Develop Reference

Update view's canvas outside of onDraw on Android

I need update a small portion of a custom view in order to display a small animation. The rest portion of the view has only static image. The most straightforward would be to obtain the canvas of the view and update only that particular portion directly. But I can't really find anyway to get the view's canvas object outside of the view::onDraw method.
The only alternative I know is this: call view::invalidate() with a specified rectangle to minimize the drawing flicker. I have the code to update the entire view within onDraw. So the best thing to do is to detect the clipping rect and only run the code to update the specified area, in order to minimize CPU usage as well?

I guess I will try to answer this question myself to the best my knowledge so far.
There is no direct access to the canvas outside of the onDraw method.
Although we can detect the clipping rect with the function Canvas.getClipBounds(), the getClipBounds function always return the entire view area if GPU is enabled. When GPU is not used, getClipBounds() returns the actual dirty area. Since there is a GPU in most phones, it makes the function getClipBounds pretty much useless.

onDraw() - Do I have to clear canvas first?

I have created a custom view to be shown in the action bar. It mostly works except sometimes on start I see a mirror copy of whatever I draw. The copy overlaps the original one but a few pixels away.
My onDraw() override is quite simple. All it does is draws an image that is centered in the canvas.
I am a bit confused. Am I expected to clear the canvas first in onDraw() method? Thank you in advance for your help.

It is confusing, but you'll notice in the custom view samples (that come with the sdk), the onDraw() method first calls canvas.drawColor() before anything else.
I assume it's not done automatically because it would be wasteful in the case where what you were drawing filled the entire view anyway. I just wish it was more clear that it is necessary in most cases.

Here is what I found out that others may find useful. If you set a background using setBackground(), the framework will actually draw the background before calling onDraw(). This background is drawn in the center. I too was drawing the background in onDraw() but I was drawing it from top-left. Hence, I was seeing the ghost image.
As Krylez mentioned, you may wish to call drawColor() as your first call in onDraw().
What I did was overrode setBackground() and its variations, and stored the background bitmap in a local variable.
Regards,
Peter

Draw a shape that tracks my finger

I know how to draw paths on a canvas and understand how to undo/redo. But now I want to draw shapes (lines, circles, rectangles) that dynamically resize depending on how I drag them - just like we have in MS Paint.
If I use "drawLine", the line is there permanently with no way of erasing it and redrawing it to my new finger location. Same with circle as I want the circle to constantly change width as I drag my finger. So the old one has to erased (keeping all the other paths on the bitmap intact) and the new one drawn in its place.
Been searching a lot for this, but haven't come across how t do it. Maybe I'm using the wrong keywords, but I don't know. Any pointers?

Each time you move the finger, call the underlying view's invalidate() function, it will trigger erasing the entire background
public void invalidate ()
Since: API Level 1
Invalidate the whole view. If the view is visible, onDraw(android.graphics.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().
Then redraw your shape based on your finger's new position.

Managed to do it. I misunderstood the way the offscreen drawing thing worked. The idea is to write to the bitmap only after you have the shape you want - ie on "touch up". Till then, draw the shape using the "canvas" object in on Draw...

Should I take view translation into account when overriding onDraw?

Should I take view translation into account when overriding onDraw()? Or it is already applied to canvas translation?
I.e. should I execute
canvas.translate(getTranslationX(), getTranslationY());
in first line of onDraw()?
Do all transformations applied to canvas in onDraw() call persist in subsequent calls? I.e. should I suppose canvas is each time untransformed?

No, you do not need to do this. A View's translation is applied before onDraw is called.
As HardCoder points out, state changes that you make to the Canvas passed to onDraw will not persist to the next call to onDraw.

As far as I know the canvas is not persistant therefore you should execute the translate. However you can save the canvas and restore it:
http://maohao.wordpress.com/2009/09/30/canvas-save-canvas-restore/
Android View.onDraw() always has a clean Canvas
http://blahti.wordpress.com/2010/12/20/moving-views-in-android-part-1/

Scrollview Optimizations

I have a scrollview that contains a custom view. The custom view is bigger than the area of the screen and draws properly.
The scrollview however tends to call onDraw() non-stop when scrolling, and I can't seem to make it smooth.
I used ScrollView.getDrawingRect() to calculate the visible portion of the screen and only draw to that, but it still returns the entire viewport (so it's optimized to not draw offscreen areas), and not the delta between the last position and the current one. Ideally I'd want to draw only the delta, and not the entire visible window.
If anyone can point me to more information about how to use the drawing caches, and if that will help optimize scrolling, I'd love to implement it, or any other possible solutions it would be greatly appreciated.

When content is scrolled, the entire viewport needs to be redrawn because all of the content has moved. I don't think there's anything that needs to be done to optimize a ScrollView - if scrolling is slow then it's the drawing method of your custom view that is too slow.
Try to avoid object creation in your draw methods which is usually the main culprit for poor drawing performance.
Edit: Also the scrollview could blit the old content up or down quickly that is still drawn on the screen, and then request a redraw of only the "new" portion of the screen. (only applies to opaque views).

I encountered the same problem. I solved it by using the function setDrawingCacheEnabled(true). By enabling this setting, your canvas view will be cached as bitmap, so you don't have to call canvas' draw method each time onDraw() is called.
In your custom view's constructor, you will need something like this:
public CustomView(Context context) {
setDrawingCacheEnabled(true);
drawnFlag = false;
}
In your onDraw method, you will need something like this:
public void onDraw(Canvas canvas) {
if (! drawnFlag) {
canvas.drawPath(...);
canvas.drawPath(...);
drawnFlag = true;
}
}
Now, scrolling on this custom view should be smooth since we only call the drawing methods once.

Afaik the ScrollView sets a proper clip rect on the canvas your view gets in onDraw so you only need to draw what's inside that rect.
You could also implement cache bitmaps based on the clip rect's size.