iOS小明开发笔记(二十七) (Autolayout(VFL)一)

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。