调用按钮内部imageView的尺寸

1.新建一个继承自UIButton的类

2.重写

- (CGRect)imageRectForContentRect:(CGRect)contentRect
{
    CGFloat imageW = 40;
    CGFloat imageH = 47;
    CGFloat imageX = (contentRect.size.width - imageW) * 0.5;
    CGFloat imageY = 20;
    return CGRectMake(imageX, imageY, imageW, imageH);
}