代码创建的NSCollectionView已注册但makeItem仍返回nil

我正在开发一款基于macOS 10.10 SDK、使用Xcode 9.3（Swift 4）的macOS应用，全程采用代码创建视图而非xib文件。我想要创建NSCollectionView，已将NSCollectionViewItem的子类通过collectionView.register(:,forItemWithIdentifier)注册到NSCollectionView，但在数据源中调用collectionView.makeItem(withIdentifier:,for:)时，该方法始终返回nil。我已找到类似问题，其解决方案为调用register方法，而我已执行该操作。

附可复现问题的最小示例代码：

import Cocoa

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
    @IBOutlet weak var window: NSWindow!

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        window.contentViewController = TestViewController()
    }

    func applicationWillTerminate(_ aNotification: Notification) {
        // Insert code here to tear down your application
    }
}

let cellIdentifier = NSUserInterfaceItemIdentifier(rawValue: "testIdentifier")

class TestViewController: NSViewController, NSCollectionViewDataSource {
    override func loadView() {
        self.view = NSView()
    }

    override func viewDidLoad() {
        let scroll = NSScrollView()
        self.view.addSubview(scroll)
        scroll.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint(item: scroll, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: 0).isActive = true
        NSLayoutConstraint(item: scroll, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0).isActive = true
        NSLayoutConstraint(item: scroll, attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0).isActive = true
        NSLayoutConstraint(item: scroll, attribute: .right, relatedBy: .equal, toItem: self.view, attribute: .right, multiplier: 1, constant: 0).isActive = true
        NSLayoutConstraint(item: scroll, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 500).isActive = true
        NSLayoutConstraint(item: scroll, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 500).isActive = true

        let collection = NSCollectionView()
        scroll.documentView = collection
        collection.register(TestViewItem.self, forItemWithIdentifier: cellIdentifier)
        collection.dataSource = self
        collection.collectionViewLayout = NSCollectionViewFlowLayout()
        collection.reloadData()
    }

    func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
        let item = collectionView.makeItem(withIdentifier: cellIdentifier, for: indexPath)
        return item
    }
}

class TestViewItem: NSCollectionViewItem {
    override func loadView() {
        self.view = NSView()
    }
}

断点显示调用makeItem后返回值始终为nil，请问我哪里操作错误？

嘿，我帮你找到了问题的核心！你的代码里有几个关键的布局设置缺失，导致NSCollectionView无法正确初始化并创建item：

1. NSCollectionView缺少必要的约束和配置 #

你把collectionView设为scrollView的documentView后，没有给它禁用自动约束转换，也没有建立和scrollView的约束关系。macOS的NSScrollView需要documentView有明确的尺寸约束才能正常工作，否则collectionView的内部布局会失效，进而无法创建注册的item。

2. 流布局未设置itemSize #

默认的NSCollectionViewFlowLayout的itemSize是极小的（甚至是0x0），就算item创建成功，你也看不到它；而且这种情况下，collectionView可能会因为没有可显示的内容而跳过item的创建流程。

3. TestViewItem的view无标识性设置 #

你的TestViewItem的view是空白的NSView，没有背景色或尺寸，就算创建成功也无法直观看到结果。

修复后的完整代码 #

import Cocoa

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
    @IBOutlet weak var window: NSWindow!

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        window.contentViewController = TestViewController()
    }

    func applicationWillTerminate(_ aNotification: Notification) {
        // Insert code here to tear down your application
    }
}

let cellIdentifier = NSUserInterfaceItemIdentifier(rawValue: "testIdentifier")

class TestViewController: NSViewController, NSCollectionViewDataSource {
    override func loadView() {
        self.view = NSView()
    }

    override func viewDidLoad() {
        let scroll = NSScrollView()
        self.view.addSubview(scroll)
        scroll.translatesAutoresizingMaskIntoConstraints = false
        
        // 用数组激活约束，更简洁高效
        NSLayoutConstraint.activate([
            scroll.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            scroll.topAnchor.constraint(equalTo: view.topAnchor),
            scroll.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            scroll.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            scroll.widthAnchor.constraint(equalToConstant: 500),
            scroll.heightAnchor.constraint(equalToConstant: 500)
        ])

        let collection = NSCollectionView()
        scroll.documentView = collection
        // 关键：禁用自动约束转换，否则无法添加自定义约束
        collection.translatesAutoresizingMaskIntoConstraints = false
        
        // 给collectionView添加约束，让它和scrollView的contentView适配
        NSLayoutConstraint.activate([
            collection.widthAnchor.constraint(equalTo: scroll.contentView.widthAnchor),
            collection.heightAnchor.constraint(greaterThanOrEqualTo: scroll.contentView.heightAnchor)
        ])
        
        collection.register(TestViewItem.self, forItemWithIdentifier: cellIdentifier)
        collection.dataSource = self
        
        // 设置流布局的itemSize，确保item有可见尺寸
        let flowLayout = NSCollectionViewFlowLayout()
        flowLayout.itemSize = CGSize(width: 80, height: 80)
        collection.collectionViewLayout = flowLayout
        
        collection.reloadData()
    }

    func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
        // 这里可以加个安全转换，避免意外nil
        guard let item = collectionView.makeItem(withIdentifier: cellIdentifier, for: indexPath) as? TestViewItem else {
            fatalError("Failed to dequeue TestViewItem")
        }
        return item
    }
}

class TestViewItem: NSCollectionViewItem {
    override func loadView() {
        self.view = NSView()
        // 设置背景色，方便直观看到item
        self.view.wantsLayer = true
        self.view.layer?.backgroundColor = NSColor.red.cgColor
    }
}

额外注意事项 #

• 优先使用NSLayoutConstraint.activate([...])来批量添加约束，比逐个设置isActive更符合现代Swift的写法，也更高效。
• 纯代码创建NSCollectionViewItem时，确保loadView正确初始化view，并且给view设置可识别的样式（比如背景色），方便调试。
• 始终给纯代码创建的视图设置translatesAutoresizingMaskIntoConstraints = false，否则系统自动生成的约束会和自定义约束冲突，导致布局异常。

这样修改后，makeItem就能正常返回TestViewItem的实例，你也能看到红色的item在collectionView中显示了。

内容的提问来源于stack exchange，提问作者Marco83