Android N 边形 View

（1）正多边形的中心点为（0，0）
for (i = 0; i < n; i++) {
  printf("%f %f\n", r * Math.cos(2 * Math.PI * i / n), r * Math.sin(2 * Math.PI * i / n));
}
x轴坐标为 r * Math.cos(2 * Math.PI * i / n)
y轴坐标为 r * Math.sin(2 * Math.PI * i / n)

（2）正多边形的中心点为（a，b）
for (i = 0; i < n; i++) {
  printf("%f %f\n",a + r * Math.cos(2 * Math.PI * i / n), b+ r * Math.sin(2 * Math.PI * i / n));
}

x轴坐标为 a + r * Math.cos(2 * Math.PI * i / n)
y轴坐标为 b + r * Math.sin(2 * Math.PI * i / n)

public class TestView extends View {

    private static final int DEF_SIDE_LENGTH = 6;
    private static final int DEF_SIZE = 66;
    private int sideLength;
    private Paint paint;
    private Context context;
    private Path path;
    private int bgRes;
    private int radius;

    public TestView(Context context) {
        super(context);
        init(context, null);
    }

    public TestView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs);
    }

    public TestView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(context, attrs);
    }

    private void init(Context context, @Nullable AttributeSet attrs) {
        this.context = context;
        TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.TestView);
        sideLength = typedArray.getInt(R.styleable.TestView_side_length, DEF_SIDE_LENGTH);
        bgRes = typedArray.getResourceId(R.styleable.TestView_bg_res, 0);
        typedArray.recycle();
        if(bgRes != 0){
            initPaint();
        }
    }

    private void initPaint() {
        paint = new Paint();
        paint.setColor(ContextCompat.getColor(context, android.R.color.black));
        paint.setStrokeWidth(10f);
        paint.setAntiAlias(true);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), bgRes);
        BitmapShader shader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        paint.setShader(shader) ;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if(bgRes == 0){
            return;
        }
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        if(widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY){
            int width = MeasureSpec.getSize(widthMeasureSpec);
            int height = MeasureSpec.getSize(heightMeasureSpec);
            radius = Math.min(width, height) / 2;
        }else {
            float scale = context.getResources().getDisplayMetrics().density;
            int size = (int) (DEF_SIZE * scale + 0.5f);
            radius = size / 2;
            widthMeasureSpec = MeasureSpec.makeMeasureSpec(size, MeasureSpec.EXACTLY);
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(size, MeasureSpec.EXACTLY);
            setMeasuredDimension(widthMeasureSpec, heightMeasureSpec);
        }
        initPoint();
    }


    public void setSideLength(int sideLength) {
        this.sideLength = sideLength;
        initPoint();
        invalidate();
    }

    private void initPoint() {
        if(null == path){
            path = new Path();
        }
        path.reset();
        for (int i = 1; i <= sideLength; i++) {
           float x = (float)(radius * Math.cos(2 * Math.PI * i / sideLength)) + radius;
            float y = (float)(radius * Math.sin(2 * Math.PI * i / sideLength)) + radius;
            if(i == 1){
                path.moveTo(x, y);
            }else {
                path.lineTo(x, y);
            }
        }
        path.close();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(bgRes != 0){
            canvas.drawPath(path, paint);
        }
    }

}

 <declare-styleable name="TestView">
        <attr name="side_length" format="integer" />
        <attr name="bg_res" format="reference" />
    </declare-styleable>