Android SDK - 不在触摸中心缩放

Question

我的应用程序有点问题。我刚刚实现了双指缩放，但是当我双指缩放时，图像不会在 2 个点的中心缩放，如下图所示。图 1 是我的手指在捏合之前的位置，图 2 显示它没有像它应该的那样在我的手指中心扩展。

图 1 与图 2:

这是我的代码:

        package com.jpiionefourone.guitarimage;

    import android.content.Context;
    import android.graphics.Canvas;
    import android.graphics.drawable.Drawable;
    import android.util.AttributeSet;
    import android.view.MotionEvent;
    import android.view.ScaleGestureDetector;
    import android.view.View;

    public class MultiTouchView extends View {
        private Drawable mIcon;
        private float mPosX;
        private float mPosY;

        private float mLastTouchX;
        private float mLastTouchY;

        private static final int INVALID_POINTER_ID = -1;

        // The ‘active pointer’ is the one currently moving our object.
        private int mActivePointerId = INVALID_POINTER_ID;

        private ScaleGestureDetector mScaleDetector;
        private float mScaleFactor = 1.f;


        public MultiTouchView(Context context) {
            this(context, null, 0);
        }

        public MultiTouchView(Context context, AttributeSet attrs) {
            this(context, attrs, 0);
        }

        public MultiTouchView(Context context, AttributeSet attrs, int defStyle) {
            super(context, attrs, defStyle);
            mIcon = context.getResources().getDrawable(R.drawable.guitar);
            mIcon.setBounds(0, 0, mIcon.getIntrinsicWidth(), mIcon.getIntrinsicHeight());

            // Create our ScaleGestureDetector
            mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());

        }

        @Override
        public boolean onTouchEvent(MotionEvent ev) {
            // Let the ScaleGestureDetector inspect all events.
            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;
                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);

                // Only move if the ScaleGestureDetector isn't processing a gesture.
                if (!mScaleDetector.isInProgress()) {
                    final float dx = x - mLastTouchX;
                    final float dy = y - mLastTouchY;

                    mPosX += dx;
                    mPosY += dy;

                    invalidate();
                }

                mLastTouchX = x;
                mLastTouchY = y;

                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 = (ev.getAction() & 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;
        }

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

            canvas.save();
            canvas.translate(mPosX, mPosY);
            canvas.scale(mScaleFactor, mScaleFactor);
            mIcon.draw(canvas);
            canvas.restore();
        }

        private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
            @Override
            public boolean onScale(ScaleGestureDetector detector) {
                mScaleFactor *= detector.getScaleFactor();

                // Don't let the object get too small or too large.
                mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));

                invalidate();
                return true;
            }
        }
    }

谢谢，

贾里德

Answer 1

在您的 ScaleListener 类中，您只调整 mScaleFactor 因此，实际上，您的图像确实不会围绕多点触控的中心缩放手势!

此外，您需要更新 mPosX 和 mPosY 以使用不同的缩放中心。

对于我自己的一个应用程序，我使用了这样的东西:

final float scale = detector.getScaleFactor();
mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor * scale, 5.0f));

if (mScaleFactor < 5f) {
   // 1 Grabbing
   final float centerX = detector.getFocusX();
   final float centerY = detector.getFocusY();
   // 2 Calculating difference
   float diffX = centerX - mPosX;
   float diffY = centerY - mPosY;
   // 3 Scaling difference
   diffX = diffX * scale - diffX;
   diffY = diffY * scale - diffY;
   // 4 Updating image origin
   mPosX -= diffX;
   mPosY -= diffY;
}

基本上，它通过以下方式计算图像原点的相对变化

1. 获取多点触控手势的来源 (getFocus())
2. 计算图像来源(diffX 和 diffY)之间的差异
3. 应用比例因子
4. 并使用缩放后的差异更新图像的原点

这(仍然)适用于我的情况。我不知道我们是否都使用 (d) 相同的坐标系，但像这样的东西应该可以帮助您在多点触控手势的中心正确缩放图像。