How to zoom list item from center of pinch

Question

I am working on a simple book reader app for Android. I have used the listview for this.

What I am doing is loading the remote images from server into my Listview (Using the picasso library) and is working fine.

I also wanted to zoom functionality in my Reader, well I used ZoomListView class to achieve this.

public class ZoomListView extends ListView {
private static final int INVALID_POINTER_ID = -1;
private int mActivePointerId = INVALID_POINTER_ID;
private ScaleGestureDetector mScaleDetector;

private float mScaleFactor = 1.f;
private float maxWidth = 0.0f;
private float maxHeight = 0.0f;
private float mLastTouchX;
private float mLastTouchY;
private float mPosX;
private float mPosY;
private float width;
private float height;

public ZoomListView(Context context) {
    super(context);
    mScaleDetector = new ScaleGestureDetector(getContext(),
            new ScaleListener());
}

public ZoomListView(Context context, AttributeSet attrs) {
    super(context, attrs);
    mScaleDetector = new ScaleGestureDetector(getContext(),
            new ScaleListener());
}

public ZoomListView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    mScaleDetector = new ScaleGestureDetector(getContext(),
            new ScaleListener());
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    width = MeasureSpec.getSize(widthMeasureSpec);
    height = MeasureSpec.getSize(heightMeasureSpec);
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

@Override
public boolean onTouchEvent(@NonNull MotionEvent ev) {
    super.onTouchEvent(ev);
    final int action = ev.getAction();
    mScaleDetector.onTouchEvent(ev);
    switch (action & MotionEvent.ACTION_MASK) {
    case MotionEvent.ACTION_DOWN: {
        final float x = ev.getX();
        final float y = ev.getY();

        mLastTouchX = x;
        mLastTouchY = y;

        mActivePointerId = ev.getPointerId(0);
        break;
    }

    case MotionEvent.ACTION_MOVE: {
        final int pointerIndex = ev.findPointerIndex(mActivePointerId);
        final float x = ev.getX(pointerIndex);
        final float y = ev.getY(pointerIndex);
        final float dx = x - mLastTouchX;
        final float dy = y - mLastTouchY;

        mPosX += dx;
        mPosY += dy;

        if (mPosX > 0.0f)
            mPosX = 0.0f;
        else if (mPosX < maxWidth)
            mPosX = maxWidth;

        if (mPosY > 0.0f)
            mPosY = 0.0f;
        else if (mPosY < maxHeight)
            mPosY = maxHeight;

        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: {
        final int pointerIndex = (action & MotionEvent.ACTION_POINTER_INDEX_MASK) >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
        final int pointerId = ev.getPointerId(pointerIndex);
        if (pointerId == mActivePointerId) {
            final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
            mLastTouchX = ev.getX(newPointerIndex);
            mLastTouchY = ev.getY(newPointerIndex);
            mActivePointerId = ev.getPointerId(newPointerIndex);
        }
        break;
    }
    }

    return true;
}

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.save(Canvas.MATRIX_SAVE_FLAG);
    canvas.translate(mPosX, mPosY);
    canvas.scale(mScaleFactor, mScaleFactor);
    canvas.restore();
}

@Override
protected void dispatchDraw(@NonNull Canvas canvas) {
    canvas.save(Canvas.MATRIX_SAVE_FLAG);
    if (mScaleFactor == 1.0f) {
        mPosX = 0.0f;
        mPosY = 0.0f;
    }
    canvas.translate(mPosX, mPosY);
    canvas.scale(mScaleFactor, mScaleFactor);
    super.dispatchDraw(canvas);
    canvas.restore();
    invalidate();
}

private class ScaleListener extends
        ScaleGestureDetector.SimpleOnScaleGestureListener {
    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        mScaleFactor *= detector.getScaleFactor();
        mScaleFactor = Math.max(1.0f, Math.min(mScaleFactor, 4.0f));
        maxWidth = width - (width * mScaleFactor);
        maxHeight = height - (height * mScaleFactor);
        invalidate();
        return true;
    }
}

}

Here is my XML layout (I have showed only the ZoomListView):

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ffffff"
    android:gravity="top"
    android:visibility="visible" >

    <libraries.ZoomListView
        android:id="@+id/lstv_book_reader"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="0dp"
        android:paddingTop="0dp" >
    </libraries.ZoomListView>
</RelativeLayout>
</RelativeLayout>

This enabled me to zoom my book reader. My book reader looks like

Problem: I am getting issue in my pinch zoom. When I am pinching the zoom is starting from top left of the listview.

Let me describe with images

1 Pinch

2 Current Output

Note: This Zoomed the listview from topleft corner

3 Recommended Output

Answer 1

Approach one:

To zoom from the center of list,get the zoom center by calling detector.getFocusX(); and detector.getFocusY();.Now use this center in your scaling logic.

Change your onScale() implementation-something like below:

    mPosX = detector.getFocusX();
    mPosY = detector.getFocusY();

Approach two (which I am currently using):

• Put your view in scroll view.
• Resize your view.
• Calculate how much the center has moved.(Take the diff of detector.getFocusX() and detector.getFocusY() before and after the resizing)
• use parent.scrollBy(dx,dy) to recenter the view.

If it helps-this is some part from my centering code.

    if (isZoomOnGoing && mPreviousWidth != 0 && Math.abs(mCenterDiff) > 0) {
            float ratioBeforeAdjustment = mCurrentZoomFocus / (float) (mTotalWidth - mCurrentZoomFocus);//
            float ratioDiff = reducePrecision(mStartZoomRatio) - reducePrecision(ratioBeforeAdjustment);
            float x1 = mStartZoomRatio * mTotalWidth / (1.0f + mStartZoomRatio);
            float y1 = mTotalWidth - x1;
            float currentRatio = x1 / y1;//
            int currentScrollBy = (int) (x1 - mCurrentZoomFocus);
            int scrollDiff = 0;
            if (mPreviousScrollBy != 0 && Math.abs(ratioDiff) > 1) {
                scrollDiff = currentScrollBy - mPreviousScrollBy;
            }
            //    if (scrollDiff>5) {//To avoid flickering//TODO do we need this
            parent.scrollBy(currentScrollBy, 0);
//            } else {
//                Log.d(TAG, "drawActualGraph skipping scroll because scrollDiff <5 ");
//            }
            mPreviousScrollBy = currentScrollBy;

Answer 2

it means you want to change the default property that is zoom (scaling) always starts from (0,0) that is:

_____________
|0,0 x       |
|y           |
|            |   Suppose this the device screen
|            |
|            |
______________

And you want to scale it from:

_____________
|            |
|            |
|            |   Suppose this the device screen
|          y |
|       X 0,0|
______________

So to achieve this: Change you code from this:

canvas.scale(mScaleFactor, mScaleFactor);

to this:

canvas.scale(mScaleFactor, -mScaleFactor);

also see this question which relates with your question: Android: Drawing to canvas, way to make bottom left correspond to (0,0)? I have i you got your solution from this example and code snippet.

Answer 3

Try to use getPivotX() and getPivotY(). By def they are 0 , 0 that's why it starts scaling from top left corner.
Try to get pivots from where it's touched and change your
canvas.scale(...) to
canvas.scale(mScaleFactor, mScaleFactor, pivotx, pivoty);