Oded's ZoomableScrollView

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.view.View;
import android.view.ViewParent;
import android.widget.FrameLayout;

public class ZoomableScrollView extends FrameLayout {

    private static final int INVALID_POINTER_ID = 1;
    private int mActivePointerId = INVALID_POINTER_ID;

    private float mScaleFactor = 1;
    private ScaleGestureDetector mScaleDetector;
    private Matrix mScaleMatrix = new Matrix();
    private Matrix mScaleMatrixInverse = new Matrix();

    private float mPosX;
    private float mPosY;
    private Matrix mTranslateMatrix = new Matrix();
    private Matrix mTranslateMatrixInverse = new Matrix();

    private float mLastTouchX;
    private float mLastTouchY;

    private float mFocusY;
    private float mFocusX;

    private int mCanvasWidth;
    private int mCanvasHeight;

    private float[] mInvalidateWorkingArray = new float[6];
    private float[] mDispatchTouchEventWorkingArray = new float[2];
    private float[] mOnTouchEventWorkingArray = new float[2];

    private boolean mIsScaling;

    public ZoomableScrollView(Context context) {
        super(context);
        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
        mTranslateMatrix.setTranslate(0, 0);
        mScaleMatrix.setScale(1, 1);
    }

    public ZoomableScrollView(Context context, AttributeSet attributeSet) {
        super(context, attributeSet);
        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
        mTranslateMatrix.setTranslate(0, 0);
        mScaleMatrix.setScale(1, 1);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                child.layout(l, t, l+child.getMeasuredWidth(), t += child.getMeasuredHeight());
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int height = 0;
        int width = 0;
        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
                height += child.getMeasuredHeight();
                width = Math.max(width, child.getMeasuredWidth());
            }
        }
        mCanvasWidth = width;
        mCanvasHeight = height;
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        canvas.save();
        canvas.translate(mPosX, mPosY);
        canvas.scale(mScaleFactor, mScaleFactor, mFocusX, mFocusY);
        super.dispatchDraw(canvas);
        canvas.restore();
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        mDispatchTouchEventWorkingArray[0] = ev.getX();
        mDispatchTouchEventWorkingArray[1] = ev.getY();
        mDispatchTouchEventWorkingArray = screenPointsToScaledPoints(mDispatchTouchEventWorkingArray);
        ev.setLocation(mDispatchTouchEventWorkingArray[0],
                mDispatchTouchEventWorkingArray[1]);
        return super.dispatchTouchEvent(ev);
    }

    /**
     * Although the docs say that you shouldn't override this, I decided to do
     * so because it offers me an easy way to change the invalidated area to my
     * likening.
     */
    @Override
    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {

        mInvalidateWorkingArray[0] = dirty.left;
        mInvalidateWorkingArray[1] = dirty.top;
        mInvalidateWorkingArray[2] = dirty.right;
        mInvalidateWorkingArray[3] = dirty.bottom;

        mInvalidateWorkingArray = scaledPointsToScreenPoints(mInvalidateWorkingArray);
        dirty.set(Math.round(mInvalidateWorkingArray[0]), Math.round(mInvalidateWorkingArray[1]),
                Math.round(mInvalidateWorkingArray[2]), Math.round(mInvalidateWorkingArray[3]));

        location[0] *= mScaleFactor;
        location[1] *= mScaleFactor;
        return super.invalidateChildInParent(location, dirty);
    }

    private float[] scaledPointsToScreenPoints(float[] a) {
        mScaleMatrix.mapPoints(a);
        mTranslateMatrix.mapPoints(a);
        return a;
    }

    private float[] screenPointsToScaledPoints(float[] a){
        mTranslateMatrixInverse.mapPoints(a);
        mScaleMatrixInverse.mapPoints(a);
        return a;
    }

    @SuppressLint("ClickableViewAccessibility")
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        mOnTouchEventWorkingArray[0] = ev.getX();
        mOnTouchEventWorkingArray[1] = ev.getY();

        mOnTouchEventWorkingArray = scaledPointsToScreenPoints(mOnTouchEventWorkingArray);

        ev.setLocation(mOnTouchEventWorkingArray[0], mOnTouchEventWorkingArray[1]);
        mScaleDetector.onTouchEvent(ev);

        final int action = ev.getAction();
        switch (action & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();

                mLastTouchX = x;
                mLastTouchY = y;

                // Save the ID of this pointer
                mActivePointerId = ev.getPointerId(0);
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                // Find the index of the active pointer and fetch its position
                final int pointerIndex = ev.findPointerIndex(mActivePointerId);
                final float x = ev.getX(pointerIndex);
                final float y = ev.getY(pointerIndex);

                if (mIsScaling && ev.getPointerCount() == 1) {
                    // Don't move during a QuickScale.
                    mLastTouchX = x;
                    mLastTouchY = y;

                    break;
                }

                float dx = x - mLastTouchX;
                float dy = y - mLastTouchY;

                float[] topLeft = {0f, 0f};
                float[] bottomRight = {getWidth(), getHeight()};
                /*
                 * Corners of the view in screen coordinates, so dx/dy should not be allowed to
                 * push these beyond the canvas bounds.
                 */
                float[] scaledTopLeft = screenPointsToScaledPoints(topLeft);
                float[] scaledBottomRight = screenPointsToScaledPoints(bottomRight);

                dx = Math.min(Math.max(dx, scaledBottomRight[0] - mCanvasWidth), scaledTopLeft[0]);
                dy = Math.min(Math.max(dy, scaledBottomRight[1] - mCanvasHeight), scaledTopLeft[1]);

                mPosX += dx;
                mPosY += dy;

                mTranslateMatrix.preTranslate(dx, dy);
                mTranslateMatrix.invert(mTranslateMatrixInverse);

                mLastTouchX = x;
                mLastTouchY = y;

                invalidate();
                break;
            }

            case MotionEvent.ACTION_UP: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_POINTER_UP: {
                // Extract the index of the pointer that left the touch sensor
                final int pointerIndex = (action & MotionEvent.ACTION_POINTER_INDEX_MASK) >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
                final int pointerId = ev.getPointerId(pointerIndex);
                if (pointerId == mActivePointerId) {
                    // This was our active pointer going up. Choose a new
                    // active pointer and adjust accordingly.
                    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                    mLastTouchX = ev.getX(newPointerIndex);
                    mLastTouchY = ev.getY(newPointerIndex);
                    mActivePointerId = ev.getPointerId(newPointerIndex);
                }
                break;
            }
        }
        return true;
    }

    private float getMaxScale() {
        return 2f;
    }

    private float getMinScale() {
        return 1f;
    }

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScaleBegin(ScaleGestureDetector detector) {
            mIsScaling = true;

            mFocusX = detector.getFocusX();
            mFocusY = detector.getFocusY();

            float[] foci = {mFocusX, mFocusY};
            float[] scaledFoci = screenPointsToScaledPoints(foci);

            mFocusX = scaledFoci[0];
            mFocusY = scaledFoci[1];

            return true;
        }

        @Override
        public void onScaleEnd(ScaleGestureDetector detector) {
            mIsScaling = false;
        }

        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            mScaleFactor *= detector.getScaleFactor();
            mScaleFactor = Math.max(getMinScale(), Math.min(mScaleFactor, getMaxScale()));
            mScaleMatrix.setScale(mScaleFactor, mScaleFactor, mFocusX, mFocusY);
            mScaleMatrix.invert(mScaleMatrixInverse);
            invalidate();

            return true;
        }
    }

}