在iOS中，圖形的繪製是基於UIView來繪製的，所以，我們要先建立一個用來繪製圖形的UIView類。

UIView的類中有一個方法是專門用來繪製圖形的：

/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
    // Drawing code
}
*/

/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
 */
- (void)drawRect:(CGRect)rect {
    // Drawing code
    
    //獲取當前裝置的上下文（畫布）
    CGContextRef context = UIGraphicsGetCurrentContext();
    //畫筆移到某一點
    CGContextMoveToPoint(context, 0, 0);
    //從一點到某一點畫條線
    CGContextAddLineToPoint(context, 100, 100);
    //給線渲染塗色
    CGContextStrokePath(context);
    
    //矩形框
    CGContextAddRect(context, CGRectMake(0, 0, 100, 100));
    CGContextStrokeRect(context, CGRectMake(0, 0, 100, 100));
    //實心矩形
    CGContextFillRect(context, CGRectMake(100, 100, 100, 100));
    //空心圓
    CGContextStrokeEllipseInRect(context, CGRectMake(100, 200, 100, 100));
    //實心圓
    CGContextFillEllipseInRect(context, CGRectMake(100, 300, 100, 100));
    
    //繪製文字
    NSString * str = @"Hello world!";
    [str drawAtPoint:CGPointMake(125, 400) withAttributes:nil];
    //繪製圖片
    [img drawAtPoint:CGPointMake(100, 500)];
    [img drawAsPatternInRect:CGRectMake(100, 400, 50, 50)]; //平鋪填充
    CGContextDrawImage(context, CGRectMake(100, 500, 80, 80), [img CGImage]);//自由縮放
}