Layout Support 获取上下bar的长度

Layout Support

This protocol is implemented by the UIViewController properties topLayoutGuide and bottomLayoutGuide to support using Auto Layout with a view controller’s view, starting in iOS . You can use layout guides as layout items in the NSLayoutConstraint factory methods.

Layout Support
length
Provides the length, in points, of the portion of a view controller’s view that is overlaid by translucent or transparent UIKit bars. (required)

Declaration
SWIFT
var length: CGFloat { get }
OBJECTIVE-C
@property(nonatomic, readonly) CGFloat length
Discussion
In iOS  and later, a view controller’s view occupies the full height of the screen when there are no opaque UIKit bars (such as opaque navigation or tab bars). To keep your content within the area of a view that is not overlaid by translucent or transparent UIKit bars, employ the topLayoutGuide and bottomLayoutGuide properties in the UIViewController class and take advantage of Auto Layout. Query these properties within your implementation of the UIViewController method viewDidLayoutSubviews.

The guides work as follows:

The top layout guide indicates the distance, in points, between the top of a view controller’s view and the bottom of the bottommost bar that overlays the view

The bottom layout guide indicates the distance, in points, between the bottom of a view controller’s view and the top the bar (such as a tab bar) that overlays the view.

  If you are not using Auto Layout, you can make use of the length property and perform layout calculations yourself. Refer to the code snippets in the descriptions for the topLayoutGuide and bottomLayoutGuide properties, which explain how to obtain the numbers you need.

Import Statement
import UIKit

Availability
Available in iOS 7.0 and later.

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https://developer.apple.com/library/ios/documentation/UIKit/Reference/UILayoutSupport_Protocol/index.html