/**
 * 在图片上作画
 * 
 * @author admin
 * @time 2015年2月8日 上午1:12:18
 */
public class MainActivity extends Activity implements OnTouchListener {

    private ImageView mImageView;
    private Bitmap bitmap;
    private Canvas canvas;
    private float downX;
    private float downY;
    private Paint paint;
    private EditText etPaintWidth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 找到控件
        mImageView = (ImageView) findViewById(R.id.iv);

        etPaintWidth = (EditText) findViewById(R.id.et_paintWidth);
        // 为图片控件设置触摸事件
        mImageView.setOnTouchListener(this);

    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:// 按下
            if (bitmap == null) {
                // 创建一张空白图片,宽,高和ImageView一样
                bitmap = Bitmap.createBitmap(mImageView.getWidth(),
                        mImageView.getHeight(), Config.ARGB_8888);

                // 把空白图片设置给Canvas画画板, canvas所绘制的东西都绘制到当前图片Bitmap上
                canvas = new Canvas(bitmap);

                // 创建一个画笔
                paint = new Paint();
                // 设置画笔的颜色
                paint.setColor(Color.RED);
                // 设置画笔画线时的粗细
                
                String spaintWidth = etPaintWidth.getText().toString().trim();
                int paintWidth = Integer.valueOf(spaintWidth);
                paint.setStrokeWidth((float) paintWidth);

                // 给画布画一个背景色
                canvas.drawColor(Color.YELLOW);
            }

            downX = event.getX();
            downY = event.getY();

            System.out.println("按下了...:X:" + downX + ",Y:" + downY);
            break;
        case MotionEvent.ACTION_MOVE:
            float moveX = event.getX();
            float moveY = event.getY();
            // 开始画线, 这一行代码执行完毕, Bitmap中有一条线了.
            canvas.drawLine(downX, downY, moveX, moveY, paint);
            
            mImageView.setImageBitmap(bitmap);

            downX = moveX;
            downY = moveY;
            break;
        case MotionEvent.ACTION_UP:
            System.out.println("抬起了.......");
            break;
        }
        return true;
    }

    /**
     * 将手绘的图片保存到SD卡中
     * 
     * @param v
     */
    public void save(View v) {
        if (bitmap != null) {
            FileOutputStream fos = null;
            try {
                fos = new FileOutputStream("/mnt/sdcard/hehe.jpg");
                bitmap.compress(CompressFormat.JPEG, 100, fos);
                Toast.makeText(this, "保存成功", Toast.LENGTH_LONG).show();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } finally {
                try {
                    if (fos != null) {
                        fos.close();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <EditText
        android:id="@+id/et_paintWidth"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:hint="请输入画笔的粗细度" />

    <ImageView
        android:id="@+id/iv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/et_paintWidth" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:onClick="save"
        android:text="保存" />

</RelativeLayout>

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />