VendingMachineTestCases.swift

//test case #1
func testVMInvalidSelection{

  //Prepare
  let vendingMachine = VendingMachine()
  var didReceiveInvalidSelectionError = false
  
  //Execute
  do{
    try vendinMachine.vend("Coke")
  }
  catch VendingMachineError.InvalidSelection{
    didReceiveInvalidSelectionError = true;
  }
  catch {
    //Important to catch the failure cases, to make sure we are not receiving unexpected Errors
    XCTAssertTrue(false, "Unexpected Exception \(error)")
  }

  //Verfiy
  XCTAssertTrue(didReceiveInvalidSelectionError, "Expected Invalid Selection Error");
}

func testVMReturnCorrectNumberOfInsufficientCoinsError() {
  //Preprare
   let vendingMachine = VendingMachine()
   var howManyCoins = 0
   
   //Execute
   do{
       try vendingMachine.vend(itemNamed: "Candy Bar")
   }
   catch  VendingMachineError.InsufficientFunds(let coinsNeeded) {
       howManyCoins = coinsNeeded
   }
   catch {
       //Important to catch the failure cases, to make sure we are not receiving unexpected Errors
       XCTAssertTrue(false, "Unexpected Exception \(error)")
    }
    
    //Verify
    XCTAssertTrue(howManyCoins==12, "Invalid number of coins needed")
}

func testVMInventoryUpdatedCorrectly(){
    //Prepare    
    let vendingMachine = VendingMachine()
    vendingMachine.coinsDeposited = 12
        
    let item = vendingMachine.inventory["Candy Bar"]
    let itemCountBeforeVend = item!.count
    
    //Execute
    do{
        try vendingMachine.vend(itemNamed: "Candy Bar")
    }
    catch {
        
    }
        
    let itemAfterVend = vendingMachine.inventory["Candy Bar"]
    let itemCountAfterVend = itemAfterVend!.count
    
    //Verify    
    XCTAssertEqual(itemCountBeforeVend, itemCountAfterVend+1, "Invalid Item count before and after end")
    //Since, the method relates to fetching items from inventory, Important to make sure that other details of the items are not tampered
    XCTAssertEqual(itemBeforeVend.price , itemAfterVend.price, "Invalid Price before and after end")
}