iPhone中 rotation 处理

只支持某个页面旋转

最近有人问我一个关于iPhone上旋转的需求：App中只对某个页面支持旋转，其它页面一律只支持 portrait。比如，A push B，在B页面旋转后，页面会自适应；当B处于 landscape 时，pop回到A时，A还要处于 portrait。
一开始，我的答案时重载 - (UIInterfaceOrientationMask)supportedInterfaceOrientations，对方试了一下没有效果。亲身实践了一下，单设置此方法确实不行。了解到对方的项目中页面并没有完全使用 AutoLayout，但还是实现了 rotation 相关的方法，于是使用在 AppDelegate 中实现: 

- (UIInterfaceOrientationMask)application:(UIApplication *)application 
  supportedInterfaceOrientationsForWindow:(UIWindow *)window

具体步骤是在 AppDelegate 中添加属性 UIInterfaceOrientationMask，在每个 VC 出现和消失的时候，更新该属性。回到这个例子，可以在 AppDelegate 初始化 UIInterfaceOrientationMask 为 UIInterfaceOrientationMaskPortrait，接着再在B中更新： 

- (void)viewWillAppear:(BOOL)animated{
    //rotation support
    AppDelegate *delegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
    [delegate updateSupportedOrientation:UIInterfaceOrientationMaskAllButUpsideDown];
}
- (void)viewWillDisappear:(BOOL)animated{
    //rotation support
    AppDelegate *delegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
    [delegate updateSupportedOrientation:UIInterfaceOrientationMaskPortrait];
}

这个方法的原理在于旋转的时候，UIWindow首先接收到相应，然后再传递给 rootViewController 以及子 viewController。而如果实现了协议方法 

UIInterfaceOrientationMaskAllButUpsideDown:supportedInterfaceOrientationsForWindow:

那么每次系统检测到旋转事件后，都会调用该方法。

正常的旋转处理流程

iOS8开始，和 rotation 相关的几个方法已经废弃：

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation;  
- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration;  
- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration;  
- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation;

尤其是第一个，在编译阶段 Xcode 会提示该方法已经废弃。其它3个方法，如果没有使用新的处理rotation的方法时，还会继续执行。
如今，推荐使用的处理 rotation 的方法如下：

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator;

使用coordinator可以处理不同方向下的constraints或是页面布局，这里主要注意的是如何正确的判断当前所处的模式。Apple推荐的是使用 UITraitCollection / UITraitEnvironment 来检测当前的方向，下面举个例子:

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)
        coordinator.animate(alongsideTransition: { context in
            self.p_updateContstraints()
        }) { contenxt in

        } 
    }

    fileprivate func p_updateContstraints() {
        let constraints = self.viewRed.constraints
        self.viewRed.removeConstraints(constraints)
        self.p_removeAnchorConstranits(target: self.viewRed, from: self.view)

        let widthConstraint = NSLayoutConstraint(item: self.viewRed!, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .width, multiplier: 1.0, constant: 50)
        let heightConstraint = NSLayoutConstraint(item: self.viewRed!, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .height, multiplier: 1.0, constant: 50)
        self.viewRed.addConstraints([widthConstraint, heightConstraint])

        let trait = self.traitCollection
        let safeLayoutGuide = self.view.safeAreaLayoutGuide
        if trait.userInterfaceIdiom == .phone {
            if trait.verticalSizeClass == .compact && trait.horizontalSizeClass == .regular{
                //plus landscape: left-top
                self.viewRed.topAnchor.constraint(equalTo: safeLayoutGuide.topAnchor, constant: 0).isActive = true
                self.viewRed.leadingAnchor.constraint(equalTo: safeLayoutGuide.leadingAnchor, constant: 0).isActive = true
            } else if trait.verticalSizeClass == .compact && trait.horizontalSizeClass == .compact {
                //landscape: left-bottom
                self.viewRed.bottomAnchor.constraint(equalTo: safeLayoutGuide.bottomAnchor, constant: 0).isActive = true
                self.viewRed.leadingAnchor.constraint(equalTo: safeLayoutGuide.leadingAnchor, constant: 0).isActive = true
            } else if trait.verticalSizeClass == .regular && trait.horizontalSizeClass == .compact {
                //portrait: right-top
                self.viewRed.topAnchor.constraint(equalTo: safeLayoutGuide.topAnchor, constant: 0).isActive = true
                self.viewRed.trailingAnchor.constraint(equalTo: safeLayoutGuide.trailingAnchor, constant: 0).isActive = true
            }
        }else if trait.userInterfaceIdiom == .pad{

        }else {

        }
    }
    fileprivate func p_removeAnchorConstranits(target: UIView, from superView: UIView ){
        let sConstraints = superView.constraints
        var toDeleteConstraints:[NSLayoutConstraint] = []
        for constraint in sConstraints {
            let firstView = constraint.firstItem as? UIView
            let secondView = constraint.secondItem as? UIView
            if firstView == target || secondView == target {
                toDeleteConstraints.append(constraint)
            }
        }
        superView.removeConstraints(toDeleteConstraints)
    }

上述代码主要是在iPhone portrait/landscape和plus landscape模式下，使红色方块处于不同的位置。因为使用了 safearealayoutguide，只对iOS11有效。

参考

uiviewcontroller
Preventing a View From Rotating
Size Class

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