iOS 如何设置再UIlabel设置背景透明度的时候,文字不透明

Question

UILabel *leftLabel = [[UILabel alloc]initWithFrame:CGRectMake(0+20, UIViewHeight - 20, UIViewWidth, 20)];

leftLabel.textColor = [UIColor whiteColor];

// leftLabel.backgroundColor = [UIColor lightGrayColor];

leftLabel.backgroundColor = [[UIColor blackColor]colorWithAlphaComponent:0.7f];