如何实现iOS图书动画:第1部分

原文链接 : How to Create an iOS Book Open Animation: Part 1

原文作者 : Vincent Ngo
译文出自 : 开发技术前线 www.devtf.cn
译者 : kmyhy
校对者: 这里校对者的github用户名
状态 : 校对中

In this two-part tutorial series, you’ll develop a nifty iOS book open animation and page flip animation similar to Paper by FiftyThree:

本教程分为2个部分，教你开发一个漂亮的iOS图书打开和翻页动画，就像你在Paper 53中所见到的一样：

In Part 1 you’ll learn how to customize your collection view layouts and apply depth and shadow to make the app look realistic.

在第1部分，你将学习到如何定制化Collection View Layout，并通过使用深度和阴影使App看起来更真实。

In Part 2, you’ll learn to create custom transitions between different controllers in a sensible way and integrate gestures to create natural, intuitive transitions between views.

在第2部分，你将学习如何以一种合理的方法在两个不同的控制器之间创建自定义的过渡特效，以及利用手势在两个视图间创建自然的、直观的过渡效果。

This tutorial is for intermediate to advanced developers; you’ll be working with custom transitions and custom collection view layouts.
If you’ve never worked with a collection view before, start with some of our other iOS tutorials first.

本教程适用于中级-高级的开发者；你将使用自定义过渡动画和自定义Collection View Layout。如果你从来没有用过Colleciton View，请先参考其他iOS教程。

Note: Full credit goes to Attila Hegedüs for creating this awesome
sample project.

注意：感谢Attila Hegdüs创建了本教程中的示例项目。

Getting Started

开始

Download the starter project for this tutorial here; extract the contents of the zip file, and open Paper.xcodeproj in Xcode.

从此处下载本教程的开始项目；解开zip压缩包，用Xcode打开Paper.xcodeproj。

编译项目，在模拟器中运行App;你将看到如下画面：

The app is pretty much fully built; you can scroll through your library of books and select one of your favorite books to view. But when was the last time you read a book which had its pages side-by-side? With a bit of UICollectionView know-how, you can dress up the page view quite a bit!

这个App的功能已经很完善了，你可以在你的书库中滚动，查看图书，选中某本图书进行浏览。但当你读一本书的时候，为什么它的书页都是并排放置的？通过一些UICollectionView的知识，你可以让这些书页看起来更好一些！

The Project Structure

项目结构

Here’s a quick rundown of the most important bits of the starter project:

关于这个开始项目，有几个重要的地方需要解释：

The Data Models folder contains three files:

Books.plist contains sample book data. Each book contains an image
cover along with an array of images to represent pages.
BookStore.swift is a singleton that is only created once in the life
cycle of the app. The BookStore’s job is to load data from
Books.plist and create Book objects.
Book.swift is a class that stores information related to the book,
such as retrieving the book cover, the image for each page index, and
the number of pages.

Data Models文件夹包含3个文件：

Books.plist 中包含了几本用于演示的图书信息。每本图书包含一张封面图片，以及一个表示每一页的内容的图片的数组。
BookStore.swift实现了单例，在整个App声明周期中只能创建一次对象。BookStore的职责是从Books.plist中加载数据并创建Book类实例。
Book.swift用于存放图书相关信息的类，比如图书的封面，每一页的图片，以及页号。

The Books folder contains two files:

BooksViewController.swift is a subclass of
UICollectionViewController. This class is responsible for displaying
your list of books horizontally.
BookCoverCell.swift displays all your book covers; it’s used by
BooksViewController.

Books文件夹包含了两个文件：

BooksViewController.swift是一个UICollectionViewController子类。负责以水平方式显式图书列表。
BookCoverCell.swift负责显示图书的封面，这个类被BooksViewController类所引用。

In the Book folder you’ll find the following:

BookViewController.swift is also a subclass of UICollectionViewController. Its purpose is to display the pages of the book when you select a book from BooksViewController.
BookPageCell.swift is used by BookViewController to display all the pages in a book.

在Book文件夹中则包括：

BookViewController.swift也是UICollectionViewController的子类。当用户在BooksViewController中选定的一本书后，它负责显示图书中的书页。
BookPageCell.swift被BookViewController用于显示图书中的书页。

Here’s what’s in the last folder, Helpers:

UIImage+Helpers.swift is an extension for UIImage. The extension contains two utility methods, one to round the corners of an image, and another to scale an image down to a given size.

在最后一个文件夹Helper中包含了：

UIImage+Helpers.swift是UIImage的扩展。该扩展包含了两个实用方法，一个用于让图片呈圆角显示，一个用于将图片缩放到指定大小。

That’s all! Enough of the review — it’s time to lay down some code!

