Swift NSExpression error handling [duplicate] - ios

Question

I am trying this code that is a calculator. How can I handle input from the user that is not valid?
//ANSWER: Bridging header to Objective-C// https://github.com/kongtomorrow/TryCatchFinally-Swift
Here is the same question but in objc but I want to do this in swift. Catching NSInvalidArgumentException from NSExpression
All I want to show is a message if it doesn't work, but now I am getting an exception when the user doesn't input the correct format.
import Foundation
var equation:NSString = "60****2" // This gives a NSInvalidArgumentException',
let expr = NSExpression(format: equation) // reason: 'Unable to parse the format string
if let result = expr.expressionValueWithObject(nil, context: nil) as? NSNumber {
let x = result.doubleValue
println(x)
} else {
println("failed")
}

Answer 1

More "Swifty" solution:
#implementation TryCatch
+ (BOOL)tryBlock:(void(^)())tryBlock
error:(NSError **)error
{
#try {
tryBlock ? tryBlock() : nil;
}
#catch (NSException *exception) {
if (error) {
*error = [NSError errorWithDomain:#"com.something"
code:42
userInfo:#{NSLocalizedDescriptionKey: exception.name}];
}
return NO;
}
return YES;
}
#end
This will generate Swift code:
class func tryBlock((() -> Void)!) throws
And you can use it with try:
do {
try TryCatch.tryBlock {
let expr = NSExpression(format: "60****2")
...
}
} catch {
// Handle error here
}

Answer 2

This is still an issue in Swift 2. As noted, the best solution is to use a bridging header and catch the NSException in Objective C.
https://medium.com/swift-programming/adding-try-catch-to-swift-71ab27bcb5b8 describes a good solution, but the exact code doesn't compile in Swift 2 because try and catch are now reserved keywords. You'll need to change the method signature to workaround this. Here's an example:
// https://medium.com/swift-programming/adding-try-catch-to-swift-71ab27bcb5b8
#interface TryCatch : NSObject
+ (void)tryBlock:(void (^)())try catchBlock:(void (^)(NSException *))catch finallyBlock:(void (^)())finally;
#end
#implementation TryCatch
+ (void)tryBlock:(void (^)())try catchBlock:(void (^)(NSException *))catch finallyBlock:(void (^)())finally {
#try {
try ? try() : nil;
}
#catch (NSException *e) {
catch ? catch(e) : nil;
}
#finally {
finally ? finally() : nil;
}
}
#end

Answer 3

A nice solution editing from https://github.com/kongtomorrow/TryCatchFinally-Swift:
First create TryCatch.h & TryCatch.m and bridge them to Swift:
TryCatch.h
#import <Foundation/Foundation.h>
void tryCatch(void(^tryBlock)(), void(^catchBlock)(NSException *e), void(^finallyBlock)());
TryCatch.m
#import <Foundation/Foundation.h>
void tryCatch(void(^tryBlock)(), void(^catchBlock)(NSException *e), void(^finallyBlock)()) {
#try {
tryBlock();
}
#catch (NSException *exception) {
catchBlock(exception);
}
#finally {
finallyBlock();
}
}
Then create the class TryCatch in Swift:
func `try`(`try`:()->()) -> TryCatch {
return TryCatch(`try`)
}
class TryCatch {
let tryFunc : ()->()
var catchFunc = { (e:NSException!)->() in return }
var finallyFunc : ()->() = {}
init(_ `try`:()->()) {
tryFunc = `try`
}
func `catch`(`catch`:(NSException)->()) -> TryCatch {
// objc bridging needs NSException!, not NSException as we'd like to expose to clients.
catchFunc = { (e:NSException!) in `catch`(e) }
return self
}
func finally(finally:()->()) {
finallyFunc = finally
}
deinit {
tryCatch(tryFunc, catchFunc, finallyFunc)
}
}
Finally, use it! :)
`try` {
let expn = NSExpression(format: "60****2")
//let resultFloat = expn.expressionValueWithObject(nil, context: nil).floatValue
// Other things...
}.`catch` { e in
// Handle error here...
print("Error: \(e)")
}