android - Android 使用 Canvas 应用覆盖导致空指针异常

Question

这让我今天有点发疯。我已经实现了这个类，它使我能够在图像文件上画线。但是我希望能够在创建时添加模板掩码。无论我尝试在类中还是从调用类中尝试，它都会导致空指针异常，但是如果我将调用放入调用类中的按钮，那么我可以让它工作吗？

这是我正在使用的课程。你可以看到注释掉的部分，如果我从这里调用“coverScreen()”，它会导致 NPE。如果我调用“coverScreen()”，也可以从创建 maskBoard 的主 Activity 中导致 NPE。但是如前所述，如果我添加一个按钮，然后将“coverScreen()”放入此按钮 onclick，它会起作用吗？

public class MaskBoard extends View {
    private String TAG = "Test APP - maskboard";
    private int mx = -1;
    private int my = -1;
    private Bitmap mainImage = null;
    private Bitmap bm = null;
    private Paint mPaint = null;
    public final int CLOSER = 50;
    public final int CLOSE = 100;

    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;


    public MaskBoard(Context c, AttributeSet s) {
        super(c, s);

        String fileToMask = "/__temppic.png";
        File imagePath = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES + "/temp/");

        File imgFile = new File(imagePath+fileToMask);
        if (imgFile.exists()) {
            bm = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
            Log.i(TAG, "found image");
        }

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);

        mPaint = new Paint();
        mPaint.setAntiAlias(false);
        mPaint.setDither(true);
        mPaint.setColor(0xFFFE0000); // FFFF0000

        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(50);




        /* coverScreen() */

        setEraser();
    }


    public void coverScreen() {

        Bitmap maskImage = BitmapFactory.decodeResource(getResources(), R.drawable.mask); //get mask image
        maskImage = Bitmap.createScaledBitmap(maskImage, bm.getWidth(), bm.getHeight(), true);
        mPaint.setXfermode(null);
        mCanvas.drawBitmap(maskImage,0,0,mPaint);
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));

    }

    public void setEraser() {

        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    }

    public void setPaint() {
        mPaint.setXfermode(null);
    }  



    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        Log.i(TAG, "onsizechanged");
         mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
         mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        Log.i(TAG, "onDraw");
        canvas.drawRect(0,0, getWidth(), getHeight(), mPaint);
        canvas.drawColor(0xFFAAAAAA);
        canvas.drawBitmap(bm, mx, my, null);
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
        Log.i(TAG, " touchdown: X(" + x + "), Y(" + y + ")");
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
            Log.i(TAG, " touchmove: X(" + x + "), Y(" + y + ")");
        }
    }



    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
               // coverScreen() ;
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }
}