xcode treating catching an error as a test failure - xcode

Question

I've written the following function:
func sendMessage(message:Dictionary<String,AnyObject>) {
do {
let json = try NSJSONSerialization.dataWithJSONObject(message, options: [])
let jsonStr = NSString(data: json, encoding: NSUTF8StringEncoding)
self.wsSource.writeString(jsonStr as! String)
} catch let err as NSError {
Rc2LogError ("error sending json message on websocket:\(err)")
}
}
Here is a unit test that should pass:
func testSendMessageFailure() {
let dict = ["foo":wspace!, "bar":22]
session!.sendMessage(dict)
XCTAssertNil(wsSrc.lastStringWritten)
}
But the test fails with the message:
test failure: -[Rc2SessionTests testSendMessageFailure()] failed: failed: caught "NSInvalidArgumentException", "Invalid type in JSON write (Rc2.Rc2Workspace)"
That isn't a failure. It is what I expect! Does this mean it is impossible to write a unit test that calls a function that has an error handler that handles the error?
How can I tell XCTest/Xcode that a caught error/exception does not mean a test failed?

Answer 1

do-catch statements are only for catching error thrown by the function. They do not catch exceptions. Exceptions are used to show that there is a programming error.
If obj will not produce valid JSON, an exception is thrown. This exception is thrown prior to parsing and represents a programming error, not an internal error. You should check whether the input will produce valid JSON before calling this method by using isValidJSONObject:.
Source: NSJSONSerialization
What is happening is that message is not an object that can be represented in json. My guess would be that wspace contains a type that can not be converted to json.