2. EventEmitters

EventEmitters implent the default Observer (or publish and subscribe) Pattern. Checkout wikipedia for a short overview of the pattern.

Usage of nope.eventemitters

To work with eventemitters we have to import these:

import * as nope from "nope-js-node";

Afterward we are able to create our first Event-Emitter.

// Create our Event-Emitter:
const obs = new nope.eventemitters.NopeEventEmitter<number>()

emit: Emit Events.

To em, use the method emit.

// Set the content to "1337"
emitter.emit(5);

// There is no observer, of that event
// lets create one:
const observer = emitter.subscribe(event => console.log('emitted value =',event));

Because events are not persistent, our observer will be informed, the next time an event is emitted.

setter: Define a specific setter for the observable.

You can specify a specifc getter for the observable for instance, to limit the number to the following constrains > 0 and < 10.

---

The setter function will receive multiple parameters, as listed below:

1. value,
2. options containing:
   • sender: The Element, which changed the data
   • timestamp: The timestamp of the change
   • args: additional args.

---

The setter function have to return a dict with the following keys: | key | type | content | | - | - | - | | valid | bool | A Flag, to show whether the data are valid or not. If the data is invalid, the observable wont store them | | value | any | The Data that has been adapted |

---

Below, we will implement an example to show the setter above.

emitter.setter = (value, options) => {
  // Print, which data we received
  console.log("setter received", value, options);
  // Show the result of our comparison
  console.log("data is valid:", (value > 0) && (value < 10));
  return {
    // The Value
    value: value,
    // Valid
    valid: (value > 0) && (value < 10)
  }
}

// Emit "1337" ==> But our setter will prevent using this value because it isnt valid.
emitter.emit(1337);

This will print:

setter received 1337 {}
data is valid: false

Because our setter defined the data as invalid, the observer wont receive information.

Now we can reset the setter by assigning null. This disables the setter again:

emitter.setter = null

// Emit "1337" we do not have any setter ==> we will use this parameter
emitter.emit(1337);

This will print:

emitted value =1337

getter: Define a specific getter for the observable.

You can specify a specifc getter for the emitter e.g., to allways return a string

// Define a getter
emitter.getter = (value) => "Allways this result";

// Now lets emit something
emitter.emit(1234)

This will print:

emitted value = Allways this result

To remove such a getter just set the getter property to null. This wont trigger the emitter.

Subscriptions

You can use an emitter to get informed about changes (we already did that to get an log message):

1. define a callback, which receives both, the value and the options.
2. subscribe the emitter.