前言

使用系统的转场动画，不能控制动画整个动画过程，没法针对系统导航栏进行颜色过渡效果，故此篇文章采取自定义转场动画的方式，把控整个动画过程，实现导航栏颜色平滑的过渡效果，实际效果如下所示。

正文

接下来开始进入正题，本篇文章主要涉及到自定义转场动画的实现和通过runtime给分类添加属性。

首先我们要自己实现push和pop时候的转场动画，实现转场动画主要是实现UINavigationController的代理方法

下面的代理方法返回了一个遵循UIViewControllerAnimatedTransitioning协议的对象，新建一个类KJPushAnimator并遵循该协议，该协议中有两个required方法

1、 - (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext;
2、 - (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext;

第一个方法表示动画的持续时间，第二方法就是实现我们转场时候的动画了。
代码如下：

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
    return 0.25;
}

动画时间定义为0.25秒,然后另一个协议方法的实现如下

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {

    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    
    
    UIView *containerView = [transitionContext containerView];
    UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
    UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
    toView.kj_x = [UIScreen mainScreen].bounds.size.width;
    
    [containerView addSubview:fromView];
    [containerView addSubview:toView];
    
    UINavigationBar *navigationBar = toVC.navigationController.navigationBar;
    
    [navigationBar kj_setBackgroundColor:fromVC.kj_navigationBarTintColor];
    [navigationBar kj_setNavigationBarAlpha:fromVC.kj_navigationBarAlpha];
    [navigationBar kj_setNavigationTitleColor:fromVC.kj_navigationTitleColor];
    
    [UIView animateWithDuration:kTransitionDuration delay:0 options:UIViewAnimationOptionCurveLinear animations:^{
        
        fromView.kj_x = -100;
        toView.kj_x = 0;
        
        [navigationBar kj_setBackgroundColor:toVC.kj_navigationBarTintColor];
        [navigationBar kj_setNavigationBarAlpha:toVC.kj_navigationBarAlpha];
        [navigationBar kj_setNavigationTitleColor:toVC.kj_navigationTitleColor];
        
    } completion:^(BOOL finished) {
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
        [navigationBar kj_setBackgroundColor:toVC.kj_navigationBarTintColor];
        [navigationBar kj_setNavigationBarAlpha:toVC.kj_navigationBarAlpha];
        [navigationBar kj_setNavigationTitleColor:toVC.kj_navigationTitleColor];
    }];
}

首先拿到两个转场相关的两个viewController（fromVC和toVC）以及它们的view（fromView和toView），这里containerView是用来承载fromView和toView的容器父视图，然后将fromView和toView add到containerView上，注意添加的顺序，注意到系统的push动画是从右往左出现要push的vc，所以toView初始的x坐标是屏幕宽度。然后是动画开始前设置navigationBar的相关属性，因为是从fromVC push 到toVC，故navigationBar的设置先跟fromVC关联，然后简单的使用UIView动画模仿系统的push动画，并设置navigationBar的颜色和透明度等。

实现完push转场，在来看pop转场实现：
新建一个类KJPopAnimator遵循UIViewControllerAnimatedTransitioning
- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
return 0.25;
}
同样动画时间定义为0.25秒,另一个协议方法如下:

• (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {

    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController  *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    
    UIView *containerView = [transitionContext containerView];
    UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
    toView.kj_x = -100;
    
    UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
    fromView.layer.shadowRadius = 8;
    fromView.layer.shadowColor = [UIColor blackColor].CGColor;
    fromView.layer.shadowOpacity = 0.5;
    
    UIView *maskView = [[UIView alloc] initWithFrame:[UIScreen mainScreen].bounds];
    maskView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.15];
    
    
    [containerView addSubview:toView];
    [containerView addSubview:maskView];
    [containerView addSubview:fromView];
    
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"shadowOpacity"];
    animation.duration = kTransitionDuration;
    animation.removedOnCompletion = YES;
    animation.toValue = @0;
    animation.delegate = self;
    [fromView.layer addAnimation:animation forKey:nil];
    
    UINavigationBar *navigationBar = toVC.navigationController.navigationBar;
    [navigationBar kj_setBackgroundColor:fromVC.kj_navigationBarTintColor];
    [navigationBar kj_setNavigationBarAlpha:fromVC.kj_navigationBarAlpha];
    [navigationBar kj_setNavigationTitleColor:fromVC.kj_navigationTitleColor];
  
    
    [UIView animateWithDuration:kTransitionDuration delay:0 options:UIViewAnimationOptionCurveLinear animations:^{
        toView.kj_x = 0;
        fromView.kj_x = [UIScreen mainScreen].bounds.size.width;
        maskView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0];
        
        [navigationBar kj_setBackgroundColor:toVC.kj_navigationBarTintColor];
        [navigationBar kj_setNavigationBarAlpha:toVC.kj_navigationBarAlpha];
        [navigationBar kj_setNavigationTitleColor:toVC.kj_navigationTitleColor];
        
    } completion:^(BOOL finished) {
        [maskView removeFromSuperview];
        fromView.layer.shadowOpacity = 0;
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
    }];
  

  }

实现上大致和push差不多，这里为了和系统的pop动画保持一致，还加了pop时的视图阴影以及一个黑色透明的遮罩层（读者可以自己打开一个app观察系统的pop动画，建议手势左划观察），另外需要注意的一点是，pop是从fromVC到toVC的过程，所以这里视图添加的顺序和push不一样，containerView先add toView然后在add fromView，确保fromView在最上面。

