使按钮列表水平与自动布局对齐

我正在通过自动布局以编程方式创建许多UIButton：

for (int i=0; i<buttonsArray.count; i++) {

    UIButton *button = (UIButton*)[UIButton buttonWithType:UIButtonTypeRoundedRect];
    [button setTitle:[NSString stringWithFormat:@" %@ ",titleString]  forState:UIControlStateNormal];
    [button sizeToFit];
    button.translatesAutoresizingMaskIntoConstraints = NO;
    button.tag =i;     
    [view addSubview:button];

    // check if first button or button would exceed maxWidth
    if ((i == 0) || (runningWidth + button.frame.size.width > maxWidth)) {
        // wrap around into next line
        runningWidth = button.frame.size.width;

        if (i== 0) {
            // first button (top left)
            // horizontal position: same as previous leftmost button (on line above)
            NSLayoutConstraint *horizontalConstraint = [NSLayoutConstraint constraintWithItem:button
                                                                                    attribute:NSLayoutAttributeLeft
                                                                                    relatedBy:NSLayoutRelationEqual
                                                                                       toItem:reuseView
                                                                                    attribute:NSLayoutAttributeLeft
                                                                                   multiplier:1.0f
                                                                                     constant:5];
            [reuseView addConstraint:horizontalConstraint];

            // vertical position:
            NSLayoutConstraint *verticalConstraint = [NSLayoutConstraint constraintWithItem:button
                                                                                  attribute:NSLayoutAttributeTop
                                                                                  relatedBy:NSLayoutRelationEqual
                                                                                     toItem:reuseView
                                                                                  attribute:NSLayoutAttributeLeft
                                                                                 multiplier:1.0f
                                                                                   constant:5];
            [reuseView addConstraint:verticalConstraint];


        } else {
            // put it in new line
            UIButton *previousLeftmostButton = [buttons objectAtIndex:indexOfLeftmostButtonOnCurrentLine];

            // horizontal position: same as previous leftmost button (on line above)
            NSLayoutConstraint *horizontalConstraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:previousLeftmostButton attribute:NSLayoutAttributeLeft multiplier:1.0f constant:0.0f];
            [reuseView addConstraint:horizontalConstraint];

            // vertical position:
            NSLayoutConstraint *verticalConstraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:previousLeftmostButton attribute:NSLayoutAttributeBottom multiplier:1.0f constant:verticalSpaceBetweenButtons];
            [reuseView addConstraint:verticalConstraint];

            indexOfLeftmostButtonOnCurrentLine = i;
        }
    } else {
        // put it right from previous buttom
        runningWidth += button.frame.size.width + horizontalSpaceBetweenButtons;

        UIButton *previousButton = [buttons objectAtIndex:(i-1)];

        // horizontal position: right from previous button
        NSLayoutConstraint *horizontalConstraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:previousButton attribute:NSLayoutAttributeRight multiplier:1.0f constant:horizontalSpaceBetweenButtons];
        [reuseView addConstraint:horizontalConstraint];

        // vertical position same as previous button
        NSLayoutConstraint *verticalConstraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:previousButton attribute:NSLayoutAttributeTop multiplier:1.0f constant:0.0f];
        [reuseView addConstraint:verticalConstraint];
    }

    [buttons addObject:button];
}

所以我得到这样的界面：

但是，我需要在视图中心水平对齐按钮吗？任何想法如何做到这一点？