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.

Motion to draw numbers on android

im new to this android things. And i have to develop an application that can help an autism to learn numbers. I have a few ideas and I've been trying to learn and implement the code. But it's failed. The question is how can i apply the motion code or sprite to draw a numbers or letter? For example like this, i wanna make the penguin move through the line and draw a number nine.
There is example from mybringback.com which is the image move to draw a rectangle. How can i implement it to draw a number? Im sorry if i asking too much, i just trying to get some ideas.

I think that you should first build an utility program, in order to create the "path vector".
What I mean by path vector is simply a vector of Points (where a point has x value, and y value). And your utility should let you draw whatever you want, with a simple pen. You should draw on surface and store points when mouse is down, and ignore points when mouse is up.
Then, in the main program, you will just have to read at the path of your number/letter.
I've tried to implement something like this for the Sugar OLPC platform, without serializing path into files : I was able to draw, and to view the animation. And I used the process I've just described you.
Hope it can help you.
P.S : I used the word mouse, but you guessed that I talk about finger ...

There are various ways to achieve animation effects. One approach that is quite versatile involves creating a custom View or SurfaceView in which you Override the onDraw method. Various tutorials can be found on this; the official Android discussion of it is here:
http://developer.android.com/guide/topics/graphics/2d-graphics.html#on-view
Your implementation will look something like this:
// Find elapsed time since previous draw
// Compute new position of drawable/bitmap along figure
// Draw bitmap in appropriate location
// Add line to buffer containing segments of curve drawn so far
// Render all segments in curve buffer
// Take some action to call for the rendering of the next frame (this may be done in another thread)
Obviously a simplification. For a very simplistic tutorial, see here:
http://www.techrepublic.com/blog/software-engineer/bouncing-a-ball-on-androids-canvas/1733/
Note that different implementations of this technique will require different levels of involvement by you; for example, if you use a SurfaceView, you are in charge of calling the onDraw method, whereas subclassing the normal View lets you leave Android in charge of redrawing (at the expense of limiting your ability to draw on a different thread). In this respect, Google remains your friend =]

android: Smoothest and most efficient way to animate drawn circles on a canvas

What is the best way to draw circles on a canvas that should have an alpha layer and change sizes? Should I use a View or a Surfaceview? The circles should also be clickable. And it should be smooth transitions when changing color size and position?
Should I put this in a runnable or use invlaidate in onDraw?
I would prefer that something like this also worked smoothly in low-end devices.
Any suggestions? I'm new to this kind of animations in Android.

If you are constantly drawing and taking user input at the same time, I would use a SurfaceView. However, if the only draw changes you plan on making to the circles happen when you touch them, then a simple View onDraw() override would probably do the trick. In the end it will just depend on what all is going on.
The point of the SurfaceView is to have that separate thread for drawing. If what you're doing could be in any way considered "game-like," then go for a SurfaceView; otherwise, stick with a View.
I say this because I'm currently working on a project with constant drawing using a View. The shapes that I'm drawing respond to touch and you can scroll through the View while it is still invalidating over and over. All this with a View and it still runs just fine on lower-end devices (I've only gone back to GingerBread, though).
Good luck!
I should also mention that in the project drawing in a View, almost everything has various alpha values and what not and runs fine.

Draw an image floating over another in Android

I want to be able to dynamically place image(s) over another image in my app.
Consider the first image as background and the other images to be on top level, I will also need to move those top level images (change their x and y on the screen) by code too.
Imagine, for example, a sea in which the user places fish and sea animals, then those sea animals start to move here and there on the screen: it will be like that.
How can I do this? If you don't know but remember any simple program or demo that does that, it will be also very welcome!
Thank you!

There is, of course, more than one way to do this, but I would say that the best way to do it would be to create a custom View (class that derives from View) and have this handle your bitmap drawing and all of your touch events.
There's a lot of code to write for loading the Bitmaps and keeping track of all of their positions (and then drawing them to the canvas in onDraw), but if you start really small by just allowing one image to be drawn and dragged around the screen, you can build on that and keep your code organized.
You would need to override onDraw(Canvas) and onTouchEvent(MotionEvent) in your custom View. You'll load your bitmaps with BitmapFactory (decodeResource method if you're including your images as resources in your project) and you'll need to remember to call recycle on your bitmaps when you're no longer using them.
In onDraw, you draw your bitmaps to the canvas at a specific location using Canvas.drawBitmap. There are two overloads of this method you can choose from, one that takes the top and left coordinates of the bitmap as floats (and therefore performs no scaling or stretching) and one that takes a destination and source rectangle to perform scaling, stretching and placement.
I always use the latter as it gives me finer tuned control. If you choose this route, you'll want to keep two Rect instances and a Bitmap instance for each image being drawn, update them in the touch events and draw them to the canvas in the draw event.
When something changes inside your view (as in the case of a touch event), call invalidate() method and the framework will know to redraw everything which triggers your onDraw method.

How to animate views?

I'm working on a game that in some ways is similar to Tetris (imagine a 2D array of colored squares that sometimes move around)
I am trying to animate the individual squares so they will smoothly slide down from coordinate to the next. Since I wanted to use Android's built-in tweening feature, the animation has to apply to the whole View (rather than parts of it). This doesn't work well for me because I only want some of the colored squares to slide down, and the rest of them to stay still.
The (theoretical) solution I came up with to resolve this is to make 2 Views, layered directly on top of each other. The top view is for animating squares when they need to move, and the bottom layer is for the static squares. The animation-layer is transparent until I am ready to animate something. I then simply turn on the colored square in the animation-layer, tween it to the new location, and turn it back off when done. In the same time span, the static-layer just turns squares on and off at the right time to make the whole thing look seamless to the end user.
The proposed solution is just a theory, since I haven't been able to make it work correctly yet. Since I have been having trouble, I was wondering if this is even the best way to solve the problem? Perhaps there is a more elegant solution that I am over looking? Anyone know of a better way?

If you just want to animate a single element check out the namespace android.view.animation.Animation. You can also use Drawable shapes and draw them directly. Finally, if you want a simulation then you will have to look into threading. Basically you will create a timer to update the canvas for you based on an interval. There are some other view canvases you can use as well like the GLView canvas.