实现完push和pop动画后就要运用到开始说的UINavigationController的代理方法了，创建一个继承自UINavigationController的子类KJNavigationController，在viewDidLoad方法中设置self.delegate = self;然后实现代理方法

- (nullable id <UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
                                            animationControllerForOperation:(UINavigationControllerOperation)operation
                                                         fromViewController:(UIViewController *)fromVC
                                                           toViewController:(UIViewController *)toVC {
    if (operation == UINavigationControllerOperationPush) {
        
        return [KJPushAnimator new];
    } else if (operation == UINavigationControllerOperationPop) {
        return [KJPopAnimatior new];
    }
    
    return nil;
}

通过operation判断是push行为还是pop，传入对应的动画驱动，此时我们就自己模仿了系统的转场动画，但是系统默认还有手势左划pop回上一个页面，这就需要实现UINavigationController的里一个代理方法- (nullable id <UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController interactionControllerForAnimationController:(id <UIViewControllerAnimatedTransitioning>) animationController

该代理返回的是一个实现了UIViewControllerInteractiveTransitioning的对象，系统有一个已经实现好的类UIPercentDrivenInteractiveTransition，直接使用就

- (void)setupGesture { 
    UIScreenEdgePanGestureRecognizer *edgePan = [[UIScreenEdgePanGestureRecognizer alloc] initWithTarget:self action:@selector(edgePanGesture:)];
    edgePan.edges = UIRectEdgeLeft;
[self.view addGestureRecognizer:edgePan];
}

设置从屏幕左边缘触发，然后看edpePanGesuture方法

- (void)edgePanGesture:(UIScreenEdgePanGestureRecognizer *)sender {
    UIGestureRecognizerState state = sender.state;
    
    CGFloat offsetX = MAX(0, [sender translationInView:sender.view].x);
    CGFloat width = [UIScreen mainScreen].bounds.size.width;
    CGFloat percent = offsetX / width;
    switch (state) {
        case UIGestureRecognizerStateBegan:
        {
            self.interactive  = [[UIPercentDrivenInteractiveTransition alloc] init];
            self.interactive.completionCurve = UIViewAnimationCurveLinear;
            [self popViewControllerAnimated:YES];
        }
            break;
        case UIGestureRecognizerStateChanged:
        {
            
            [self.interactive updateInteractiveTransition:percent];
            
        }
            break;
        case UIGestureRecognizerStateEnded:
        {
            if (percent > 0.5) {
                [self.interactive finishInteractiveTransition];
            } else {
                [self.interactive cancelInteractiveTransition];
            }
            
            self.interactive = nil;
        }
            break;
        default:
        {
            [self.interactive cancelInteractiveTransition];
            self.interactive = nil;
        }
            break;
    }
}

首先拿到手指滑动的偏移量offsetX，计算手指滑动距离与屏幕尺寸的百分比，然后判断手势的状态，在手势began时，调用popViewControllerAnimated方法(这一步很重要),在changed时，告诉UIPercentDrivenInteractiveTransition对象pop完成的百分比，在手势end时，判断手指滑动是否超过屏幕一般，超过一般则pop完成，调用finishInteractiveTransition,没有则表示取消这次pop，调用cancelInteractiveTransition，并且注意将self.interactive置为nil。

以上实现这些都是为了能够得到push和pop时的转场进行的百分比（进度），接来下就是对navigationBar的一些配置。

给navigationBar创建一个分类，设置navigationBar的颜色、透明度以及标题颜色

这里不是直接对navigationBar进行这些颜色设置，而是在navigationBar上插入一个子视图，通过设置这个子视图的颜色来显示navigationBar的颜色

- (UIView *)backgroundView {
    UIView *backgroundView = objc_getAssociatedObject(self, _cmd);
    if (!backgroundView) {
        [self setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
        backgroundView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, CGRectGetWidth([UIScreen mainScreen].bounds), CGRectGetHeight(self.bounds) + 20)];
        backgroundView.userInteractionEnabled = NO;
        backgroundView.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin|UIViewAutoresizingFlexibleTopMargin|UIViewAutoresizingFlexibleBottomMargin;
        [self.subviews.firstObject insertSubview:backgroundView atIndex:0];
        [self setBackgroundView:backgroundView];
    }
    self.backgroundColor = [UIColor clearColor];
    return backgroundView;
}
- (void)setBackgroundView:(UIView *)maskLayer {
    objc_setAssociatedObject(self, @selector(backgroundView), maskLayer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

上图中将_UIBarBackground的color设置为clearColor,后面的navigationBar的颜色实际上就是插入的backgroundView的颜色

最后再给UIViewController添加一个分类，方便直接设置navigationBar的相关属性

就是一些setter和getter方法，主要是用户没有手动设置时给定一个默认置，代码比较简单就不做详细解释了，读者此时可以回到KJPushAnimator和KJPopAnimator中看看navigationBar的相关设置，注意动画前后navigationBar的设置。

然后实际项目中的设置navigationBar的颜色，和标题颜色就非常简单了，#improt "UIViewController+KJNavigationBar.h"然后在viewDidLoad中

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.kj_navigationBarTintColor = [UIColor cyanColor];
    self.kj_navigationTitleColor = [UIColor whiteColor];
}

实际效果就如同文章开始的gif一样

本文完整Demo请点击这里