如何用约束使多个按钮居中？

我写了一个简单的addButtons方法来添加以屏幕为中心的按钮。

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();
    // Perform any additional setup after loading the view, typically from a nib

    for (int i = 0; i < 6; i++)
    {
        addButtons(View,i);
    }
}

void addButtons(UIView backView, int buttonNumber) {

    //number of buttons in one line
    int cols = 3;

    //add it to buttonY to change the start position of Y
    var startYPadding = 30;

    var buttonWidth = 90;
    var buttonHeight = 70;

    var colMargin = (backView.Bounds.Size.Width - (cols * buttonWidth)) / (cols + 1);

    var col = buttonNumber % cols;
    var row = buttonNumber / cols;

    //if you want the paddings between buttons use below code
    //var buttonX = col * (buttonWidth + colMargin)+ colMargin;
    //var buttonY = row * (buttonHeight + colMargin) + startYPadding;

    //if you dont want the padding between buttons use below code
    var buttonX = (backView.Bounds.Size.Width - (cols * buttonWidth))/2 + col*buttonWidth;
    var buttonY = row * buttonHeight + startYPadding;

    UILabel btn = new UILabel(new CGRect(buttonX, buttonY, buttonWidth, buttonHeight));
    btn.BackgroundColor = UIColor.Red;
    btn.TextAlignment = UITextAlignment.Center;
    btn.Text = "botton" + buttonNumber;
    backView.Add(btn);
}

如果你有什么问题可以问我。