android - 如何使用按钮清除android中的 Canvas ？

Question

我想清除 Canvas 中所有绘制的内容。我尝试了几种方法，但没有一个为我点击。如何使用清除按钮清除 Canvas android中的所有内容？

我有清除按钮来清除 canvas android 中的内容。但我显然不知道如何删除内容。我有撤消和重做按钮，除了这个清除按钮外，它工作正常。有人可以告诉我如何使用此清除按钮并删除内容？我的代码在这里。

public class MainActivity extends Activity {

    private ArrayList<Path> undonePaths = new ArrayList<Path>();
    private ArrayList<Path> paths = new ArrayList<Path>();
    /*
     * private Button alphabetsButton; private Button lettersButton; private
     * Button clearButton;
     */
    private FrameLayout frmLayout;
    Canvas canvas;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        frmLayout = (FrameLayout) findViewById(R.id.frameLayout);
        final DrawingPanel dp = new DrawingPanel(MainActivity.this);
        frmLayout.addView(dp);

        /*
         * alphabetsButton=(Button) findViewById(R.id.button1);
         * lettersButton=(Button) findViewById(R.id.button2);
         * 
         * 
         * 
         * alphabetsButton.setOnClickListener(new OnClickListener() {
         * 
         * @Override public void onClick(View v) { // TODO Auto-generated method
         * stub startActivity(new Intent(MainActivity.this,Letters.class));
         * finish(); } });
         */

        ((Button) findViewById(R.id.Clear)).setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

                paths = new ArrayList<Path>();
                paths.clear();

            }
        });

        ((Button) findViewById(R.id.Undo)).setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if (paths.size() > 0) {
                    undonePaths.add(paths.remove(paths.size() - 1));
                    dp.invalidate();
                }
            }
        });

        ((Button) findViewById(R.id.Redo)).setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if (undonePaths.size() > 0) {
                    paths.add(undonePaths.remove(undonePaths.size() - 1));
                    dp.invalidate();
                }
            }
        });
    }

    class DrawingPanel extends View implements OnTouchListener {

        private Canvas mCanvas;
        private Path mPath;
        private Paint mPaint, circlePaint, outercirclePaint;
        private Bitmap mBitmap;
        private int width;
        private int height;

        // private ArrayList<Path> undonePaths = new ArrayList<Path>();
        private float xleft, xright, xtop, xbottom;

        public DrawingPanel(Context context) {
            super(context);
            setFocusable(true);
            setFocusableInTouchMode(true);
            this.setOnTouchListener(this);
            circlePaint = new Paint();
            mPaint = new Paint();
            outercirclePaint = new Paint();
            outercirclePaint.setAntiAlias(false);
            circlePaint.setAntiAlias(false);
            mPaint.setAntiAlias(false);
            mPaint.setColor(0xFF000000);
            outercirclePaint.setColor(0x44FFF000);
            circlePaint.setColor(0xF57F35);
            outercirclePaint.setStyle(Paint.Style.FILL_AND_STROKE);
            circlePaint.setStyle(Paint.Style.FILL);
            mPaint.setStyle(Paint.Style.STROKE);
            mPaint.setStrokeJoin(Paint.Join.MITER);
            mPaint.setStrokeCap(Paint.Cap.ROUND);
            mPaint.setStrokeWidth(20);
            outercirclePaint.setStrokeWidth(15);
            mCanvas = new Canvas();
            mPath = new Path();
            paths.add(mPath);
        }

        public void colorChanged(int color) {
            mPaint.setColor(color);
        }

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

        @Override
        protected void onDraw(Canvas canvas) {

            for (Path p : paths) {
                canvas.drawPath(p, mPaint);
            }

        }

        private float mX, mY;
        private static final float TOUCH_TOLERANCE = 0;

        private void touch_start(float x, float y) {
            mPath.reset();
            mPath.moveTo(x, y);
            mX = x;
            mY = 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;
            }
        }

        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 = new Path();
            paths.add(mPath);
        }

        @Override
        public boolean onTouch(View arg0, MotionEvent event) {
            float x = event.getX();
            float y = event.getY();

            switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                // if (x <= cx+circleRadius+5 && x>= cx-circleRadius-5) {
                // if (y<= cy+circleRadius+5 && cy>= cy-circleRadius-5){
                // paths.clear();
                // return true;
                // }
                // }
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
            }
            return true;
        }
    }

}

Answer 1

你应该避免在调用clear()之前再次实例化paths，否则你会在一个空的数组列表:

((Button) findViewById(R.id.Clear)).setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            if (paths != null)
               paths.clear();
            if (dp != null)
               dp.invalidate();
        }
    });