bindTo rx_text of a UITextField did not trigger the UITextField's emit event
<p>I'm a new one to learn RxSwift.   </p>

<p>I modified the Simple Numbers example in the RxSwift Example App, which will add three numbers into a result number. </p>

<p>I add a <code>testStr</code> <code>UITextField</code>, and an <code>upperCase</code> <code>UILabel</code>. I map <code>testStr</code> to uppercase and <code>bindTo</code> the <code>upperCase</code> label, that's good. And I also map <code>testStr</code> to its <code>length</code>, and <code>bindTo</code> the <code>num1</code> field. Strange things happen, although the contents of the <code>num1</code> field changes, it does not emit any event, so it has no effect on the <code>result</code> label. Even if I input some number into another number field, the <code>result</code> number does not count the <code>num1</code>.</p>

<p>Have I made any wrong use of <code>bindTo</code>? In what way can I make the <code>num1</code> emit an event?</p>

<p>Thanks!!!</p>

<pre><code>@IBOutlet weak var num1: UITextField!
@IBOutlet weak var num2: UITextField!
@IBOutlet weak var num3: UITextField!
@IBOutlet weak var result: UILabel!

@IBOutlet weak var testStr: UITextField!
@IBOutlet weak var upperCase: UILabel!

let disposeBag = DisposeBag()

override func viewDidLoad() {
    super.viewDidLoad()

    // Do any additional setup after loading the view, typically from a nib.
    Observable.combineLatest(num1.rx_text, num2.rx_text, num3.rx_text) {
        (textval1, textval2, textval3) -&gt; Int in
            return (Int(textval1) ?? 0) + (Int(textval2) ?? 0) + (Int(textval3) ?? 0)
        }
        .map{$0.description}
        .bindTo(result.rx_text)
        .addDisposableTo(disposeBag)

    let obStr = testStr.rx_text

    obStr
        .map {$0.uppercaseString}
        .bindTo(upperCase.rx_text)
        .addDisposableTo(disposeBag)

    obStr
        .map{ $0.characters.count }
        .map{ $0.description }
        .bindTo(num1.rx_text)
        .addDisposableTo(disposeBag)
}
</code></pre>