ios - 如何在 Swift if 语句中捕获 0 的返回值？

Question

我有一个 init 调用，它要么延迟加载我的种子数据，要么加载我的数据库中存在的数据:

    if let savedPages = loadPages() {
        pages += savedPages
    } else {
        createPages()
    }

我的 createPages() 方法没有被调用，因为 loadPages() 至少返回了 0 个结果。

func loadPages() -> [NSManagedObject]? {

    // Load all those pages!
    let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    let managedContext = appDelegate.managedObjectContext
    let fetchRequest = NSFetchRequest(entityName: "Page")
    do {
        let results = try managedContext.executeFetchRequest(fetchRequest)
        return results as? [NSManagedObject]
    } catch let error as NSError {
        print("Could not fetch \(error), \(error.userInfo)")
    }
    return nil
}

如何使我的 else 语句 捕获以便调用 createPages()？

Answer 1

选项 1:

在loadPages()中添加返回

if results.count == 0 { 返回 nil }

func loadPages() -> [NSManagedObject]? {

    // Load all those pages!
    let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    let managedContext = appDelegate.managedObjectContext
    let fetchRequest = NSFetchRequest(entityName: "Page")
    do {
        let results = try managedContext.executeFetchRequest(fetchRequest)
        ///
        if results.count == 0 { return nil }
        ///
        return results as? [NSManagedObject]
    } catch let error as NSError {
        print("Could not fetch \(error), \(error.userInfo)")
    }
    return nil
}

选项 2:

在if let之后添加where

if let savedPages = loadPages() where savedPages.count > 0 {
    pages += savedPages
} else {
    createPages()
}