绘图——Android绘图基础：Canvas、Paint等

Android的绘图应该继承View组件，并重写它的onDraw(Canvas canvas)方法即可。

重写onDraw(Canvas canvas)方法时涉及一个绘图API：Canvas，Canvas代表了“依附”于指定View的画布，它提供了如表7.1所示的方法绘制各种图形。

表7.1  Canvas的绘制方法

方法签名	简要说明
drawArc(RecF oval,float startAngle,flaot sweepAngle,boolean useCenter,Paint paint)	绘制弧度
drawBitmap(Bitmap bitmap,Rect src,Rect dst,Paint  paint)	在指定点绘制从源位图中“挖取”的一块
drawBitmap(Bitmap bitmap,float left,float top,Paint paint)	在指定点绘制位图
drawCircle(float cx,float cy,float radius,Paint paint)	在指定点绘制一个圆形
drawLine(float startX,float startY,float stopX,float stopY,Paint paint)	绘制一条线
drawLines(float[]pts,int offset,int count,Paint paint)	绘制多条线
drawOval(RectF oval,Paint paint)	绘制椭圆
drawPath(Path path,Paint paint)	沿着指定path绘制任意形状
drawPoint(float x,float y,Paint)	绘制一个点
drawPoints(float[] pts,int offset,int count,Paint paint)	绘制多个点
drawRec(float left,float top,float right,float bottom,Paint paint)	绘制矩形
drawRoundRect(RectF rect,float rx,float ry,Paint paint)	绘制圆角矩形
drawText(String text,int start,int end,Paint paint)	绘制字符串
drawTextOnPath(String text,Path path,float hOffset,float vOffset,Paint paint)	沿着路径绘制字符串
clipRect(float left,float top,float right,float bottom)	剪切一个矩形区域
clipRegion(Region region)	剪切指定区域

除了表7.1所定义的各种方法之外，Canvas还提供了如下方法进行坐标变换。

• rotate(float degrees,float px,float py)：对Canvas执行旋转变换。
• scale(float sx,float sy,float px,float py)：对Canvas执行缩放变换。
• skew(float sx,float sy)：对Canvas执行倾斜变换。
• translate(float dx,float dy)：移动Canvas。向右移动dx距离（dx为负数即向左移动）；向下移动dy距离（dy为负数即向上移动）。

Canvas提供的上面的方法还涉及一个API：Paint，Paint代表了Canvas上的画笔，因此Paint类主要用于设置绘制风格，包括画笔颜色、画笔笔触粗细、填充风格等。Paint提供了如表7.2所示的方法。

表7.2  Paint的常用方法

方法签名	简要说明
setARGB(int a,int g,int b)/setColor(int color)	设置颜色
setAlpha(int a)	设置透明度
setAntiAlias(boolean aa)	设置是否抗锯齿
setColor(int color)	设置颜色
setPathEffect(PathEffect effect)	设置绘制路径时的路径效果
setShader(Shader shader)	设置画笔的填充效果
setShadowLayer(float radius,float dx,float dy,int color)	设置阴影
setStrokeWidth(float width)	设置画笔的笔触宽度
setStrokeJoin(Paint.Join join)	设置画笔转弯处的连接风格
setStyle(Paint.Style style)	设置Paint的填充风格
setTextAlign(Paint.Align align)	设置绘制文本时文本的对齐方式
setTextSize(float textSize)	设置绘制文本时的文字大小

在Canvas提供的绘制方法中还用到了一个API：Path，Path代表了任意多条直线连接而成的任意图形，当Canvas根据Path绘制时，它可以绘制出任意的形状。

下面的程序示范了如何在Android应用中绘制基本的集合图形，该程序的关键在于一个自定义的View组件，程序重写该View组件的onDraw(Canvas)方法，接下来在该Canvas上绘制了大量几何图形。这个自定义View的代码如下。

package com.example.studydrawable;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.LinearGradient;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.RectF;
import android.graphics.Shader;
import android.util.AttributeSet;
import android.view.View;

public class MyView extends View {

    public MyView(Context context, AttributeSet attrs) {
        super(context, attrs);
        // TODO Auto-generated constructor stub
    }

    @Override
    //重写该方法，进行绘图
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        super.onDraw(canvas);
        //把整张画布绘制成白色
        canvas.drawColor(Color.WHITE);

