如何使用NSManagedObject的新fetchRequest函数创建获取请求？ [英] How do I make a fetch request using NSManagedObject's new fetchRequest function?

问题描述

在iOS 10中，CoreData团队为NSManagedObject添加了一个新的fetchRequest方法。它看起来像这样：

In iOS 10 the CoreData team added a new "fetchRequest" method to NSManagedObject. It looks like this:

public class func fetchRequest() -> NSFetchRequest<NSFetchRequestResult>

根据我的理解，我们可以将其替换为：

Which, from what I understand, allows us to replace this:

let request = NSFetchRequest<MyEntity>(entityName: "MyEntity")

这个：

let request = MyEntity.fetchRequest()

然而，当我尝试提出这样一个简单的请求时：

However, when I try to make a simple request like this:

let request = MyEntity.fetchRequest()
do {
    results = try request.execute()
} catch let error {
    print("failed to fetch coffee object: \(error)")
}

I收到以下错误：

错误域= NSCocoaErrorDomain代码= 134060（null）
UserInfo = {message =无法获取在
范围内没有NSManagedObjectContext}

Error Domain=NSCocoaErrorDomain Code=134060 "(null)" UserInfo={message=Cannot fetch without an NSManagedObjectContext in scope}

因此，显然错误是说我需要将NSManagedObjectContext带入范围。我一直在寻找示例，但似乎可以找到如何使用新API功能执行请求的完整示例。

So, clearly the error is stating that I need to bring an NSManagedObjectContext into scope. I've been looking for examples but can seem to find a full example of how to perform a request using the new API features.

使用最新的Core Data API功能，如何进行简单的提取请求？基本问题是我如何将NSmanagedObjectCotnext带入范围。

Using the latest Core Data API features, how do I make a simple fetch request? The underlying question is how do I bring my NSmanagedObjectCotnext into scope.

我应该注意到我能够使用传统语法成功发出请求。

I should note that I am able to successfully make a request using the traditional syntax.

推荐答案

好的，我想出了两种方法。 第一种方法，与我的示例代码类似，只是简单：

Ok, I figured out two ways of doing this. The first approach, which is similar to my example code is just simply:

var moc: NSManagedObjectContext!
let request = MyEntity.fetchRequest()
var results : [MyEntity]

 do {
     results = try moc.fetch(request)
 } catch { // error stuff}

第二种方法，已显示在WWDC 2016中，在我们的获取请求对象上使用名为execute的函数。

The second approach, which was shown in WWDC 2016 uses a function named "execute" on our fetch request object.

如果您查看有关NSFetchRequest的详细信息，您将看到一条注释，明确指出该操作必须在一个区块中执行。

If you view the details on NSFetchRequest you'll see a comment that clearly states that the operation must be performed with in a block.

   // Executes the fetch request using the current managed object context. This method must be called from within a block submitted to a managed object context.
    @available(iOS 10.0, *)
    public func execute() throws -> [ResultType]

我突然意识到这是执行抓取的方式：

It's dawned on me that this was the way to perform the fetch:

var moc: NSManagedObjectContext!
let request = MyEntity.fetchRequest()
var results : [MyEntity]

moc.perform {
    self.results = try! request.execute()
}

对于那些自己推荐的人

如果要滚动自己的代码，可能会发现代码无法编译。自动生成的代码中包含一个名为fetchRequest的新方法，编译器使用该方法传递类型。这是代码的样子。

If you are rolling your own code you may find that your code does not compile. Included in the auto-generated code is a new method called fetchRequest that the compiler uses to pass the type along. Here's what the code looks like.

@nonobjc public class func fetchRequest() -> NSFetchRequest<MyEntity> {
    return NSFetchRequest<MyEntity>(entityName: "MyEntity");
}

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