How to properly handle NSFileHandle exceptions in Swift 2.0?

Question

First of all, I am new to iOS and Swift and come from a background of Android/Java programming. So to me the idea of catching an exception from an attempt to write to a file

Answer 1

a second (recoverable) solution would be to create a very simple ObjectiveC++ function that takes a block and returns an exception.

create a file entitled: ExceptionCatcher.h and add import it in your bridging header (Xcode will prompt to create one for you if you don't have one already)

//
//  ExceptionCatcher.h
//

#import <Foundation/Foundation.h>

NS_INLINE NSException * _Nullable tryBlock(void(^_Nonnull tryBlock)(void)) {
    @try {
        tryBlock();
    }
    @catch (NSException *exception) {
        return exception;
    }
    return nil;
}

Using this helper is quite simple, I have adapted my code from above to use it.

func appendString(string: String, filename: String) -> Bool {
    guard let fileHandle = NSFileHandle(forUpdatingAtPath: filename) else { return false }
    guard let data = string.dataUsingEncoding(NSUTF8StringEncoding) else { return false }

    // will cause seekToEndOfFile to throw an excpetion
    fileHandle.closeFile()

    let exception = tryBlock {
        fileHandle.seekToEndOfFile()
        fileHandle.writeData(data)
    }
    print("exception: \(exception)")

    return exception == nil
}

Answer 2

seekToEndOfFile() and writeData() are not marked as throws (they don't throw an NSError object which can be caught in with a do-try-catch block), which means in the current state of Swift, the NSExceptions raised by them cannot be "caught".

If you're working on a Swift project, you could create an Objective-C class which implements your NSFileHandle methods that catches the NSExceptions (like in this question), but otherwise you're out of luck.

Answer 3

This can be achieved without using Objective C code, here is a complete example.

class SomeClass: NSObject {
    static func appendString(string: String, filename: String) -> Bool {
        guard let fileHandle = NSFileHandle(forUpdatingAtPath: filename) else { return false }
        guard let data = string.dataUsingEncoding(NSUTF8StringEncoding) else { return false }

        // will cause seekToEndOfFile to throw an excpetion
        fileHandle.closeFile()

        SomeClass.startHandlingExceptions()
        fileHandle.seekToEndOfFile()
        fileHandle.writeData(data)
        SomeClass.stopHandlingExceptions()

        return true
    }

    static var existingHandler: (@convention(c) NSException -> Void)?
    static func startHandlingExceptions() {
        SomeClass.existingHandler = NSGetUncaughtExceptionHandler()
        NSSetUncaughtExceptionHandler({ exception in
            print("exception: \(exception))")
            SomeClass.existingHandler?(exception)
        })
    }

    static func stopHandlingExceptions() {
        NSSetUncaughtExceptionHandler(SomeClass.existingHandler)
        SomeClass.existingHandler = nil
    }
}

Call SomeClass.appendString("add me to file", filename:"/some/file/path.txt") to run it.