利用Canvas实现在屏幕随机位置绘制10个大小(边长为10-160)颜色随机的空心五角星

Android不但能够显示图片，而且还能够直至一些简单的二维绘图，其实对于我们常见的一些Android基本组件，如TextView、Button等都是系统绘制出来的，其绘制的操作在View类的onDraw(Canvas canvas) 方法中，每个组件都需要实现onDraw(Canvas canvas) 方法进行自定义绘制。

下面实现的是利用Canvas在屏幕随机位置绘制10个大小(边长为10-160)和颜色随机的空心五角星，先看下面的效果图：

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利用中学三角函数相关知识，以最左边顶点为基准，五角星的各个拐点点坐标表示如下（草图）：

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XML文件 :

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/wujiaoxing"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.example.myapplication.CanvasView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</RelativeLayout>

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MainActavity.java :

package com.example.myapplication;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

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CanvasView.java :

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
import java.util.Random;

public class CanvasView extends View {
    public CanvasView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //自定义相关三角函数值
        float sin18 = (float) Math.sin(18 * Math.PI / 180);
        float cos18 = (float) Math.cos(18 * Math.PI / 180);
        float sin36 = (float) Math.sin(36 * Math.PI / 180);
        float cos36 = (float) Math.cos(36 * Math.PI / 180);
        float sin27 = (float) Math.sin(27 * Math.PI / 180);
        float cos27 = (float) Math.cos(27 * Math.PI / 180);
        float sin54 = (float) Math.sin(54 * Math.PI / 180);
        float cos54 = (float) Math.cos(54 * Math.PI / 180);
        Paint paint = new Paint();//定义一个画笔对象
        paint.setAntiAlias(true);//设置抗锯齿
        paint.setStyle(Paint.Style.STROKE);//设置画笔风格
        paint.setStrokeWidth(8);//设置画笔粗细

        for(int i=0;i<10;i++) {
            //随机颜色(rgb)生成
            int red = new Random().nextInt(255)+1;
            int green = new Random().nextInt(255)+1;
            int blue = new Random().nextInt(255)+1;
            paint.setColor(Color.rgb(red,green,blue));
            //随机(x,y),a生成
            float x = (float) (new Random().nextInt(800) + 40);
            float y = (float) (new Random().nextInt(130)*10+100);
            float a= (float) (new Random().nextInt(150)+11);
            //五角星边的绘制
            canvas.drawLine(x, y, x + a, y, paint);
            canvas.drawLine(x + a, y, x + a + a * sin18, y - a * cos18, paint);
            canvas.drawLine(x + a + a * sin18, y - a * cos18, x + a + 2 * a * sin18, y, paint);
            canvas.drawLine(x + a + 2 * a * sin18, y, x + 2 * a + 2 * a * sin18, y, paint);
            canvas.drawLine(x + 2 * a + 2 * a * sin18, y, (x + 2 * a + 2 * a * sin18 - a * cos36), y + a * sin36, paint);
            canvas.drawLine(x + 2 * a + 2 * a * sin18 - a * cos36, y + a * sin36, x + (2 * a + 2 * a * sin18) * sin54, y + (2 * a + 2 * a * sin18) * cos54, paint);
            canvas.drawLine(x + (2 * a + 2 * a * sin18) * sin54, y + (2 * a + 2 * a * sin18) * cos54, x + a * sin54 - a * sin27 + a * cos27, y + a * (cos54 + cos27 - sin27), paint);
            canvas.drawLine(x + a * sin54 - a * sin27 + a * cos27, y + a * (cos54 + cos27 - sin27), x + a * sin54 - a * sin27, y + a * (cos54 + cos27), paint);
            canvas.drawLine(x + a * sin54 - a * sin27, y + a * (cos54 + cos27), x + a * sin54, y + a * (cos54), paint);
            canvas.drawLine(x + a * sin54, y + a * (cos54), x, y, paint);
            }
        }
}