这就是整个开始项目的大致介绍——接下来该是我们写点代码的时候了！

Customizing the Book Layout

定制化图书界面

First you need to to override the default layout for BooksViewController‘s collection view. The existing layout shows three big book covers that takes up the whole screen. You’ll scale it down a bit to make it look more pleasant, like so:

首先我们需要在BooksViewController中覆盖Collection View的默认布局方式。但当前的布局是在屏幕上显示3张图书封面的大图。为了美观，我们将这些图片缩减到一定大小，如下图所示：

As you scroll, the cover image nearest the center of the screen grows in size to indicate it’s the active selection. As you keep scrolling, the book cover shrinks in size to indicate you’re setting it aside.

当我们滑动图片，移动到屏幕中心的图片将被放大，以表示该图书为选中状态。如果继续滑动，该图书的封面又会缩小到一边，表示我们放弃选择该图书。

Create a group named Layout under the App\Books group. Next right-click the Layout folder and select New File…, then select the iOS\Source\Cocoa Touch Class template and click Next. Name the class BooksLayout, make it a subclass of UICollectionViewFlowLayout, and set Language to Swift.

在App\Books文件夹下新建一个文件夹组：Layout。在Layout上点击右键，选择New File...，然后选择iOS\Source\Cocoa Touch Class模板，并点击Next。类名命名为BooksLayout，继承UICollectionViewFlowLayout类，语言设置为Swift。

Next you need to instruct BooksViewController‘s collection view to use your new layout.

然后需要告诉BooksViewController中的Collection View，适用我们新建的BooksLayout。

Open Main.storyboard, click on BooksViewController then click on the Collection View. In the Attributes Inspector, set Layout to Custom and Class to BooksLayout as shown below:

打开Main.storyboard，展开BooksViewController对象，然后选择Collection View。在属性面板中，设置Layout 属性为 Custom，设置Class属性为BooksLayout，如下图所示：

Open BooksLayout.swift and add the following code above the BooksLayout class declaration.

打开BooksLayout.swift，在BooksLayout类声明之上加入以下代码：

private let PageWidth: CGFloat = 362
private let PageHeight: CGFloat = 568

These two constants will be used to set the size of the cell.
Now add the following initialization method within the class curly braces:

这个两个常量将用于设置单元格的的大小。
现在，在类定义内部定义如下初始化方法：

required init(coder aDecoder: NSCoder) {
  super.init(coder: aDecoder)
 
  scrollDirection = UICollectionViewScrollDirection.Horizontal //1
  itemSize = CGSizeMake(PageWidth, PageHeight) //2
  minimumInteritemSpacing = 10 //3
}

Here’s what the code above does:
Sets the collection view’s scroll view direction to horizontal.
Sets the size of the cell to the page width of 362 and to a height of 568.
Set the minimum spacing between cells to 10.

上述代码作用如下：

设置Collectioin View的滚动方向为水平方向。
设置单元格的大小为PageWidth和PageHeight，即362x568。
设置两个单元格间距10。

Next, add the following code after init(coder:):

然后，在init(coder:)方法中加入代码：

override func prepareLayout() {
  super.prepareLayout()
 
  //The rate at which we scroll the collection view.
  //1
  collectionView?.decelerationRate = UIScrollViewDecelerationRateFast
 
  //2
  collectionView?.contentInset = UIEdgeInsets(
    top: 0,
    left: collectionView!.bounds.width / 2 - PageWidth / 2,
    bottom: 0,
    right: collectionView!.bounds.width / 2 - PageWidth / 2
  )
}

prepareLayout() gives you the chance to perform any calculations before you come up with any layout information for each cell.

prepareLayout()方法允许我们在每个单元格的布局信息生效之前可以进行一些计算。

Taking each numbered comment in turn:

对应注释中的编号，以上代码分别说明如下：

Sets how fast the collection view will stop scrolling after a user lifts their finger. By setting it to UIScrollViewDecelerationRateFast the scroll view will decelerate much faster. Try playing around with Normal vs Fast to see the difference!
Sets the content inset of the collection view so that the first book cover will always be centered.

