Swift idiomatic error checking

问题:

Let's say that you have a function like this:

func getSomething(error: NSErrorPointer) -> Something

and you typically use it this way:

var error : NSError? = nil
let a = getSomething(&error)

What is an idiomatic way to check for error here? More specific questions:

1. If error == nil can we assume that a will never be nil and vice versa?
2. What should we check first: error (for its nilness) or a (to confirm that it's not a nil)?
3. Can a != nil && error != nil be true in some cases?

Thank you!

回答1:

Compare Handling Error Objects Returned From Methods in the "Error Handling Programming Guide":

Important: Success or failure is indicated by the return value of the method. Although Cocoa methods that indirectly return error objects in the Cocoa error domain are guaranteed to return such objects if the method indicates failure by directly returning nil or NO, you should always check that the return value is nil or NO before attempting to do anything with the NSError object.

So for Cocoa/Cocoa Touch methods you should always check the return value first. It is guaranteed that error != nil if the method fails, but it is not explicitly guaranteed that error == nil if the method succeeds.

Examples:

JSON Serialization

var error : NSError?
if let jsonObj = NSJSONSerialization.JSONObjectWithData(jsonData, options: nil, error: &error) {
    // success
} else {
    // failure
    println("Invalid JSON data: \(error!.localizedDescription)")
}

Core Data fetch request

var error : NSError?
if let result = context.executeFetchRequest(request, error: &error) {
    // success, result has zero or more elements
} else {
    // failure
    println("Fetch failed: \(error!.localizedDescription)")
}

Copying files

var error : NSError?
if !NSFileManager.defaultManager().copyItemAtPath(srcPath, toPath: dstPath, error: &error) {
    println("Cannot copy file: \(error!.localizedDescription)")
}

Of course you can define your own rules for your own functions, but I would follow the same Apple guidelines.

---

Update: As of Swift 2, Cocoa methods that produce errors are translated to Swift functions that throw an error, and this error must be handled with try-catch. Here is the Swift 2 version of above examples:

JSON Serialization

do {
    let jsonObj = try NSJSONSerialization.JSONObjectWithData(jsonData, options: [])
    // success
} catch let error as NSError {
    // failure
    print("Invalid JSON data: \(error.localizedDescription)")
}

Core Data fetch request

do {
    let result = try context.executeFetchRequest(request)
    // success, result has zero or more elements
} catch let error as NSError {
    // failure
    print("Fetch failed: \(error.localizedDescription)")
}

Copying files

do {
    try NSFileManager.defaultManager().copyItemAtPath(srcPath, toPath: dstPath)
} catch let error as NSError {
    print("Cannot copy file: \(error.localizedDescription)")
} 

回答2:

if a function returns an optional value i.e

func someFunc(someVar: String) -> NSData? {
  // some code
}

(the optional means that it can return a nil) then error checking is pretty simple

if let data = someFunc("someString") {
  // this means there was NO error as the function didn't return a nil
}
else {
  // This means there was an error
}

This video is a pretty good reference for error checking and handling in swift https://youtu.be/m8szaLqHVDs