        Paint paint=new Paint();
        //去锯齿
        paint.setAntiAlias(true);
        paint.setColor(Color.BLUE);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(3);
        //绘制圆形
        canvas.drawCircle(40, 40, 30, paint);
        //绘制正方形
        canvas.drawRect(10, 80,70,140,paint);
        //绘制矩形
        canvas.drawRect(10, 150,70,190,paint);
        RectF re1=new RectF(10,200,70,230);
        //绘制圆角矩形
        canvas.drawRoundRect(re1, 15, 15, paint);
        RectF rel1=new RectF(10,240,70,270);
        //绘制椭圆
        canvas.drawOval(rel1, paint);
        //定义一个Path对象，封闭成一个三角形
        Path path1=new Path();
        path1.moveTo(10, 340);
        path1.lineTo(70, 340);
        path1.lineTo(40, 290);
        path1.close();
        //根据Path进行绘制，绘制三角形
        canvas.drawPath(path1,paint);
        //定义一个Path对象，封闭成一个五角形
        Path path2=new Path();
        path2.moveTo(26, 360);
        path2.lineTo(54, 360);
        path2.lineTo(70, 392);
        path2.lineTo(40, 420);
        path2.lineTo(10, 392);
        path2.close();
        //根据path进行绘制，绘制五角形
        canvas.drawPath(path2, paint);
        //----------设置填充风格后绘制----------------
        paint.setStyle(Paint.Style.FILL);
        paint.setColor(Color.RED);
        canvas.drawCircle(120, 40, 30, paint);
        //绘制正方形
        canvas.drawRect(90, 80,150,140,paint);
        //绘制矩形
        canvas.drawRect(90, 150,150,190,paint);
        RectF re2=new RectF(90,200,150,230);
        //绘制圆角矩形
        canvas.drawRoundRect(re2, 15, 15,paint);
        RectF re21=new RectF(90,240,150,270);
        //绘制椭圆
        canvas.drawOval(re21, paint);
        Path path3=new Path();
        path3.moveTo(90, 340);
        path3.lineTo(150, 340);
        path3.lineTo(120, 290);
        path3.close();
        //绘制三角形
        canvas.drawPath(path3, paint);
        Path path4=new Path();
        path4.moveTo(106, 360);
        path4.lineTo(134, 360);
        path4.lineTo(150, 392);
        path4.lineTo(120, 420);
        path4.lineTo(90, 392);
        path4.close();
        //绘制五角形
        canvas.drawPath(path4, paint);
        //---------设置渐变器后绘制----------
        //为Paint设置渐变器
        Shader mShader=new LinearGradient(0,0,40,60,new int[]{Color.RED,Color.GREEN,Color.BLUE,Color.YELLOW}
        ,null,Shader.TileMode.REPEAT);
        paint.setShader(mShader);
        //设置阴影
        paint.setShadowLayer(45,10, 10, Color.GRAY);
        //绘制圆形
        canvas.drawCircle(200, 40, 30, paint);
        //绘制正方形
        canvas.drawRect(170,80,230,140, paint);
        //绘制矩形
        canvas.drawRect(170,150,230,190, paint);
        RectF re3=new RectF(170,200,230,230);
        //绘制圆角矩形
        canvas.drawRoundRect(re3, 15, 15, paint);
        RectF re31=new RectF(170,240,230,270);
        //绘制椭圆
        canvas.drawOval(re31, paint);
        Path path5=new Path();
        path5.moveTo(170, 340);
        path5.lineTo(230, 340);
        path5.lineTo(200, 290);
        path5.close();
        //根据Path进行绘制，绘制三角形
        canvas.drawPath(path5, paint);
        Path path6=new Path();
        path6.moveTo(186, 360);
        path6.lineTo(214, 360);
        path6.lineTo(230, 392);
        path6.lineTo(200, 420);
        path6.lineTo(170, 392);
        path6.close();
        //根据Path进行绘制，绘制五角形
        canvas.drawPath(path6,paint);
        //------设置字符大小后绘制--------
        paint.setTextSize(24);
        paint.setShader(null);
        //绘制7个字符串
        canvas.drawText(getResources().getString(R.string.circle), 240, 50, paint);
        canvas.drawText(getResources().getString(R.string.square), 240, 120, paint);
        canvas.drawText(getResources().getString(R.string.rect), 240, 175, paint);
        canvas.drawText(getResources().getString(R.string.round_rect), 230, 220, paint);
        canvas.drawText(getResources().getString(R.string.oval), 240, 260, paint);
        canvas.drawText(getResources().getString(R.string.triangle), 240, 325, paint);
        canvas.drawText(getResources().getString(R.string.circle), 240, 390, paint);

    }

}

上面的程序中大量调用了Canvas的方法来绘制几何图形，而且程序的粗体字代码还为Paint画笔设置了使用渐变、阴影，因此接下来绘制的几何图形将采用渐变填充，而且具有阴影。使用一个Activity来显示上