设置当用户手指离开屏幕后，Collection
View停止滚动的速度。默认的设置为UIScrollViewDecelerationRateFast，这是一个较快的速度。你可以尝试着设置为Normal 和 Fast，看看二者之间有什么区别。
设置Collection View的contentInset，以使第一本书的封面位于Collection View的中心。

Now you need to handle the layout information of each cell.
Add the following code below prepareLayout():

现在我们需要处理每一个单元格的布局信息。
在prepareLayout()方法下面，加入以下代码：

override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
  //1
  var array = super.layoutAttributesForElementsInRect(rect) as! [UICollectionViewLayoutAttributes]
 
  //2
  for attributes in array {
    //3
    var frame = attributes.frame
    //4
    var distance = abs(collectionView!.contentOffset.x + collectionView!.contentInset.left - frame.origin.x)
    //5
    var scale = 0.7 * min(max(1 - distance / (collectionView!.bounds.width), 0.75), 1)
    //6
    attributes.transform = CGAffineTransformMakeScale(scale, scale)
  }
 
  return array
}

layoutAttributesForElementsInRect(_:) returns an array of UICollectionViewLayoutAttributes objects, which provides the layout attributes for each cell. Here’s a breakdown of the code:

layoutAttributesForElementsInRect(_:) 方法返回一个UICollectionViewLayoutAttributes对象数组，其中包含了每一个单元格的布局属性。以上代码稍作说明如下：

Calling the superclass of layoutAttributesForElementsInRect returns an array that contains all default layout attributes for each cell.
Loop through each attribute in the array.
Grab the frame for the current cell attribute.
Calculate the distance between the book cover — that is, the cell — and the center of the screen.
Scale the book cover between a factor of 0.75 and 1 depending on the distance calculated above. You then scale all book covers by 0.7 to keep them nice and small.
Finally, apply the scale to the book cover.

调用父类的layoutAttributesForElementsInRect方法，已获得默认的单元格布局属性。
遍历数组中的每个单元格布局属性。
从单元格布局属性中读取frame。
计算两本书的封面之间的间距——即两个单元格之间的间距——以及屏幕的中心点。
以0.75~1之间的比率缩放封面，具体的比率取决于前面计算出来的间距。然后为了美观，将所有的封面都缩放70%。
最后，应用仿射变换。

Next, add the following code right after layoutAttributesForElementsInRect(_:):

接下来，在layoutAttributesForElementsInRect(_:)方法后增加如下代码：

override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
  return true
}

Returning true forces the layout to recalculate its attributes every time the collection view’s bound changes. A UICollectionView changes its bounds while scrolling, which is perfect for recalculating the cell’s attribute.

返回true表示每当Collection View的bounds发生改变时都强制重新计算布局属性。Collection View在滚动时会改变它的bounds，因此我们需要重新计算单元格的布局属性。

Build and run your app; you’ll see that book in the middle of the view is larger than the others:

编译运行程序，我们将看到位于中央的封面明显比其他封面要大上一圈：

Scroll through the books to see how each book cover scales up and down. But wouldn’t it be great if the book could snap into place, indicating the selection?
The next method you’ll add will do just that!

拖动Colleciton View，查看每本书放大、缩小。但仍然有一点稍显不足，为什么不让书本能够卡到固定的位置呢？
接下来我们介绍的这个方法就是干这个的。

Snapping to a Book

对齐书本

targetContentOffsetForProposedContentOffset(_:withScrollingVelocity:) determines at which point the collection view should stop scrolling, and returns a proposed offset to set the collection view’s contentOffset. If you don’t override this method, it just returns the default offset.

targetContentOffsetForProposedContentOffset(_:withScrollingVelocity:)方法用于计算每本书应该在对齐到哪个位置，它返回一个偏移位置，可用于设置Collection View的contentOffset。如果你不覆盖这个方法，它会返回一个默认的值。

Add the following code after shouldInvalidateLayoutForBoundsChange(_:):

在shouldInvalidateLayoutForBoundsChange(_:)方法后添加如下代码：

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
  // Snap cells to centre
  //1
  var newOffset = CGPoint()
  //2
  var layout = collectionView!.collectionViewLayout as! UICollectionViewFlowLayout
  //3
  var width = layout.itemSize.width + layout.minimumLineSpacing
  //4
  var offset = proposedContentOffset.x + collectionView!.contentInset.left
 
  //5
  if velocity.x > 0 {
    //ceil returns next biggest number
    offset = width * ceil(offset / width)
  } else if velocity.x == 0 { //6
    //rounds the argument
    offset = width * round(offset / width)
  } else if velocity.x < 0 { //7
    //removes decimal part of argument
    offset = width * floor(offset / width)
  }
  //8
  newOffset.x = offset - collectionView!.contentInset.left
  newOffset.y = proposedContentOffset.y //y will always be the same...
  return newOffset
}

Here’s how you calculate the proposed offset for your book covers once the user lifts their finger:

这段代码计算当用户手指离开屏幕时，封面应该位于哪个偏移位置：

Create a new CGPoint called newOffset.
Grab the current layout of the collection view.
Get the total width of a cell.
Calculate the current offset with respect to the center of the screen.
If velocity.x > 0, the user is scrolling to the right. Think of offset/width as the book index you’d like to scroll to.
If velocity.x = 0, the user didn’t put enough oomph into scrolling, and the same book remains selected.
If velocity.x < 0, the user is scrolling left.
Update the new x offset and return. This guarantees that a book will always be centered in the middle.

