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UIScrollView在iOS11新增的contentInsetAdjustmentBehavior属性

2017-07-06

在iOS11中，苹果对UIScrollView新增了contentInsetAdjustmentBehavior属性来取代之前的UIViewController的automaticallyAdjustsScrollViewInsets属性。

在iOS11之前的项目中，如果我们需要将滚动视图的内容布局从屏幕左上角开始，也就是内容穿透UINavigationBar的效果，只需要设置UIViewController的automaticallyAdjustsScrollViewInsets属性为NO即可，告诉系统不要自动调整滚动视图的contentInset。但是在iOS11中，你将发现尽管将automaticallyAdjustsScrollViewInsets设置为NO了，但是滚动视图的内容还是往下偏移了64pt，从导航栏底部开始显示，通过查看automaticallyAdjustsScrollViewInsets的声明发现在iOS11中该属性已经被废弃以及被UIScrollView的contentInsetAdjustmentBehavior属性所替代了。

@property(nonatomic,assign) BOOL automaticallyAdjustsScrollViewInsets API_DEPRECATED_WITH_REPLACEMENT("Use UIScrollView's contentInsetAdjustmentBehavior instead", ios(7.0,11.0),tvos(7.0,11.0)); // Defaults to YES

我们再看下contentInsetAdjustmentBehavior的声明

@property(nonatomic) UIScrollViewContentInsetAdjustmentBehavior contentInsetAdjustmentBehavior API_AVAILABLE(ios(11.0),tvos(11.0));

contentInsetAdjustmentBehavior对应的是一个UIScrollViewContentInsetAdjustmentBehavior枚举类型的值，该枚举定义如下:

typedef NS_ENUM(NSInteger, UIScrollViewContentInsetAdjustmentBehavior) {
    UIScrollViewContentInsetAdjustmentAutomatic, // Similar to .scrollableAxes, but will also adjust the top & bottom contentInset when the scroll view is owned by a view controller with automaticallyAdjustsScrollViewContentInset = YES inside a navigation controller, regardless of whether the scroll view is scrollable
    UIScrollViewContentInsetAdjustmentScrollableAxes, // Edges for scrollable axes are adjusted (i.e., contentSize.width/height > frame.size.width/height or alwaysBounceHorizontal/Vertical = YES)
    UIScrollViewContentInsetAdjustmentNever, // contentInset is not adjusted
    UIScrollViewContentInsetAdjustmentAlways, // contentInset is always adjusted by the scroll view's safeAreaInsets
} API_AVAILABLE(ios(11.0),tvos(11.0));

通过查看UIScrollViewContentInsetAdjustmentBehavior的定义，我们很容易知道想要实现前面所说的效果，只需将滚动设置的contentInsetAdjustmentBehavior属性设置为对应的UIScrollViewContentInsetAdjustmentNever即可。

if (@available(iOS 11.0, *)) {
    scrollView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;
} else {
    self.automaticallyAdjustsScrollViewInsets = NO;
}

在iOS11之前，系统会通过动态调整UIScrollView的contentInset属性来实现视图内容的偏移，但是在iOS11中，系统不再调整UIScrollView的contentInset，而是通过contentInsetAdjustmentBehavior和adjustedContentInset来配合完成。

iOS