初学Android,图形图像之使用双缓冲画图(二十七)

　当数据量很大时，绘图可能需要几秒钟甚至更长的时间，而且有时还会出现闪烁现象，为了解决这些问题，可采用双缓冲技术来绘图。
　双缓冲即在内存中创建一个与屏幕绘图区域一致的对象，先将图形绘制到内存中的这个对象上，再一次性将这个对象上的图形拷贝到屏幕上，这样能大大加快绘图的速度。双缓冲实现过程如下：
　　1、在内存中创建与画布一致的缓冲区
　　2、在缓冲区画图
　　3、将缓冲区位图拷贝到当前画布上

　　4、释放内存缓冲区

下面的例子(一个画图板)将实现双缓冲画图

先自定义一个View(Bitmap将会绘制到这个View上)

package Wangli.Graphics.HandDraw;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class DrawView extends View {
    float preX;
    float preY;
    private Path path;
    public Paint paint = null;
    final int VIEW_WIDTH = 320;
    final int VIEW_HEIGHT = 480;
    //定义一个内存中的图片,该图片将作为缓冲区
    Bitmap cacheBitmap = null;
    //定义cacheBitmap上的canvas对象
    Canvas cacheCanvas = null;
	public DrawView(Context context, AttributeSet attrs) {
		super(context, attrs);
		//创建一个与该View相同大小的缓存区
		cacheBitmap = Bitmap.createBitmap(VIEW_WIDTH,VIEW_HEIGHT,Config.ARGB_8888);
		cacheCanvas = new Canvas();
		path = new Path();
		//设置cacheCanvas将会绘制到内存中的cacheBitmap上
		cacheCanvas.setBitmap(cacheBitmap);
		//设置画笔的颜色
		paint = new Paint(Paint.DITHER_FLAG);
		paint.setColor(Color.RED);
		//设置画笔的风格
		paint.setStyle(Paint.Style.STROKE);
		paint.setStrokeWidth(1);
		//反锯齿
		paint.setAntiAlias(true);
		paint.setDither(true);
	}
    public boolean onTouchEvent(MotionEvent event)
    {
    	//获取拖动事件发生的位置
    	float x = event.getX();
    	float y = event.getY();
    	switch(event.getAction())
    	{
    	    case MotionEvent.ACTION_DOWN:
    		     path.moveTo(x, y);
    		     preX = x;
    		     preY = y;
    		     break;
    	    case MotionEvent.ACTION_MOVE:
   		         path.quadTo(preX, preY, x, y);
   		         preX = x;
   		         preY = y;
   		         break;
    	    case MotionEvent.ACTION_UP:
    	    	 cacheCanvas.drawPath(path, paint);
    	    	 path.reset();
    	    	 break;
    	}
    	invalidate();
    	//返回true表明处理方法已经处理该事件
    	return true;
    }
    public void onDraw(Canvas canvas)
    {
    	Paint bmpPaint = new Paint();
    	//将cacheBitmap绘制到该View组件上
    	canvas.drawBitmap(cacheBitmap, 0, 0, bmpPaint);
    	//沿着path绘制
    	canvas.drawPath(path, paint);
    }
}

定义菜单资源文件my_menu

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
	<item android:title="@string/color">
		<menu>
			<!-- 定义一组单选菜单项 -->
			<group android:checkableBehavior="single">
				<!-- 定义多个菜单项 -->
				<item
					android:id="@+id/red" android:title="@string/color_red"/>
				<item
					android:id="@+id/green" android:title="@string/color_green"/>
				<item
					android:id="@+id/blue" android:title="@string/color_blue"/>
			</group>
		</menu>
	</item>
	<item android:title="@string/width">
		<menu>
			<!-- 定义一组菜单项 -->
			<group>
				<!-- 定义3个菜单项 -->
				<item
					android:id="@+id/width_1" android:title="@string/width_1"/>
				<item
					android:id="@+id/width_3" android:title="@string/width_3"/>
				<item
					android:id="@+id/width_5" android:title="@string/width_5"/>						
			</group>
		</menu>
	</item>
	<item
		android:id="@+id/blur" android:title="@string/blur"/>	
	<item
		android:id="@+id/emboss" android:title="@string/emboss"/>							
</menu>

strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="hello">Hello World, HandDraw!</string> 
  <string name="app_name">手绘</string> 
  <string name="width_1">1像素</string> 
  <string name="width_3">3像素</string> 
  <string name="width_5">5像素</string> 
  <string name="color_red">红色</string> 
  <string name="color_green">绿色</string> 
  <string name="color_blue">蓝色</string> 
  <string name="color">画笔颜色</string> 
  <string name="width">画笔宽度</string> 
  <string name="blur">模糊效果</string> 
  <string name="emboss">浮雕效果</string> 
</resources>

主界面处理菜单事件　

package Wangli.Graphics.HandDraw;

import android.app.Activity;
import android.graphics.BlurMaskFilter;
import android.graphics.Color;
import android.graphics.EmbossMaskFilter;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;

public class HandDraw extends Activity {
    /** Called when the activity is first created. */
	EmbossMaskFilter emboss;
	BlurMaskFilter blur;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        emboss = new EmbossMaskFilter(new float[]{1.5f,1.5f,1.5f},0.6f,6,4.2f);
        blur = new BlurMaskFilter(8,BlurMaskFilter.Blur.NORMAL);
    }
    //负责创建选项菜单
    public boolean onCreateOptionsMenu(Menu menu)
    {
    	MenuInflater inflator = new MenuInflater(this);
    	//状态R.menu.context对应菜单,并添加到中
    	inflator.inflate(R.menu.my_menu,menu);
    	return super.onCreateOptionsMenu(menu);
    }
    //菜单项被单击后的回调方法
    public boolean onOptionsItemSelected(MenuItem mi)
    {
    	DrawView dv = (DrawView)findViewById(R.id.draw);
    	//判断单击的是哪个菜单项,并有针对性地做出响应
    	switch(mi.getItemId())
    	{
    	    case R.id.red:
    	    	dv.paint.setColor(Color.RED);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.green:
    	    	dv.paint.setColor(Color.GREEN);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.blue:
    	    	dv.paint.setColor(Color.BLUE);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.width_1:
    	    	dv.paint.setStrokeWidth(1);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.width_3:
    	    	dv.paint.setStrokeWidth(3);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.width_5:
    	    	dv.paint.setStrokeWidth(5);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.blur:
    	    	dv.paint.setMaskFilter(blur);
    	    	mi.setChecked(true);
    	    	break;
    	    case R.id.emboss:
    	    	dv.paint.setMaskFilter(emboss);
    	    	mi.setChecked(true);
    	    	break;
    	}
    	return true;
    }
}