self.edgesForExtendedLayout

https://github.com/HeshamMegid/HMSegmentedControl
读HMSegmentedControl中，发现如下一行代码

self.edgesForExtendedLayout = UIRectEdgeNone;

查了一下用法：
http://blog.sina.com.cn/s/blog_6291e42d0101f6b0.html

    self.title = @"HMSegmentedControl Demo";
    self.view.backgroundColor = [UIColor whiteColor];
    self.edgesForExtendedLayout = UIRectEdgeNone;
    CGFloat viewWidth = CGRectGetWidth(self.view.frame);
    // Minimum code required to use the segmented control with the default styling.
    HMSegmentedControl *segmentedControl = [[HMSegmentedControl alloc] initWithSectionTitles:@[@"Trending", @"News", @"Library"]];
    segmentedControl.frame = CGRectMake(0, 20, viewWidth, 40);
    segmentedControl.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleWidth;
    [segmentedControl addTarget:self action:@selector(segmentedControlChangedValue:) forControlEvents:UIControlEventValueChanged];
    [self.view addSubview:segmentedControl];

    self.title = @"HMSegmentedControl Demo";
    self.view.backgroundColor = [UIColor whiteColor];
    // 注释zhe
    //self.edgesForExtendedLayout = UIRectEdgeNone;
    CGFloat viewWidth = CGRectGetWidth(self.view.frame);
    // Minimum code required to use the segmented control with the default styling.
    HMSegmentedControl *segmentedControl = [[HMSegmentedControl alloc] initWithSectionTitles:@[@"Trending", @"News", @"Library"]];
    segmentedControl.frame = CGRectMake(0, 84, viewWidth, 40);
    segmentedControl.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleWidth;
    [segmentedControl addTarget:self action:@selector(segmentedControlChangedValue:) forControlEvents:UIControlEventValueChanged];
    [self.view addSubview:segmentedControl];

两者效果图一样，如下图

没有遇到该方法之前，我之前用的是第二种方式，通过添加y值来达到效果。