声明一个CGPoint。
获得Collection View的当前布局。
获得单元格的总宽度。
计算相对于屏幕中央的currentOffset。
如果velocity.x>0，表明用户向右滚动，用offset除以width，得到书的索引，并滚动到相应的位置。
如果velocity.x=0，表明用户是无意识的滚动，原来的选择不会发生改变。
如果velocity.x<0，表明用户向左滚动。
修改newOffset.x，然后返回newOffset。这样就保证书本总是对齐到屏幕的中央。

Build and run your app; scroll through them again and you should notice that the scrolling action is a lot snappier:

编译运行程序；再次滚动封面，你会注意到滚动动作将变得更整齐了。

To finish up this layout, you need to create a mechanism to restrict the user to click only the book in the middle. As of right now, you can currently click any book regardless of its position.

要完成这个布局，我们还需要使用一种机制，以限制用户只能点击位于中央的封面。目前，不管哪个位置的封面都是可点击的。

Open BooksViewController.swift and place the following code under the comment // MARK: Helpers:

打开BooksViewController.swift，在注释"//MARK:Helpers"下面加入以下代码：

func selectedCell() -> BookCoverCell? {
  if let indexPath = collectionView?.indexPathForItemAtPoint(CGPointMake(collectionView!.contentOffset.x + collectionView!.bounds.width / 2, collectionView!.bounds.height / 2)) {
    if let cell = collectionView?.cellForItemAtIndexPath(indexPath) as? BookCoverCell {
      return cell
    }
  }
  return nil
}

selectedCell() will always return the middle cell.
Next, replace openBook(_:) with the following:

selectedCell()方法返回位于中央的那个单元格。
替换openBook(_:)方法的代码如下：

func openBook() {
  let vc = storyboard?.instantiateViewControllerWithIdentifier("BookViewController") as! BookViewController
  vc.book = selectedCell()?.book
  // UICollectionView loads it's cells on a background thread, so make sure it's loaded before passing it to the animation handler
  dispatch_async(dispatch_get_main_queue(), { () -> Void in
    self.navigationController?.pushViewController(vc, animated: true)
    return
  })
}

This simply uses the new selectedCell method you wrote rather than taking a book as a parameter.
Next, replace collectionView(_:didSelectItemAtIndexPath:) with the following:

这里，直接调用新的selectedCell方法，并用它的book属性代替原来的book参数。
然后，将collectionView(_:didSelectItemAtIndexPath:)方法替换为：

override func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
  openBook()
}

This simply removes the code that opened the book at the selected index; now you'll always open the book in the center of the screen.
Build and run your app; you'll notice now that the book in the center of the view is always the one that opens.

这里，我们简单地删除了原来的打开某个索引处的图书的代码，而直接打开了当前位于屏幕中央的图书。
编译运行程序，我们将看到每次打开的图书总是位于屏幕中央的那本。

You're done with BooksLayout. It's time to make the on-screen book more realistic, and let the user flip the pages in the book!

BooksLayout的工作就到这里了。后面我们将使这本电子书显得更真实，能够让用户”翻动“书里的每一页！

Book Flipping Layout

翻页布局

Here's the final effect you're shooting for:

最终实现的效果如下：

Now that looks more like a book! :]
这看起来就像是一本真正的书! :]

Create a group named Layout under the Book group. Next, right-click the Layout folder and select New File..., then select the iOS\Source\Cocoa Touch Class template and click Next. Name the class BookLayout, make it a subclass of UICollectionViewFlowLayout, and set Language to Swift.

在Book文件夹下新建一个Layout文件夹。在Layout文件夹上右键，选择New File...，然后适用iOS\Source\Cocoa Touch Class模板，然后点Next。类名命名为BookLayout，继承于UICollectionViewFlowLayout，语言选择Swift。

Just as before, your book collection view needs to use the new layout. Open Main.storyboard and select the Book View Controller Scene. Select the collection view and set the Layout to Custom.

