画饼图

 第一步, 获取上下文
        第二步,拼接路径 ,绘制第一个扇形
         获取上下文
         CGContextRef ctx =  UIGraphicsGetCurrentContext();
         CGPoint center = CGPointMake(125, 125);
         CGFloat radius = 100;
         CGFloat startA = 0;
         CGFloat endA = 0;
         CGFloat angle = 25 / 100.0 * M_PI * 2;
         endA = startA + angle;
         拼接路径
         UIBezierPath *path = [UIBezierPath bezierPathWithArcCenter:center 
                                                               radius:radius 
                                                               startAngle:startA 
                                                               endAngle:endA 
                                                               clockwise:YES];
         添加一根线到圆心
         [path addLineToPoint:center];
         把路径添加到上下文
         CGContextAddPath(ctx, path.CGPath);
         把上下文渲染到View
         CGContextFillPath(ctx);
        
         注意点: CGFloat angle = 25 / 100.0 * M_PI * 2; 
                100.0一定要加一个.0

        绘制第二个扇形
        一样的, 把路径描述第二个扇形就好了
        直接来个path =
        让Path指针重新指向一个新的对象.也就是把指针重复利用了
        之前的那个对象已经用完了,已经添加到了上下文当中了.
 
         第二个扇形
         startA = endA;
         angle = 25 / 100.0 * M_PI * 2;
         endA = startA + angle;
         path = [UIBezierPath bezierPathWithArcCenter:center
                                                  radius:radius 
                                                  startAngle:startA 
                                                  endAngle:endA 
                                                  clockwise:YES];
         [path addLineToPoint:center];
         把二个路径添加到上下文
         CGContextAddPath(ctx, path.CGPath);
         把上下文渲染到View
         CGContextFillPath(ctx);
 
 
        添加第二个扇形之后, 发现它们的颜色都一样,想要修改它的颜色
        在下面再写一个
        [[UIColor greenColor] set];
        下面的一个颜色把之前的东西给覆盖了.
        解决办法, 让它渲染两次
 
        第三个扇形,把第二个拷贝一下就好了
 
 
        总结:
            有没有发现在画三个扇形用太多代码了,
            里面有很多代码相似.
            是不是可以把代码给抽一下
            可以用便利数组的的方式
            发现就两个地方变了, 一个数字变了, 一个颜色变了.
 
 
        抽取代码:
            假设他给一组数据
            NSArray datas =  @[@25,@25,@50];
            把数组便利出来
            
 
         NSArray *datas =  @[@25,@25,@50];
         
         CGPoint center = CGPointMake(125, 125);
         CGFloat radius = 100;
         CGFloat startA = 0;
         CGFloat angle = 0;
         CGFloat endA = 0;
         
         for (NSNumber *number in datas) {
         
         startA = endA;
         angle = number.intValue / 100.0 * M_PI * 2;
         endA = startA + angle;
         
         描述路径
         UIBezierPath *path = [UIBezierPath bezierPathWithArcCenter:center 
                                               radius:radius
                                               startAngle:startA
                                               endAngle:endA
                                               clockwise:YES];
         
         [path addLineToPoint:center];
         [[self randomColor] set];
         [path fill];
         
         }

         - (UIColor *)randomColor{
         CGFloat r = arc4random_uniform(256)/ 255.0;
         CGFloat g = arc4random_uniform(256)/ 255.0;
         CGFloat b = arc4random_uniform(256)/ 255.0;
         return [UIColor colorWithRed:r green:g blue:b alpha:1];
         
         }
            
        随机颜色:alpha通道它的取值范围是0-255;
                OC里面的取值范围只能是0到1,把它 / 255.0就让它到这个范围了,
                arc4random_uniform(256)随机产生 0 - 255的数.
                颜色通道它的取值范围是0 到 255.
                所以说要把0 到 255转换成0 到 1
                直接是  0 ~ 255 / 255.0就可以了.
                刚好是255就是255 / 255.0 就是1,
                刚才是0 就是 0 / 255.0 就是0.