常にスクロール可能なImageView(視差効果)

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.View;

import static java.lang.Math.abs;
import static java.lang.Math.floor;

/**
 * Created by thijs on 08-06-15.
 */
public class ScrollingImageView extends View {
    private final int speed;
    private final Bitmap bitmap;

    private Rect clipBounds = new Rect();
    private int offset = 0;

    private boolean isStarted;

    public ScrollingImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ParallaxView, 0, 0);
        try {
            speed = ta.getDimensionPixelSize(R.styleable.ParallaxView_speed, 10);
            bitmap = BitmapFactory.decodeResource(getResources(), ta.getResourceId(R.styleable.ParallaxView_src, 0));
        } finally {
            ta.recycle();
        }
        start();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), bitmap.getHeight());
    }

    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (canvas == null) {
            return;
        }

        canvas.getClipBounds(clipBounds);

        int normalizedOffset = offset;
        int layerWidth = bitmap.getWidth();
        if (offset < -layerWidth) {
            offset += (int) (floor(abs(normalizedOffset) / (float) layerWidth) * layerWidth);
        }

        int left = offset;
        while (left < clipBounds.width()) {
            canvas.drawBitmap(bitmap, getBitmapLeft(layerWidth, left), 0, null);
            left += layerWidth;
        }

        if (isStarted) {
            offset -= speed;
            postInvalidateOnAnimation();
        }
    }

    private float getBitmapLeft(int layerWidth, int left) {
        float bitmapLeft = left;
        if (speed < 0) {
            bitmapLeft = clipBounds.width() - layerWidth - left;
        }
        return bitmapLeft;
    }

    /**
     * Start the animation
     */
    public void start() {
        if (!isStarted) {
            isStarted = true;
            postInvalidateOnAnimation();
        }
    }

    /**
     * Stop the animation
     */
    public void stop() {
        if (isStarted) {
            isStarted = false;
            invalidate();
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="ParallaxView">
        <attr name="speed" format="dimension" />
        <attr name="src" format="reference" />
    </declare-styleable>
</resources>

<com.....ScrollingImageView
xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/scrolling_background"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:speed="1dp"
    app:src="@drawable/scrolling_background" />

ScrollingImageView scrollingBackground = (ScrollingImageView) loader.findViewById(R.id.scrolling_background);
scrollingBackground.stop();
scrollingBackground.start();

<FrameLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

  <com......ScrollingImageView
      android:id="@+id/scrolling_background"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      app:speed="1dp"
      app:src="@drawable/scrolling_background" />

  <com.....ScrollingImageView
      android:id="@+id/scrolling_foreground"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      app:speed="2.5dp"
      app:src="@drawable/scrolling_foreground" />
</FrameLayout>