解决 button 的背景图片被拉伸

UIImage *btnImage = [UIImage imageNamed:@"common_button_red"];
CGFloat btnImageW = btnImage.size.width * 0.5;
CGFloat btnImageH = btnImage.size.height * 0.5;
UIImage *newBtnImage = [btnImage resizableImageWithCapInsets:UIEdgeInsetsMake(btnImageH, btnImageW, btnImageH, btnImageW) resizingMode:UIImageResizingModeStretch];
[_loginBT setBackgroundImage:newBtnImage forState:UIControlStateNormal];