Draws lines on the set drawable based on the user touches. sample: http://imgur.com/RNO6MA0

FinderDrawView.java

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.ViewTreeObserver;
import android.widget.ImageView;

/**
 * Draws lines on the set drawable based on the user touches. The input is limited by the bounds 
 * of the first set image, if {@link #setAdjustViewBounds} or its XML attribute is used. 
 * TODO: adjust bounds when the drawable is changed
 * Created by vedant on 13/10/15.
 */
public class FingerDrawView extends ImageView {
    private static final String TAG = "FingerDrawView";

    private static final float MINP = 0.25f;
    private static final float MAXP = 0.75f;

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

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


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

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(12);

        ViewTreeObserver vto = getViewTreeObserver();
        vto.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
            public boolean onPreDraw() {
                Drawable drawable = getDrawable();
                // wait for drawable to be set
                if (drawable != null) {
                    // Remove after the first run so it doesn't fire forever
                    getViewTreeObserver().removeOnPreDrawListener(this);
                    int w, h;
                    w = drawable.getIntrinsicWidth();
                    h = drawable.getIntrinsicHeight();
                    mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
                    mCanvas = new Canvas(mBitmap);
                    LogWrapper.d(TAG, "drawable != null Height: " + h + " Width: " + w);
                }
                return true;
            }
        });
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (mBitmap == null)
            return;
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);
    }

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

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

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mPath.reset();
                mPath.moveTo(x, y);
                mX = x;
                mY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                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;
                }
                break;
            case MotionEvent.ACTION_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();
                break;
        }
        invalidate();
        return true;
    }
}