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我正在尝试一个带有画布的简单 CustomView。这个自定义视图只是在调用时随机更改屏幕上图标图片的位置updatePosition()，并且在我的意图中应该在拖动时移动图像，在捏合时缩放......当我触摸屏幕中的位置时，应该出现在点击的位置位置。

我的尝试如下 

public class CustomView extends View {

    Bitmap mBmp;
    Random mRnd;
    Paint mPaint;

    private static final int INVALID_POINTER_ID = -1;

    private float mPosX;
    private float mPosY;

    private float mLastTouchX;
    private float mLastTouchY;
    private int mActivePointerId = INVALID_POINTER_ID;

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

    int w, h, bw, bh;
    int px = -1, py = -1;

    public CustomView(Context context) {
        this(context, null, 0);
        mBmp = BitmapFactory.decodeResource(context.getResources(),
                R.drawable.icon);

    }

    public CustomView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs, 0);

        mBmp = BitmapFactory.decodeResource(context.getResources(),
                R.drawable.icon); 
        bw = mBmp.getWidth(); 
        bh = mBmp.getHeight();
        mPaint = new Paint(); 
        mPaint.setColor(Color.CYAN);
        mPaint.setAntiAlias(true);
        mRnd = new Random();

    }

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

        canvas.save();
        Log.d("DEBUG", "X: " + mPosX + " Y: " + mPosY);
        canvas.translate(mPosX, mPosY);
        canvas.scale(mScaleFactor, mScaleFactor);

        if (px == -1 && py == -1) { 
            px = w / 2 - bw / 2; 
            py = h / 2 - bh / 2; 
        }

        canvas.drawCircle(px + (bw / 2), py + (bh / 2), 70, mPaint); 
        canvas.drawBitmap(mBmp, px, py, null); 

        canvas.restore();

    }

    public void updatePosition() {
        px = mRnd.nextInt(w - bw);
        py = mRnd.nextInt(h - bh);
        invalidate();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        w = MeasureSpec.getSize(widthMeasureSpec);
        h = MeasureSpec.getSize(heightMeasureSpec);
        setMeasuredDimension(w, h);
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        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);

            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) {
                final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                mLastTouchX = ev.getX(newPointerIndex);
                mLastTouchY = ev.getY(newPointerIndex);
                mActivePointerId = ev.getPointerId(newPointerIndex);
            }
            break;
        }
        }

        return true;
    }

    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, 10.0f));

            invalidate();
            return true;
        }
    }

}

如果我尝试拖动图像或触摸屏幕的任何位置，当 onTouchListener 导致应用程序崩溃并出现 NullPointerException 时，updatePosition() 侦听器工作

有人可以帮助我吗？谢谢

4

1 回答 1

1

看起来mScaleDetector只在其中一个构造函数中初始化。它应该在所有构造函数中初始化。一种常见的模式是创建一个init()方法并将您初始化的所有内容放在其中，然后从所有构造函数中调用该方法。

注意：发布 StackTrace 总是有帮助的……您应该能够看到调用了哪个构造函数。

于 2013-08-01T22:52:54.027 回答