レイヤーごとのBitmapを保持して重ねて描画するViewのサンプル

LayerImageView.java

import java.util.ArrayList;
import java.util.List;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.view.View;

public class LayerImageView extends View {
	private List<Layer> layers = new ArrayList<Layer>();
	private Canvas tmpCanvas = new Canvas();

	public LayerImageView(Context context, AttributeSet attrs, int defStyleAttr) {
		super(context, attrs, defStyleAttr);
	}

	public LayerImageView(Context context, AttributeSet attrs) {
		super(context, attrs);
	}

	public LayerImageView(Context context) {
		super(context);
	}

	@Override
	protected void onDraw(Canvas canvas) {
		super.onDraw(canvas);

		for (Layer l : layers) {
			tmpCanvas.setBitmap(l.bitmap);
			tmpCanvas.save();
			l.onLayerDrawingListener.draw(tmpCanvas);
			tmpCanvas.restore();
			canvas.drawBitmap(l.bitmap, 0, 0, l.paint);
		}
	}

	public Layer createLayer() {
		Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Config.ARGB_8888);
		Layer layer = new Layer(bitmap);
		return layer;
	}

	public void addLayer(Layer layer) {
		layers.add(layer);
	}

	public void addLayer(int index, Layer layer) {
		layers.add(index, layer);
	}

	public void setLayer(int index, Layer layer) {
		layers.set(index, layer);
	}

	public boolean removeLayer(Layer layer) {
		return layers.remove(layer);
	}

	public List<Layer> getLayers() {
		return layers;
	}

	public static class Layer {
		private final Paint paint = new Paint();
		private final Bitmap bitmap;
		private OnLayerDrawingListener onLayerDrawingListener;

		private Layer(Bitmap bitmap) {
			this.bitmap = bitmap;
		}

		private Layer(Bitmap bitmap, OnLayerDrawingListener callback) {
			this.bitmap = bitmap;
			this.onLayerDrawingListener = callback;
		}

		public OnLayerDrawingListener getOnLayerDrawingListener() {
			return onLayerDrawingListener;
		}

		public void setOnLayerDrawingListener(OnLayerDrawingListener layerDrawingCallback) {
			this.onLayerDrawingListener = layerDrawingCallback;
		}

		public void setBrendingMode(Mode mode) {
			paint.setXfermode(new PorterDuffXfermode(mode));
		}
	}

	public static interface OnLayerDrawingListener {
		void draw(Canvas canvas);
	}
}