Android Canvas - 缩放和翻译

Question

I have an SVG that I want to zoom and pan using gestures in Android. It works, but not smoothly enough for my tastes. So how can I improve this class to have smooth panning that works at all zoom levels and stays under your finger, scales using a pinch gesture zooms from the area of the gesture (AKA focus) and as a bonus double tapping should result in snapping the image back to 1.0? I'm using this SVG library . An acceptable answer should meet these functional requirements, not jump around unexpectedly and stick to GestureDetectors unless a case can be made that they just won't work.

public class MapView extends ViewGroup {
    private static final float MINIMUM_SCALE_FACTOR = 1.0f;
    private static final float MAXIMUM_SCALE_FACTOR = 10.0f;

    private Picture mPicture;
    private Paint mPaint;

    private ScaleGestureDetector mScaleDetector;
    private GestureDetector mMoveDetector;

    private float mScaleFactor;
    private float mScaleFocusX;
    private float mScaleFocusY;
    private float mFocusX = 0.0f;
    private float mFocusY = 0.0f;

    private float mImageHeight;
    private float mImageWidth;
    private int mViewHeight;
    private int mViewWidth;
    private RectF mDrawingRect;


    public MapView(Context context) {
        super(context);

        init(context);
    }

    public MapView(Context context, AttributeSet attrs) {
        super(context, attrs);

        init(context);

    }

    public MapView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        init(context);
    }

    public void init(Context context) {
        Log.d(getClass().getCanonicalName(), "Initialized.");

        this.setBackgroundColor(Color.WHITE);

        this.mPaint = new Paint();
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setColor(Color.BLUE);
        mPaint.setStrokeWidth(2);
        mPaint.setAntiAlias(true);

        this.mScaleFactor = MINIMUM_SCALE_FACTOR;

        // Setup Gesture Detectors
        this.mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
        this.mMoveDetector = new GestureDetector(context, new MoveListener());
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        Log.d(getClass().getCanonicalName(), "onLayout");

        this.mDrawingRect = getDrawingSquare();
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        this.mViewWidth = w;
        this.mViewHeight = h;
    }

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

        Log.d(getClass().getCanonicalName(), "onDraw()");

        canvas.save();
        canvas.scale(mScaleFactor, mScaleFactor, mScaleFocusX, mScaleFocusY);

        canvas.translate(mFocusX, mFocusY);

        canvas.drawRect((int) mDrawingRect.left, (int) mDrawingRect.top, (int) mDrawingRect.right, (int) mDrawingRect.bottom, mPaint);
        canvas.drawPicture(mPicture, mDrawingRect);

        canvas.restore();
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        mScaleDetector.onTouchEvent(ev);
        mMoveDetector.onTouchEvent(ev);

        return true;
    }

    public RectF getDrawingSquare() {
        float width = getWidth();
        float height = getHeight();
        Log.d(getClass().getCanonicalName(), "Picture width " + width + " - height " + height);

        float side = Math.min(width, height);

        return new RectF(0, 0, side, side);
    }

    public void setSVG(SVG svg) {
        Log.d(getClass().getCanonicalName(), "SVG to Picture.");

        this.mPicture = svg.getPicture();
        this.mImageHeight = mPicture.getHeight();
        this.mImageWidth = mPicture.getWidth();

        invalidate();
    }

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            mScaleFactor *= detector.getScaleFactor(); // scale change since previous event
            mScaleFocusX = detector.getFocusX();
            mScaleFocusY = detector.getFocusY();

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

            invalidate();

            return true;
        }
    }

    private class MoveListener extends GestureDetector.SimpleOnGestureListener {
//        @Override
//        public boolean onDown(MotionEvent e) {
//            return true;
//        }

        @Override
        public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
            scrollBy((int)distanceX, (int)distanceY);
//            mFocusX += distanceX;
//            mFocusY += distanceY;
//            return true;
            return true;
        }
    }
}

Answer 1

如果（重新）绘制图片的速度不够快，您可能不得不将其绘制到位图而是滚动它。