同前面一样，图书所使用的Collection View需要适用新的布局。打开Main.storyboard，然后选择Book View Controller场景，展开并选中其中的Collection View，然后设置Layout属性为Custom。

Finally, set the layout Class to BookLayout as shown below:

然后，将Layout属性下的 Class属性设置为BookLayout：

Open BookLayout.swift and add the following code above the BookLayout class declaration:

打开BookLayout.swift，在类声明之上加入如下代码：

private let PageWidth: CGFloat = 362
private let PageHeight: CGFloat = 568
private var numberOfItems = 0

You'll use these constant variables to set the size of every cell; as well, you're keeping track of the total number of pages in the book.
Next, add the following code inside the class declaration:

这几个常量将用于设置单元格的大小，以及记录整本书的页数。
接着，在类声明内部加入代码：

override func prepareLayout() {
  super.prepareLayout()
  collectionView?.decelerationRate = UIScrollViewDecelerationRateFast
  numberOfItems = collectionView!.numberOfItemsInSection(0)
  collectionView?.pagingEnabled = true
}

This is similar to what you did in BooksLayout, with the following differences:

这段代码和我们在BooksLayout中所写的差不多，仅有以下几处差别：

Set the deceleration rate to UIScrollViewDecelerationRateFast to increase the rate at which the scroll view slows down.
Grab the number of pages in the current book.
Enable paging; this lets the view scroll at fixed multiples of the collection view's frame width (rather than the default of continuous scrolling).

将减速速度设置为UIScrollViewDecelerationRateFast，以加快Scroll View滚动速度变慢的节奏。
记住本书的页数。
启用分页，这样Scroll View滚动时将以其宽度的固定倍数滚动（而不是持续滚动）。

Still in BookLayout.swift, add the following code:

仍然在BookLayout.swift中，加入以下代码：

override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
  return true
}

Again, returning true lets the layout update every time the user scrolls.

跟前面一样，返回true，以表示用户每次滚动都会重新计算布局。

Next, give the collection view a content size by overriding collectionViewContentSize() as shown below:

然后，覆盖collectionViewContentSize() ，以指定Collection View的contentSize：

override func collectionViewContentSize() -> CGSize {
  return CGSizeMake((CGFloat(numberOfItems / 2)) * collectionView!.bounds.width, collectionView!.bounds.height)
}

This returns the overall size of the content area. The height of the content will always stay the same, but the overall width of the content is the number of items — that is, pages — divided by two multiplied by the screen's width. The reason you divide by two is that book pages are double sided; there's content on both sides of the page.

这个方法返回了内容区域的整个大小。内容区域的高度总是不变的，但宽度是随着页数变化的——即书的页数除以2倍，再乘以屏幕宽度。除以2是因为书页有两面，内容区域一次显示2页。

Just as you did in BooksLayout, you need to override layoutAttributesForElementsInRect(_:) so you can add the paging effect to your cells.

就如我们在BooksLayout中所做的一样，我们还需要覆盖layoutAttributesForElementsInRect(_:)方法，以便我们能够在单元格上增加翻页效果。

Add the following code just after collectionViewContentSize():

在collectionViewContentSize()方法后加入：

override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
  //1
  var array: [UICollectionViewLayoutAttributes] = []
 
  //2
  for i in 0 ... max(0, numberOfItems - 1) {
    //3
    var indexPath = NSIndexPath(forItem: i, inSection: 0)
    //4
    var attributes = layoutAttributesForItemAtIndexPath(indexPath)
    if attributes != nil {
      //5
      array += [attributes]
    }
  }
  //6
  return array
}

Rather than calculating the attributes within this method like you did in BooksLayout, you leave this task up to layoutAttributesForItemAtIndexPath(_:), as all cells are within the visible rect at any given time in the book implementation.

不同于在BooksLayout中的将所有计算布局属性的代码放在这个方法里面，我们这次将这个任务放到layoutAttributesForItemAtIndexPath(_:)中进行，因为在图书的实现中，所有单元格都是同时可见的。

Here's a line by line explanation:

以上代码解释如下：

Create a new array to hold UICollectionViewLayoutAttributes.
Loop through all the items (pages) in the collection view.
For each item in the collection view, create an NSIndexPath.
Grab the attribute for the current indexPath. You'll override layoutAttributesForItemAtIndexPath(_:) soon.
Add the attributes to your array.
Return all the cell attributes.

