在Swift中检查nil参数以及该参数是否为空字符串

Question

There may or may not be an easier way to write this, but I feel like being a novice at Swift, I am probably missing something. I have a parameter (fileName) for a method that is an optional String? I need to check if it is nil , or if it is an empty String. This is the code I have, and it works fine, but it seems like it could be a bit more concise/readable.

func writeFile(fileName: String?, withContents contents: String, errorCallback failureCallback: RCTResponseSenderBlock, callback successCallback: RCTResponseSenderBlock) -> Void {

  // If fileName has a value -- is not nil
  if let fileName = fileName {
    // Check to see if the string isn't empty
    if count(fileName) < 1 {
      // Craft a failure message
      let resultsDict = [
        "success": false,
        "errMsg": "Filename is empty"
      ]

      // Execute the JavaScript failure callback handler
      failureCallback([resultsDict])

      return; // Halt execution of this function
    }
    // else, fileName is nil, and should return the same error message.
  } else {
    // Craft a failure message
    let resultsDict = [
      "success": false,
      "errMsg": "Filename is empty"
    ]

    // Execute the JavaScript failure callback handler
    failureCallback([resultsDict])

    return; // Halt execution of this function
  }
}

Answer 1

How about

First create a nil string optional to set up the example var fileName: String?

Now code that lets you see if filename is nil/empty in one simple line:

if (fileName ?? "").isEmpty
{
  println("empty")
}
else
{
  println("not empty")
}

This uses ?? , the Swift " nil coalescing operator ". (link)

For the expression:

a ?? b

Where a is an optional it says:

if a is not nil, return a. If a is nil, return b instead.

So

if (fileName ?? "").isEmpty

Says

First, evaluate fileName and see if it's nil. If so, replace it with an empty string.

Next, check the result to see if it's empty.

Answer 2

You have the right approach. All you need to do is combine your IF statements into one line:

func writeFile(fileName: String?, withContents contents: String, errorCallback failureCallback: RCTResponseSenderBlock, callback successCallback: RCTResponseSenderBlock) -> Void {
  // Check if fileName is nil or empty
  if (fileName == nil) || (fileName != nil && count(fileName!) < 1) {
    // Craft a failure message
    let resultsDict = [
      "success": false,
      "errMsg": "Filename is empty"
    ]

    // Execute the JavaScript failure callback handler
    failureCallback([resultsDict])

    return; // Halt execution of this function
  }

  // Perform code here since fileName has a value and is not empty
}

This code first checks if fileName is nil . If so, it goes into the failure block. If not, it goes to the second conditional. In the second conditional, it will go into the failure block if fileName has a value and is empty. Only when fileName has a value and is NOT empty will it skip the failure block.

Answer 3

You could combine your if let and if statement into something like this:

if let str = fileName where !str.isEmpty {
    println(str)
} else {
    println("empty")
}

This approach is useful if you need to use the filename after checking whether it's nil or empty.