文档解释
layoutMargins
使用这个属性用于指定视图和它的子视图之间的边距（单位使用点）,自动布局系统使用你的margins作为限制条件布局内容。例如，如果你通过格式化语句“|-[subview]-|”设置水平约束，则子视图的左右边缘距离父视图左右内边缘相应的layout margins。当你的视图边缘接近父视图的边缘并且你视图的 preservesSuperviewLayoutMargins属性为YES的时候，实际的layout margins可能会增加以防止内容与superview的边距重叠。
默认的margins 在每个边缘都是8 points
如果视图是一个控制器的root view，系统设置管理视图的margins，顶部和底部的marigns被设置为0。侧面的margins依赖于当前size class而变化，但可以使16或者20points。你不可以更改这些margins
preservesSuperviewLayoutMargins
当这个属性的值为YES的时候，一个视图布局内容时其父视图的margins也会被考虑在内。这个margins对于一个视图与其父视图边缘距离的影响小于视图相应的margin。例如，你有一个内容视图的frame恰好和父视图的bounds一样，当父视图的任何margins在内容视图和它的margins的表示范围的内部时（有重合？？？），UIKit自动调整内容视图的的布局以满足父视图的margins。调整的数量是保证内容视图恰好在父视图的margins范围内的最小数量。
属性默认值为NO。
个人理解
layoutMargins
设置一个视图的边距（视图边缘与其子视图边缘的距离），防止子视图和父视图边缘重合。在iOS 8中，可以使用layoutMargins去定义view之间的间距,该属性只对AutoLayout布局生效。
因此AutoLayout中NSLayoutAttribute的枚举值有了相应的更新：
NSLayoutAttributeLeftMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeRightMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeTopMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeBottomMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeLeadingMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeTrailingMargin NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeCenterXWithinMargins NS_ENUM_AVAILABLE_IOS(8_0),
NSLayoutAttributeCenterYWithinMargins NS_ENUM_AVAILABLE_IOS(8_0),
代码只有通过这些枚举值定义约束时，父视图定义的layoutMargins才会生效。见[self example1];消息
代码：
- (void)example1 {
UIView *blueView = [[UIView alloc] init];
blueView.backgroundColor = [UIColor blueColor];
blueView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:blueView];
NSMutableArray *constraints = [NSMutableArray array];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]];
[self.view addConstraints:constraints];
UIView *yellowView = [[UIView alloc] init];
yellowView.backgroundColor = [UIColor yellowColor];
yellowView.translatesAutoresizingMaskIntoConstraints = NO;
[blueView addSubview:yellowView];
[constraints removeAllObjects];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeLeftMargin relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeLeftMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeRightMargin relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeRightMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeTopMargin relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeTopMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeBottomMargin relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeBottomMargin multiplier:1.0 constant:0.0]];
[blueView addConstraints:constraints];
blueView.layoutMargins = UIEdgeInsetsMake(50, 50, 50, 50);
}
结果：
6C50DE61-ABB2-42C9-B21D-E427093BA09D.png
preservesSuperviewLayoutMargins
当为YES时，保证满足父视图通过layoutMargins设置的边距，如果一个视图A本身无法满足父视图B的layoutMargins并且视图的preservesSuperviewLayoutMargins属性为YES时，UIKit自动调整视图A内容视图的的布局以满足父视图的margins。
见[self example2];
设置blueView的layoutMargins都为50，yellowView高为blueView的一半且中心点重合，UIKit约束计算后得出yellowView此时上下边缘到blueView的上下边缘距离满足50 points距离的结论。但是左右边缘不满足所以当yellowView的preservesSuperviewLayoutMargins为YES时，UIKit自动调整blackView的左右边距以满足blueView的margins。当yellowView.preservesSuperviewLayoutMargins = NO;的时候blackView完全遮盖yellowView
代码：
- (void)example2 {
UIView *blueView = [[UIView alloc] init];
blueView.backgroundColor = [UIColor blueColor];
blueView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:blueView];
NSMutableArray *constraints = [NSMutableArray array];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]];
[self.view addConstraints:constraints];
UIView *yellowView = [[UIView alloc] init];
yellowView.backgroundColor = [UIColor yellowColor];
yellowView.translatesAutoresizingMaskIntoConstraints = NO;
[blueView addSubview:yellowView];
[constraints removeAllObjects];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeHeight multiplier:0.5 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]];
[blueView addConstraints:constraints];
UIView *blackView = [[UIView alloc] init];
blackView.backgroundColor = [UIColor blackColor];
blackView.translatesAutoresizingMaskIntoConstraints = NO;
[yellowView addSubview:blackView];
[constraints removeAllObjects];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blackView attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:yellowView attribute:NSLayoutAttributeTrailingMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blackView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:yellowView attribute:NSLayoutAttributeLeadingMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blackView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:yellowView attribute:NSLayoutAttributeTopMargin multiplier:1.0 constant:0.0]];
[constraints addObject:[NSLayoutConstraint constraintWithItem:blackView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:yellowView attribute:NSLayoutAttributeBottomMargin multiplier:1.0 constant:0.0]];
[yellowView addConstraints:constraints];
// 设置blueView的layoutMargins都为50，yellowView高为blueView的一半且中心点重合，UIKit约束计算后得出yellowView此时上下边缘到blueView的上下边缘距离满足50 points距离的结论。但是左右边缘不满足所以当yellowView的preservesSuperviewLayoutMargins为YES时，UIKit自动调整blackView的左右边距以满足blueView的margins。当yellowView.preservesSuperviewLayoutMargins = NO;的时候blackView完全遮盖yellowView
blueView.layoutMargins = UIEdgeInsetsMake(50, 50, 50, 50);
yellowView.layoutMargins = UIEdgeInsetsZero;
yellowView.preservesSuperviewLayoutMargins = YES;
}
656D2D8A-1B6E-4CE7-9482-9C2E5B3319C1.png
当yellowView.preservesSuperviewLayoutMargins = NO时：
944992DD-1582-4349-A0C2-B53AC147E571.png
将yellowView.preservesSuperviewLayoutMargins 设置为YES，修改：
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]];
为：
[constraints addObject:[NSLayoutConstraint constraintWithItem:yellowView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeTop multiplier:1.0 constant:0.0]];
后yellowView的顶部边缘和blueView的顶部边缘重合，不在满足blueView设置的
layoutMargins的值，所以UIKit自动调整blackView的边距：
E27D0CAC-F6C7-41E6-A2A7-C62906D29D49.png
见demo [self example3];