为什么可以用这种奇怪的方式编写UICollectionViewDiffableDataSource初始化器？

Question

在通过Paul Hudson的tutorial学习Swift时，我遇到了一些奇怪的事情。

UICollectionViewDiffableDataSource的初始化器定义为：

public init(collectionView: UICollectionView, cellProvider: @escaping UICollectionViewDiffableDataSource<SectionIdentifierType, ItemIdentifierType>.CellProvider)

据我所知，没有其他的初始化器。但是，Paul成功地将其初始化为这样，省略了cellProvider参数：

dataSource = UICollectionViewDiffableDataSource<Section, App>(collectionView: collectionView) { collectionView, indexPath, app in
    switch self.sections[indexPath.section].type {
    case "mediumTable":
        return self.configure(MediumTableCell.self, with: app, for: indexPath)
    case "smallTable":
        return self.configure(SmallTableCell.self, with: app, for: indexPath)
    default:
        return self.configure(FeaturedCell.self, with: app, for: indexPath)
    }
}

同时，雷·温德利希的tutorial会这样做：

dataSource = UICollectionViewDiffableDataSource<Section, App>(collectionView: collectionView, cellProvider: { (collectionView, indexPath, app) -> UICollectionViewCell? in
    switch self.sections[indexPath.section].type {
    case "mediumTable":
        return self.configure(MediumTableCell.self, with: app, for: indexPath)
    case "smallTable":
        return self.configure(SmallTableCell.self, with: app, for: indexPath)
    default:
        return self.configure(FeaturedCell.self, with: app, for: indexPath)
    }
})

我试图理解保罗的方式背后发生了什么样的斯威夫特“魔法”，因为他似乎放弃了cellProvider的争论，而是做了一些时髦的闭包事情。他在这里具体应用了哪些Swift规则？

Answer 1

在您的例子中，您可以看到两种方法如何使用不同的语法编写相同的代码。

在这里，您通过两个参数collectionView和cellProvider以预期的方式使用所有参数。

dataSource = UICollectionViewDiffableDataSource<Section, App>(collectionView: collectionView, cellProvider: { (collectionView, indexPath, app) -> UICollectionViewCell? in
    switch self.sections[indexPath.section].type {
    case "mediumTable":
        return self.configure(MediumTableCell.self, with: app, for: indexPath)
    case "smallTable":
        return self.configure(SmallTableCell.self, with: app, for: indexPath)
    default:
        return self.configure(FeaturedCell.self, with: app, for: indexPath)
    }
})

在这里，您使用第二个参数(cellProvider:)作为括号后面的闭包，除了最后一个参数。

dataSource = UICollectionViewDiffableDataSource<Section, App>(collectionView: collectionView) { collectionView, indexPath, app in
    switch self.sections[indexPath.section].type {
    case "mediumTable":
        return self.configure(MediumTableCell.self, with: app, for: indexPath)
    case "smallTable":
        return self.configure(SmallTableCell.self, with: app, for: indexPath)
    default:
        return self.configure(FeaturedCell.self, with: app, for: indexPath)
    }
}

请注意，这两个示例都是相同的：

{ collectionView, indexPath, app in
    switch self.sections[indexPath.section].type {
    case "mediumTable":
        return self.configure(MediumTableCell.self, with: app, for: indexPath)
    case "smallTable":
        return self.configure(SmallTableCell.self, with: app, for: indexPath)
    default:
        return self.configure(FeaturedCell.self, with: app, for: indexPath)
    }

有关Trailing Closures或What is trailing closure syntax?的更多信息

Answer 2

感谢为我指明正确方向的每一个人。查看其他更精简的示例可能会有所帮助：

func executeThis(info: String, completion: ((String) -> Void)) {
    completion(info)
}

let completion: ((String) -> Void) = { value in 
    print("Done: " + value)
}

// Pass the closure variable to the completion param:
executeThis(info:"Some Activity", completion:completion)

// Provide the closure directly as an argument:
executeThis(info:"Some Activity 2", completion: { value in
    print("Done: " + value)
})

// Trailing closure: the last param, which is a closure, is implicitly provided via "{ value in"
executeThis(info:"Some Activity 3") { value in
    print("Trailing closure done: " + value)
}