主函数

public class MainActivity extends AppCompatActivity {
    ImageView iv;
    Paint paint;
    Canvas canvas;
    Bitmap copyBimap;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv = (ImageView) findViewById(R.id.iv);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.chenwei);
        copyBimap = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), bitmap.getConfig());
        paint = new Paint();
        canvas = new Canvas(copyBimap);
        canvas.drawBitmap(bitmap, new Matrix(), paint);
        //改变画笔的颜色
        paint.setColor(Color.RED);
        //改变画笔的大小
        paint.setStrokeWidth(5);
        iv.setImageBitmap(copyBimap);

        //图片的保存
        try{
        File file = new File(getExternalCacheDir(), "text.png");
        FileOutputStream fos = new FileOutputStream(file);
        copyBimap.compress(Bitmap.CompressFormat.PNG,100,fos);}
        catch (Exception e){
            e.printStackTrace();
        }
        iv.setOnTouchListener(new View.OnTouchListener() {
            int startX;
            int startY;

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                int action = event.getAction();
                switch (action) {
                    case MotionEvent.ACTION_DOWN://按下屏幕
                        //获取按下屏幕的位置
                        startX = (int) event.getX();
                        startY = (int) event.getY();
                        break;
                    case MotionEvent.ACTION_MOVE://移动
                        //d当前手指的位置
                        int endX = (int) event.getX();
                        int endy = (int) event.getY();
                        System.out.println(startX + " " + startY + " " + endX + " " + endy);
                        //划线
                        canvas.drawLine(startX, startY, endX, endy, paint);
                        iv.setImageBitmap(copyBimap);
                        //更新初始位置
                        startX = endX;
                        startY = endy;
                        break;
                    case MotionEvent.ACTION_UP://离开屏幕
                        iv.setImageBitmap(copyBimap);
                        break;
                }
                //一定要该为true
                return true;
            }
        });

    }

}