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 开发者_运维知识库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.