近實(shí)現(xiàn)了一個(gè)完整的電子書(shū)閱讀器，支持txt和epub格式的電子書(shū)閱讀，其中epub支持圖文混排的方式展示。本文主要談?wù)勂渲袃煞N翻頁(yè)效果的實(shí)現(xiàn)，分別為仿真翻頁(yè)和水平滑動(dòng)翻頁(yè)。
仿真翻頁(yè)
最合適的方案就是使用系統(tǒng)提供的UIPageviewcontroller了，不過(guò)默認(rèn)的UIpageviewcontroller翻頁(yè)時(shí)背面是白色的，而閱讀器通常都會(huì)有背景色或背景圖片，翻頁(yè)時(shí)用戶體驗(yàn)就很糟糕，比如就像下面這樣
所以接下來(lái)主要說(shuō)說(shuō)如何修改背面顏色以達(dá)到美觀的翻頁(yè)效果。
UIpageviewcontroller有一個(gè)屬性叫做isDoubleSided，默認(rèn)為yes，也就是內(nèi)容只會(huì)在單面（正面）顯示，設(shè)置為no后，內(nèi)容便可以正面和背面雙面顯示，這時(shí)每翻一頁(yè)，pageview的下面兩個(gè)回調(diào)會(huì)調(diào)用兩次
func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController?{ // 第一次回調(diào)索取背面的controller // 第二次回調(diào)索取正面的controller}func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController?{ // 第一次回調(diào)索取背面的controller // 第二次回調(diào)索取正面的controller} 所以我們可以對(duì)正面的controller進(jìn)行反向截圖，并將其放在背面的controller上顯示，這樣整體翻頁(yè)效果就會(huì)很美觀了。
代碼示例
// 對(duì)輸入的controller進(jìn)行反向截圖func grabViewController(viewController: DUAPageViewController) -> Void { self.index = viewController.index self.chapterBelong = viewController.chapterBelong let rect = viewController.view.bounds UIGraphicsBeginImageContextWithOptions(rect.size, true, 0.0) let context = UIGraphicsGetCurrentContext() let transform = CGAffineTransform(a: -1.0, b: 0.0, c: 0.0, d: 1.0, tx: rect.size.width, ty: 0.0) context?.concatenate(transform) viewController.view.layer.render(in: context!) self.backImage = UIGraphicsGetImageFromCurrentImageContext() UIGraphicsEndImageContext() }效果像下面這樣
左右平滑翻頁(yè)
最初采用的是UIpageviewcontroller的另一種翻頁(yè)模式——滾動(dòng)模式。該模式蘋果在底層是用scrollView實(shí)現(xiàn)的。但這種模式在頁(yè)面切換時(shí)存在一些問(wèn)題，由于蘋果會(huì)對(duì)相鄰的controller進(jìn)行緩存，當(dāng)調(diào)用open func setViewControllers(_ viewControllers: [UIViewController]?, direction: UIPageViewControllerNavigationDirection, animated: Bool, completion: ((Bool) -> Swift.Void)? = nil)方法并且動(dòng)畫為true時(shí)，有時(shí)蘋果會(huì)錯(cuò)誤的認(rèn)為它已經(jīng)有了頁(yè)面的緩存而不再執(zhí)行數(shù)據(jù)源方法，從而引發(fā)一些問(wèn)題，更詳細(xì)的說(shuō)明可以看這篇文章，因此決定自己寫一個(gè)翻頁(yè)控件
DUATranslationController
DUATranslationController并沒(méi)有采用Scrollview的方式實(shí)現(xiàn)，而是基于controller容器，通過(guò)替換child controller來(lái)實(shí)現(xiàn)，具體來(lái)說(shuō)就是當(dāng)用戶點(diǎn)擊或者滑動(dòng)時(shí)，判斷需要展示上一個(gè)頁(yè)面還是下一個(gè)頁(yè)面，然后模仿UIpageviewcontroller通過(guò)回調(diào)的方式索取controller，加入到controller容器中，并通過(guò)動(dòng)畫的方式將新的controller平滑移動(dòng)進(jìn)入屏幕，舊的controller同時(shí)移出，如下是單擊手勢(shì)代碼示例（滑動(dòng)手勢(shì)涉及和用戶交互，邏輯更復(fù)雜些，但基本思路是一致的）
@objc func handleTapGes(gesture: UITapGestureRecognizer) -> Void { let hitPoint = gesture.location(in: gesture.view) let curController = self.childViewControllers.first! if hitPoint.x < gesture.view!.frame.size.width/3 {// 滑向上一個(gè)controller let lastController = self.delegate?.translationController(translationController: self, controllerBefore: curController) if lastController != nil { self.delegate?.translationController(translationController: self, willTransitionTo: lastController!) self.setViewController(viewController: lastController!, direction: .right, animated: allowAnimating, completionHandler: {(complete) in self.delegate?.translationController(translationController: self, didFinishAnimating: complete, previousController: curController, transitionCompleted: complete) }) } } if hitPoint.x > gesture.view!.frame.size.width*2/3 {// 滑向下一個(gè)controller let nextController: UIViewController? = self.delegate?.translationController(translationController: self, controllerAfter: self.childViewControllers.first!) if nextController != nil { self.delegate?.translationController(translationController: self, willTransitionTo: nextController!) self.setViewController(viewController: nextController!, direction: .left, animated: allowAnimating, completionHandler: {(complete) in self.delegate?.translationController(translationController: self, didFinishAnimating: complete, previousController: curController, transitionCompleted: complete) }) } } }// 該方法模仿UIpageviewcontroller，切換到某一個(gè)controllerfunc setViewController(viewController: UIViewController, direction: translationControllerNavigationDirection, animated: Bool, completionHandler: ((Bool) -> Void)?) -> Void { if animated == false { // 直接添加child controller ，略 }else { let oldController = self.childViewControllers.first self.addController(controller: viewController) var newVCEndTransform: CGAffineTransform var oldVCEndTransform: CGAffineTransform viewController.view.transform = .identity if direction == .left { viewController.view.transform = CGAffineTransform(translationX: screenWidth, y: 0) newVCEndTransform = .identity oldController?.view.transform = .identity oldVCEndTransform = CGAffineTransform(translationX: -screenWidth, y: 0) }else { viewController.view.transform = CGAffineTransform(translationX: -screenWidth, y: 0) newVCEndTransform = .identity oldController?.view.transform = .identity oldVCEndTransform = CGAffineTransform(translationX: screenWidth, y: 0) } UIView.animate(withDuration: animationDuration, animations: { oldController?.view.transform = oldVCEndTransform viewController.view.transform = newVCEndTransform }, completion: { (complete) in if complete { self.removeController(controller: oldController!) } if completionHandler != nil { completionHandler!(complete) } }) } } 最終效果像這樣：
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