be constructed from the buffer bitmap and used simply for plotting the lines, shapes, etc. and then the buffer bitmap would be drawn onto the other Canvas for display in the View?

I should welcome any advice!

Answers to similar questions here (and on other websites) refer to "blitting". I understand the concept but can't find anything about "blit" or "bitblt" in the Android documentation. Are Canvas.drawBitmap and Bitmap.Copy Android's equivalents?

Answer 1

I had this problem too,

I did the drawing like this:

Canvas BigCanvas = new Canvas();
Bitmap BigBitmap = new Bitmap(width,height);

int ScrollPosX , ScrollPosY  // (calculate these with the onScrollEvent handler)

void onCreate()
{
   BigCanvas.SetBitmap(BigBitmap);
}

onDraw(Canvas TargetCanvas)
{
   // do drawing stuff
   // ie.  BigCanvas.Draw.... line/bitmap/anything

   //draw to the screen with the scrolloffset

   //drawBitmap (Bitmap bitmap, Rect src, Rect dst, Paint paint)
   TargetCanvas.DrawBitmap(BigBitmap(new Rect(ScrollPosX,ScrollPosY,ScrollPosX + BigBitmap.getWidth(),ScrollPosY + BigBitmap.getHeight(),new Rect(0,0,ScreenWidth,ScreenHeight),null);
}

for smooth scrolling you'd need to make some sort of method that takes a few points after scrolling (i.e the first scroll point and the 10th) , subtract those and scroll by that number in a for each loop that makes it gradually slower ( ScrollAmount - turns - Friction ).

I Hope this gives some more insight.

Answer 2

I seem to have found an answer. I have put the bulk of the drawing code (which was previously in onDraw()) in a new doDrawing() method. This method starts by creating a new bitmap larger than the screen (large enough to hold the complete drawing). It then creates a second Canvas on which to do the detailed drawing:

    BufferBitmap = Bitmap.createBitmap(1000, 1000, Bitmap.Config.ARGB_8888);
    Canvas BufferCanvas = new Canvas(BufferBitmap);

The rest of the doDrawing() method is taken up with detailed drawing to BufferCanvas.

The entire onDraw() method now reads as follows:

    @Override protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawBitmap(BufferBitmap, (float) -posX, (float) -posY, null);
}

The position variables, posX and posY, are initialised at 0 in the application's onCreate()method. The application implements OnGestureListener and uses the distanceX and distanceY arguments returned in the OnScroll notification to increment posX and posY.

That seems to be about all that's needed to implement smooth scrolling. Or am I over-looking something!?

Answer 3

Continuation of reply to Viktor ...

In fact, the situation is more complicated. Because the doDrawing process is quite slow (taking 2-3 seconds on my slow old HTC Hero phone) I found it desirable to pop up a Toast message to advise the user that it was happening and to indicate the reason. The obvious way to do this was to create a new method containing just 2 lines:

public void redrawBuffer(String strReason) {
    Toaster.Toast(strReason, "Short");`
    doDrawing();
}

and to call this method from other places in my program instead of doDrawing().

However, I found that the Toaster either never appeared or flashed up so briefly that it could not be read. My workaround has been to use a time check Handler to force the program to sleep for 200 milliseconds between displaying the Toast and calling doDrawing(). Although this slightly delays the start of a redraw I feel this is a price worth paying in terms of the program's usability because the user knows what is going on. reDrawBuffer() now reads:

public void redrawBuffer(String strReason) {
    Toaster.Toast(strReason, "Short");
    mTimeCheckHandler.sleep(200);    
}`

and the Handler code (which is nested within my View class) is:

private timeCheckHandler mTimeCheckHandler = new timeCheckHandler();

class timeCheckHandler extends Handler {
@Override
    public void handleMessage(Message msg) {
        doDrawing();
    }
    public void sleep(long delayMillis) {
        this.removeMessages(0);
        sendMessageDelayed(obtainMessage(0), delayMillis);
    }
}`

Answer 4

No need for the activity to be restarted! (Per prepbgg's Jan 27 10 reply to his Jan 17 10 'answer') Rather than recycling the bitmap and incurring the overhead of having the activity reloaded, you can avoid having the application loaded by putting the 'android:configChanges' attribute shown below, in the 'activity' element of the AndroidManifest.xml file for the app. This tells the system the the app will handle orientation changes and that it doesn't need to restart the app.

<activity android:name=".ANote"
    android:label="@string/app_name"
    android:configChanges="orientation|screenLayout">
    <intent-filter>
       <action android:name="android.intent.action.MAIN" />
       <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

This method can be used to get a notification when the orienation is changed:

public void onConfigurationChanged(Configuration  newConfig) {
    super.onConfigurationChanged(newConfig);
    prt("onConfigurationChanged: "+newConfig);

    if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) {
      prt("  PORTRAIT");
    } else {
      prt("  LANDSCAPE");
    }
} // end of onConfigurationChanged

Answer 5

prepbgg: I don't think the code will work because canvas.drawBitmap does not draw into the bitmap but draws the bitmap on-to the canvas.

Correct me if I am wrong!