声明一个数组，用于保存所有单元格的布局属性。
遍历所有的书页。
对于CollecitonView中的每个单元格，都创建一个NSIndexPath。
通过每个NSIndexPath来获得单元格的布局属性，在后面，我们会覆盖layoutAttributesForItemAtIndexPath(_:)方法。
把每个单元格布局属性添加到数组。
返回数组。

Handling the Page Geometry

处理页面的几何计算

Before you jump straight into the implementation of layoutAttributesForItemAtIndexPath(_:), take a minute to consider the layout, how it will work, and if you can write any helper methods to keep everything nice and modular. :]

在我们开始实现layoutAttributesForItemAtIndexPath(_:)方法之前，花几分钟好好思考一下布局的问题，它是怎样实现的，如果能够写几个助手方法将会让我们的代码更漂亮和模块化。:]

The diagram above shows that every page flips with the book's spine as the axis of rotation. The ratios on the diagram range from -1.0 to 1.0. Why? Well, imagine a book laid out on a table, with the spine representing 0.0. When you turn a page from the left to the right, the "flipped" ratio goes from -1.0 (full left) to 1.0 (full right).
Therefore, you can represent your page flipping with the following ratios:

上图演示了翻页时以书籍为轴旋转的过程。图中书页的”打开度“用-1到1来表示。为什么？你可以想象一下放在桌子上的一本书，书脊所在的位置代表0.0。当你从左向右翻动书页时，书页张开的程度从-1（最左）到1（最右）。因此，我们可以用下列数字表示“翻页”的过程：

0.0 means a page is at a 90 degree angle, perpendicular to the table.
+/- 0.5 means a page is at a 45 degree angle to the table.
+/- 1.0 means a page is parallel to the table.

0.0表示一个书页翻成90度，与桌面成直角。
+/-0.5表示书页翻至于桌面成45度角。
+/-1.0表示书页翻至与桌面平行。

Note that since angle rotation is counterclockwise, the sign of the angle will be the opposite of the sign of the ratio.

注意，因为角度是按照反时针方向增加的，因此角度的符号和对应的打开度是相反的。

First, add the following helper method after layoutAttributesForElementsInRect(_:):

首先，在layoutAttributesForElementsInRect(_:)方法后加入助手方法：

//MARK: - Attribute Logic Helpers
 
func getFrame(collectionView: UICollectionView) -> CGRect {
  var frame = CGRect()
 
  frame.origin.x = (collectionView.bounds.width / 2) - (PageWidth / 2) + collectionView.contentOffset.x
  frame.origin.y = (collectionViewContentSize().height - PageHeight) / 2
  frame.size.width = PageWidth
  frame.size.height = PageHeight
 
  return frame
}

For every page, you calculate the frame with respect to the middle of the collection view. getFrame(_:) will align every page's edge to the book's spine. The only variable that changes is the collection view's content offset in the x direction.

对于每一页，我们都可以计算出相对于Collection View中心的frame。getFrame(_:)方法会将每一页的一边对齐到书脊。唯一会变的是Collectoin View的contentOffset在x方向上的改变。

Next, add the following method after getFrame(_:):

然后，在getFrame(_:)方法后添加如下方法：

func getRatio(collectionView: UICollectionView, indexPath: NSIndexPath) -> CGFloat {
  //1
  let page = CGFloat(indexPath.item - indexPath.item % 2) * 0.5
 
  //2
  var ratio: CGFloat = -0.5 + page - (collectionView.contentOffset.x / collectionView.bounds.width)
 
  //3
  if ratio > 0.5 {
    ratio = 0.5 + 0.1 * (ratio - 0.5)
 
  } else if ratio < -0.5 {
    ratio = -0.5 + 0.1 * (ratio + 0.5)
  }
 
  return ratio
}

The method above calculates the page's ratio. Taking each commented section in turn:

上面的方法计算书页翻开的程度。对每一段有注释的代码分别说明如下：

Calculate the page number of a page in the book — keeping in mind that pages in the book are double-sided. Multiplying by 0.5 gives you the exact page you're on.
Calculate the ratio based on the weighted percentage of the page you're turning.
You need to restrict the page to a ratio between the range of -0.5 and 0.5. Multiplying by 0.1 creates a gap between each page to make it look like they overlap.

算出书页的页码——记住，书是双面的。除以2就是你真正在翻读的那一页。
算出书页的打开度。注意，这个值被我们加了一个权重。
书页的打开度必须限制在-0.5到0.5之间。另外乘以0.1的作用，是为位了在页与页之间增加一条细缝，以表示它们是上下叠放在一起的。

Once you've calculated the ratio, you'll use it to calculate the angle of the turning page.
Add the following code after getRatio(_:indexPath:):

一旦我们计算出书页的打开度，我们就可以将之转变为旋转的角度。
在getRation(_:indexPath:)方法后面加入代码：

func getAngle(indexPath: NSIndexPath, ratio: CGFloat) -> CGFloat {
  // Set rotation
  var angle: CGFloat = 0
 
