手指绘图（贝塞尔曲线）

贝塞尔曲线是应用于二维图形应用程序的数学曲线。
曲线的定义有四个点：起始点、终止点（也称锚点）以及两个相互分离的中间点。
滑动两个中间点，贝塞尔曲线的形状会发生变化
</pre><p></p><pre name="code" class="java">import android.app.Activity;
import android.os.Bundle;
import android.content.Context;
import android.graphics.*;
import android.view.MotionEvent;
import android.view.View;

public class TouchScreenActivity extends Activity {    
	
      private Paint	mPaint;
	
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new TouchView(this));

        mPaint = new Paint();
        mPaint.setAntiAlias(true);  <span style="color: rgb(0, 130, 0); font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px; ">//设置抗锯齿，让绘画比较平滑 </span>
        mPaint.setDither(true);   <span style="color: rgb(0, 130, 0); font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px; ">//设置递色</span>
        mPaint.setColor(0xFF00FF00);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(10);       
    }
        
    public class TouchView extends View {
               
        private Bitmap  mBitmap;
        private Canvas  mCanvas;
        private Path    mPath;
        private Paint   mBitmapPaint;
        
        public TouchView(Context c) {
            super(c);
            /*   android.graphics.Bitmap.Config是一个枚举类型,里面定义了位图的四种格式
             *  ALPHA_8:数字为8，图形参数应该由一个字节来表示,应该是一种8位的位图
                 ARGB_4444:4+4+4+4=16，图形的参数应该由两个字节来表示,应该是一种16位的位图.
                 ARGB_8888:8+8+8+8=32，图形的参数应该由四个字节来表示,应该是一种32位的位图.
                 RGB_565:5+6+5=16，图形的参数应该由两个字节来表示,应该是一种16位的位图.
 
                                             网上讲,ALPHA_8，ARGB_4444,ARGB_8888都是透明的位图,也就是所字母A代表透明.
                 ARGB_4444:意味着有四个参数,即A,R,G,B,每一个参数由4bit表示.
                                     同理:
                ARGB_8888:意味着有四个参数,即A,R,G,B,每一个参数由8bit来表示.
                                    同理:
               RGB_565:意味着有三个参数,R,G,B,三个参数分别占5bit,6bit,5bit.
             */
            mBitmap = Bitmap.createBitmap(480, 800, 
            		  Bitmap.Config.RGB_565);
            mCanvas = new Canvas(mBitmap);
            mPath = new Path();
            mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        }
        
        @Override
        protected void onDraw(Canvas canvas) {    
            canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);            
            canvas.drawPath(mPath, mPaint);
        }
        
        private float mX, mY;
        //只有当移动的距离大于4px时,才在屏幕上绘图
        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;
        }
        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);//二次方贝塞尔曲线
                mX = x;
                mY = y;
            }
        }
        private void touch_up() {
            mPath.lineTo(mX, mY);
            mCanvas.drawPath(mPath, mPaint);
            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);
                    invalidate();
                    break;
                case MotionEvent.ACTION_MOVE:
                    touch_move(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    touch_up();
                    invalidate();
                    break;
            }
            return true;
        }
    }       
}