simple textChanged example

example.kt

interface View {
  fun textChanged(): Single<String>
  fun getText(): String
}

interface Model {
  fun validateText(text: String) Boolean
}

class Presenter(view: View, model: Model) {

  fun setupTextChanged() {
    // textChanged emits a signal every time the user changes some text.
    // Ideally, it emits the string of text in the field that was changed.
    view.textChanged()
      // Map does a simple type mapping.  Here, we are going from String to Bool
      .map { model.validateText(text) }
      // do* methods are for side effects.  Here, the side effect of invalid data
      // is that the specific error should be shown.  Think of it as a place within
      // the middle of the stream to place IO and other global-state actions
      .doOnNext { if (!it) { view.displayErrorForText() } }
      // Filters out anything that doesnt match the predicate.  In this case, simply
      // only emits true events (the text is valid)
      .filter { it }
      // Subscriber is also allowed to handle side effects and touch global state.  So
      // here we simply determine if we can enable to submit button.
      .subscribe { view.enableSubmit(checkAllFields()) }
  }
  
  fun checkAllFields() {
    /** run validation checks on all fields */
    return model.validateText(view.getText()) // etc.
  }
}