  //1
  if indexPath.item % 2 == 0 {
    // The book's spine is on the left of the page
    angle = (1-ratio) * CGFloat(-M_PI_2)
  } else {
    //2
    // The book's spine is on the right of the page
    angle = (1 + ratio) * CGFloat(M_PI_2)
  }
  //3
  // Make sure the odd and even page don't have the exact same angle
  angle += CGFloat(indexPath.row % 2) / 1000
  //4
  return angle
}

There's a bit of math going on, but it's not so bad when you break it down:

这个方法中有大量计算，我们一点点拆开来看：

Check to see if the current page is even. This means that the page is to the right of the book's spine. A page turn to the right is counterclockwise, and pages on the right of the spine have a negative angle. Recall that the ratio you defined is between -0.5 and 0.5.
If the current page is odd, the page is to the left of the book's spine. A page turn to the left is clockwise, and pages on the left side of the spine have a positive angle.
Add a small angle to each page to give the pages some separation.
Return the angle for rotation.

判断该页是否是偶数页。如果是，则该页将翻到书脊的右边。翻到右边的页是反手翻转，同时书脊右边的页其角度必然是负数。注意，我们将打开度定义为-0.5到0.5之间。
如果当前页是奇数，则该页将位于书脊左边，当书页被翻到左边时，它的按正手翻转，书脊左边的页其角度为正数。
每页之间加一个小夹角，使它们彼此分离。
返回旋转角度。

Once you have the angle, you need to transform each page. Add the following method:

得到旋转角度之后，我们可以操纵书页使其旋转。增加如下方法：

func makePerspectiveTransform() -> CATransform3D {
  var transform = CATransform3DIdentity
  transform.m34 = 1.0 / -2000
  return transform
}

Modifying the m34 of the transform matrix adds a bit of perspective to each page.

修改转换矩阵的m34属性，已达到一定的立体效果。

Now it's time to apply the rotation. Add the following code:

然后应用旋转动画。实现下面的方法：

func getRotation(indexPath: NSIndexPath, ratio: CGFloat) -> CATransform3D {
  var transform = makePerspectiveTransform()
  var angle = getAngle(indexPath, ratio: ratio)
  transform = CATransform3DRotate(transform, angle, 0, 1, 0)
  return transform
}

Here you use the two previous helper methods to calculate the transform and the angle, and create a CATransform3D to apply to the page along the y-axis.

在这个方法中，我们用到了刚才创建的两个助手方法去计算旋转的角度，然后通过一个CATransform3D对象让书页在y轴上旋转。

Now that you have all the helper methods set up, you are finally ready to create the attributes for each cell. Add the following method after layoutAttributesForElementsInRect(_:):

所有的助手方法都实现了，我们最终需要配置每个单元格的属性。在layoutAttributesForElementsInRect(_:)方法后加入以下方法：

override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes! {
  //1
  var layoutAttributes = UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)
 
  //2
  var frame = getFrame(collectionView!)
  layoutAttributes.frame = frame
 
  //3
  var ratio = getRatio(collectionView!, indexPath: indexPath)
 
  //4
  if ratio > 0 && indexPath.item % 2 == 1
     || ratio < 0 && indexPath.item % 2 == 0 {
    // Make sure the cover is always visible
    if indexPath.row != 0 {
      return nil
    }
  }   
  //5
  var rotation = getRotation(indexPath, ratio: min(max(ratio, -1), 1))
  layoutAttributes.transform3D = rotation
 
  //6
  if indexPath.row == 0 {
    layoutAttributes.zIndex = Int.max
  }
 
  return layoutAttributes
}

You'll call this method for each cell in your collection view:

在Collection View的每个单元格上，都会调用这个方法。这个方法做了如下工作：

Create a UICollectionViewLayoutAttributes object for the cell at the given NSIndexPath.
Set the frame of the attribute using the getFrame method you created to ensure it's always aligned with the book's spine.
Calculate the ratio of an item in the collection view using getRatio, which you wrote earlier.
Check that the current page is within the ratio's threshold. If not, don't display the cell. For optimization purposes (and because of common sense), you won't display the back-side of a page, but only those that are front-facing — except when it's the book's cover, which you display at all times.
Apply a rotation and transform with the given ratio you calculated.
Check if indexPath is the first page. If so, make sure its zIndex is always on top of the other pages to avoid flickering effects.

