android - 如何从捏合中心缩放列表项

Question

我正在为 Android 开发一个简单的图书阅读器应用程序。我为此使用了 ListView 。

我正在做的是将远程图像从服务器加载到我的 ListView 中(使用 picasso 库)并且工作正常。

我还想在我的阅读器中缩放功能，我使用 ZoomListView 类来实现这一点。

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;
    }
}

}

这是我的 XML 布局(我只显示了 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>

这使我能够缩放我的图书阅读器。我的图书阅读器看起来像

问题:我的双指缩放出现问题。当我捏合时，缩放从 ListView 的左上角开始。

让我用图片来描述

1 捏

2 电流输出

注意:这从左上角缩放了 ListView

3 推荐输出

Answer 1

方法一:

要从列表的中心开始缩放，通过调用 detector.getFocusX(); 和 detector.getFocusY(); 获取缩放中心。现在使用这个中心你的缩放逻辑。

更改您的 onScale() 实现 - 如下所示:

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

方法二(我目前正在使用):

• 将您的 View 置于 ScrollView 中。
• 调整 View 大小。
• 计算中心移动了多少。(在调整大小之前和之后获取 detector.getFocusX() 和 detector.getFocusY() 的差异)
• 使用 parent.scrollBy(dx,dy) 使 View 重新居中。

如果有帮助，这是我的居中代码的一部分。

    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;