Swift 2.0中的countForFetchRequest [英] countForFetchRequest in Swift 2.0

问题描述

我试图在Swift 2.0中的托管对象上下文上使用countForFetchRequest方法.

I am trying to use the countForFetchRequest method on a managed object context in Swift 2.0.

我注意到executeFetchRequest的错误处理已更改为新的do-try-catch语法:

I note that the error handling for executeFetchRequest has been changed across to the new do-try-catch syntax:

func executeFetchRequest(_ request: NSFetchRequest) throws -> [AnyObject]

，但是countForFetchRequest方法仍然使用旧式错误指针:

but the countForFetchRequest method still uses the legacy error pointer:

func countForFetchRequest(_ request: NSFetchRequest,
                    error error: NSErrorPointer) -> Int

...而且我在弄清楚如何在Swift 2.0中使用此功能时遇到了一些麻烦.

...and I am having a bit of trouble figuring out how to use this in Swift 2.0.

如果我做的是与Swift 2.0之前的版本相同的事情:

If I do the same thing as pre-Swift 2.0:

let error: NSError? = nil
let count = managedObjectContext.countForFetchRequest(fetchRequest, error: &error)

我收到错误消息说要删除&，但是如果我删除该消息，我会收到另一个错误消息说NSError无法转换为NSErrorPointer.

I get errors saying to remove the &, but if I remove that I get another error saying that NSError cannot be converted to an NSErrorPointer.

对于如何使它正常工作，我们将不胜感激.

Any help would be appreciated about how to get this working.

推荐答案

您的代码几乎是正确的，但是error必须是变量，以便以 输入&的inout参数:

Your code is almost correct, but error needs to be a variable, in order to be passed as inout-argument with &:

var error: NSError? = nil
let count = managedObjectContext.countForFetchRequest(fetchRequest, error: &error)

更新:自 Swift 3 起，countForFetchRequest 引发错误:

Update: As of Swift 3, countForFetchRequest throws an error:

do {
    let count = try managedObjectContext.context.count(for:fetchRequest)
    return count
} catch let error as NSError {
    print("Error: \(error.localizedDescription)")
    return 0
}

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