双缓冲技术主要用在绘图，动画效果上，其原理就是：将资源先加载到缓冲区，然后再将缓冲区整个加载到View上面去。双缓冲技术能够有效防止闪烁，提高显示质量。
DrawView.java:
package com.example.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 = 720;
final int VIEW_HEIGHT = 800;
//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 set) {
super(context, set);
// TODO Auto-generated constructor stub
//创建一个与该View相同大小的缓冲区
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);
//设置画笔风格
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(1);
//反锯齿
paint.setAntiAlias(true);
paint.setDither(true);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
//获取拖动事件发生位置
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();
//表明处理方法已经处理改事件
return true;
}
@Override
protected 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);
}
}
my_menu.xml:
<?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 >
<!-- 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 >
<!-- 定义三个菜单项 -->
<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>
Activity.java:package com.example.handdraw;
import android.os.Bundle;
import android.app.Activity;
import android.graphics.BlurMaskFilter;
import android.graphics.Color;
import android.graphics.EmbossMaskFilter;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
public class MainActivity extends Activity {
EmbossMaskFilter emboss;
BlurMaskFilter blur;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
emboss = new EmbossMaskFilter(new float[]{ 1.5f, 1.5f, 1.5f }, 0.6f, 6, 4.2f);
blur = new BlurMaskFilter(8, BlurMaskFilter.Blur.NORMAL);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
MenuInflater inflator = new MenuInflater(this);
inflator.inflate(R.menu.my_menu, menu);
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// TODO Auto-generated method stub
DrawView dv = (DrawView)findViewById(R.id.draw);
switch(item.getItemId())
{
case R.id.red:
dv.paint.setColor(Color.RED);
item.setChecked(true);
break;
case R.id.green:
dv.paint.setColor(Color.GREEN);
item.setChecked(true);
break;
case R.id.blue:
dv.paint.setColor(Color.BLUE);
item.setChecked(true);
break;
case R.id.width_1:
dv.paint.setStrokeWidth(1);
break;
case R.id.width_3:
dv.paint.setStrokeWidth(3);
break;
case R.id.width_5:
dv.paint.setStrokeWidth(5);
break;
case R.id.blur:
dv.paint.setMaskFilter(blur);
break;
case R.id.emboss:
dv.paint.setMaskFilter(emboss);
break;
}
return true;
}
}
main.xml
<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >
<com.example.handdraw.DrawView
android:id="@+id/draw"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />
</RelativeLayout>