创建一个UICollectionViewLayoutAttributes对象layoutAttributes，供IndexPath所指的单元格使用。
调用我们先前定义的getFrame方法设置layoutAttributes的frame，确保单元格对齐于书脊。
调用先前定义的getRatio方法算出单元格的打开度。
判断当前页是否位于正确的打开度范围之内。如果不，不显示该单元格。为了优化（也是为了符合常理），除了正面向上的书页，我们不应当显示书页的背面——书的封面例外，那个不管什么时候都需要显示。
应用旋转动画，使用前面算出的打开度。
判断是否是第一页，如果是，将它的zIndex放在其他页的上面，否则有可能出现画面闪烁的Bug。

Build and run your app, open up one of your books, flip through it and...whoa, what?

编译，运行。打开书，翻动每一页……呃？什么情况？

书被错误地从中间装订了，而不是从书的侧边装订。

As the diagram shows, each page's anchor point is set at 0.5 for both x and y. Can you tell what you need to do to fix this?

如图中所示，每个书页的锚点默认是x轴和y轴的0.5倍处。现在你知道怎么做了吗？

It's clear you need to change the anchor point of a pages to its edge. If the page is on the right hand side of a book, the anchor point should be (0, 0.5). But if the page is on the left hand side of a book, the anchor point should be (1, 0.5).

很显然，我们需要修改书页的锚点为它的侧边缘。如果这个书页是位于书的右边，则它的锚点应该是(0,0.5)。如果书页是位于书的左边，测锚点应该是(1,0.5)。

Open BookPageCell.swift and add the following code:

打开BookePageCell.swift，添加如下代码：

override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes!) {
  super.applyLayoutAttributes(layoutAttributes)
  //1
  if layoutAttributes.indexPath.item % 2 == 0 {
    //2
    layer.anchorPoint = CGPointMake(0, 0.5)
    isRightPage = true
    } else { //3
      //4
      layer.anchorPoint = CGPointMake(1, 0.5)
      isRightPage = false
    }
    //5
    self.updateShadowLayer()
}

Here you override applyLayoutAttributes(_:), which applies the layout attributes created in BookLayout.
It's pretty straightforward code:

我们重写了applyLayoutAttributes(_:)方法，这个方法用于将BookLoayout创建的布局属性应用到第一个。
上述代码非常简单：

Check to see if the current cell is even. This means that the book's spine is on the left of the page.
Set the anchor point to the left side of the cell and set isRightPage to true. This variable helps you determine where the rounded corners of the pages should be.
If the current cell is odd, then the book's spine is on the right side of the page.
Set the anchor point to the right side of the cell and set isRightPage to false.
Finally, update the shadow layer of the current page.

检查当前单元格是否是偶数，也就是说书脊是否位于书页的左边。
如果是，将书页的锚点设置为单元格的左边缘，同时isRightPage设置为true。isRightPage变量可用于决定书页的圆角样式应当在那一边应用。
如果是奇数页，书脊应当位于书页的右边。
这只书页的锚点为单元格的右边缘，isRightPage设为false。
最后，设置当前书页的阴影层。

Build and run your app; flip through the pages and things should look a little better:

编译，运行。翻动书页，这次的效果已经好了许多：

That's it for the first part of this tutorial! Take some time to bask in the glory of what you've created — it's a pretty cool effect! :]

本教程第一部分就到此为止了。你是不是觉得自己干了一件了不起的事情呢——效果做得很棒，不是吗？

接下来做什么

Where to Go From Here?

You can download the completed project from Part 1 that contains all the source code.

第一部分的完整代码在此处下载：http://cdn1.raywenderlich.com/wp-content/uploads/2015/05/Part-1-Paper-Completed.zip

You started out with the default layouts for a collection view, and learned to customize a new layout to turn it into something truly amazing! Someone using this app will feel like they are flipping through a real book. It's the little things that turn a normal reader app into something that people can feel truly connected with.

我们从一个默认的Collection View布局开始，学习了如何定制自己的layout，并用它取得了令人叫绝的效果！用户在用这个App时，会觉得自己真的是在翻一本书。其实，将一个普通的电子阅读App变成一个让用户体验更加真实的带翻页效果的App，只需要做很小的改动。
However, you're not done yet! You'll make this app even better and more intuitive in Part 2 of this tutorial, where you'll explore custom transitions between the closed and opened book views.

当然，我们还没有完成这个App。在本教程的第二部分，我们将使App变得更加完善和生动，我们将在打开书和关上书的一瞬间加入自定义动画。

Do you have any crazy layout ideas you are considering for your own app? If you have any questions, comments or other ideas from this tutorial, please join the discussion below!

在开发App过程中，你也可能曾经有过一些关于布局的“疯狂”想法。如果你对本文有任何问题、意见或想法，请加入到下面的讨论中来！

如何实现iOS图书动画:第1部分