学步园

VFL的子视图居中，网上搜了很多都没找到有用代码，自己研究出来了。代码如下

    UIActivityIndicatorView* prgrssView = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
    prgrssView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:prgrssView];
    [prgrssView release];
    NSDictionary* views = NSDictionaryOfVariableBindings(prgrssView);
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|-[prgrssView]-|" options:NSLayoutFormatAlignAllCenterX metrics:nil views:views]];

上面的代码可以让prgrssView 水平居中。垂直代码如下

    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[prgrssView]-|" options:NSLayoutFormatAlignAllCenterY metrics:nil views:views]];

以上代码测试下来只对UIActivityIndicatorView有效，如果一个view有宽度和高度则上面的居中代码会报错。可以通过另一种方式来处理，具体代码如下

        //beging
        UIImageView* imagevew = [UIImageView autolayoutView];
        [imagevew setContentMode:UIViewContentModeScaleToFill];
        [imagevew setImage:_tabCellImage];
        [imagevew setTag:AEC_TAG_IMAGE_VIEW];
        [self.view addSubview:imagevew];
        [imagevew release];
        
        NSDictionary* views = NSDictionaryOfVariableBindings(imagevew);
        //设置高度
        [self.view.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[imagevew(60)]" options:0 metrics:nil views:views]];
        //设置宽度
        [self.view.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"[imagevew(60)]" options:0 metrics:nil views:views]];
        //垂直居中
        [self.view addConstraint:[NSLayoutConstraint constraintWithItem:imagevew attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
        //水平居中
        [self.view addConstraint:[NSLayoutConstraint constraintWithItem:imagevew attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];
        //end

这样就设置了一个imageview其大小为60*60 在整个视图中居中。