自定義Pop轉場動畫

繼續使用上個程序，把push改為pop只需要做很少的工作就能完成

1、復制PushTransition.h和PushTransition.m。命名為PopTransition.h和PopTransition.m

2、在PopTransition.m中把

ViewController * fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
SecondViewController * toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];

改為：

SecondViewController * fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
ViewController * toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];

兩個類名交換位置即可

3、把所有的avatarImageView改為sourceImageView，把所有的sourceImageView改為avatarImageView，如下：

// 轉場動畫的具體內容
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext{
    // 獲取動畫的源控制器和目標控制器
    SecondViewController * fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    ViewController * toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView * container = transitionContext.containerView;
    
    // 創建一個imageView的截圖，并把原本imageView隱藏，造成以為移動的就是imageView的假象
    UIView * snapshotView = [fromVC.avatarImageView snapshotViewAfterScreenUpdates:NO];
    snapshotView.frame = [container convertRect:fromVC.avatarImageView.frame fromView:fromVC.view];
    fromVC.avatarImageView.hidden = YES;
    
    // 設置目標控制器的位置，并把透明度設為0，在后面的動畫中慢慢顯示出來變為1
    toVC.view.frame = [transitionContext finalFrameForViewController:toVC];
    toVC.sourceImageView.hidden = YES;
    
    // 都添加到container中。注意順序
//    [container addSubview:toVC.view];
    [container insertSubview:toVC.view belowSubview:fromVC.view];
    [container addSubview:snapshotView];
    
    // 執行動畫
    [UIView animateKeyframesWithDuration:[self transitionDuration:transitionContext] delay:0 options:UIViewKeyframeAnimationOptionCalculationModeLinear animations:^{
        snapshotView.frame = toVC.sourceImageView.frame;
        fromVC.view.alpha = 0;
    } completion:^(BOOL finished) {
        toVC.sourceImageView.hidden = NO;
        [snapshotView removeFromSuperview];
        fromVC.avatarImageView.hidden = NO;
        
        //一定要記得動畫完成后執行此方法，讓系統管理 navigation
        [transitionContext completeTransition: ![transitionContext transitionWasCancelled]]; // 如果參數寫成yes，當用戶取消pop時，會繼續執行動畫，也就是讓detailVC消失，設置成這個參數，會避免這樣的錯誤
    }];
}

4、修改ViewController中的判斷動畫類型的方法，加入pop的判斷：

- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController animationControllerForOperation:(UINavigationControllerOperation)operation fromViewController:(UIViewController *)fromVC toViewController:(UIViewController *)toVC{
    if (operation == UINavigationControllerOperationPush){  // 就是在這里判斷是哪種動畫類型
        return [[PushTransition alloc] init]; // 返回push動畫的類
    }else if (operation == UINavigationControllerOperationPop){  // 就是在這里判斷是哪種動畫類型
        return [[PopTransition alloc] init]; // 返回pop動畫的類{
    }else{
        return nil;
    }
}

系統默認的 Push 和 Pop 動畫都支持手勢驅動，并且可以根據手勢移動距離改變動畫完成度。幸運的是，Cocoa 已經集成了相關方法，我們只用告訴它百分比就可以了。所以下一步就是手勢驅動。

1、在 SecondViewController 的 viewDidLoad() 方法中，加入滑動手勢。

    // 加入左側邊界手勢
    UIScreenEdgePanGestureRecognizer * edgePan = [[UIScreenEdgePanGestureRecognizer alloc] initWithTarget:self action:@selector(edgePanGesture:)];
    edgePan.edges = UIRectEdgeLeft;
    [self.view addGestureRecognizer:edgePan];

2、遵循UINavigationControllerDelegate協議，因為navigationController的動畫需要在這里執行，所以需要設置代理為自己

- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    self.navigationController.delegate = self;
}

3、在手勢監聽方法中，創建 UIPercentDrivenInteractiveTransition 屬性，并實現手勢百分比更新。

// 在手勢監聽方法中，創建UIPercentDrivenInteractiveTransition屬性，并實現手勢百分比更新
- (void)edgePanGesture:(UIScreenEdgePanGestureRecognizer *)edgePan{
    // 進度值，這是左側邊界的算法，如果要改為右側邊界，改為self.view.bounds.size.width / [edgePan translationInView:self.view].x;
    CGFloat progress = [edgePan translationInView:self.view].x / self.view.bounds.size.width;
    if (edgePan.state == UIGestureRecognizerStateBegan) {
        self.percentDrivenTransition = [[UIPercentDrivenInteractiveTransition alloc] init];
        [self.navigationController popViewControllerAnimated:YES];
        
    }else if(edgePan.state == UIGestureRecognizerStateChanged){
        [self.percentDrivenTransition updateInteractiveTransition:progress];
    }else if(edgePan.state == UIGestureRecognizerStateCancelled || edgePan.state == UIGestureRecognizerStateEnded){
        if(progress > 0.5){
            [self.percentDrivenTransition finishInteractiveTransition];
        }else{
            [self.percentDrivenTransition cancelInteractiveTransition];
        }
        self.percentDrivenTransition = nil;
    }
}

4、實現返回 UIViewControllerInteractiveTransitioning 的方法并返回剛剛創建的 UIPercentDrivenInteractiveTransition屬性。

- (id<UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController interactionControllerForAnimationController:(id<UIViewControllerAnimatedTransitioning>)animationController{
    if([animationController isKindOfClass:[PopTransition class]]){
        return self.percentDrivenTransition;
    }else{
        return nil;
    }
}

5、還需要設置一下返回動畫，否則手勢驅動不會生效

- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController animationControllerForOperation:(UINavigationControllerOperation)operation fromViewController:(UIViewController *)fromVC toViewController:(UIViewController *)toVC{
    if (operation == UINavigationControllerOperationPop){
        return [[PopTransition alloc] init]; // 返回pop動畫的類
    }else{
        return nil;
    }
}

完成

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