How to Trigger Event When a Variable'S Value Is Changed

How to trigger event when a variable's value is changed?

Seems to me like you want to create a property.

public int MyProperty
{
    get { return _myProperty; }
    set
    {
        _myProperty = value;
        if (_myProperty == 1)
        {
            // DO SOMETHING HERE
        }
    }
}

private int _myProperty;

This allows you to run some code any time the property value changes. You could raise an event here, if you wanted.

Trigger an Event when a Variable Value Changes

Rather than using a field, or an auto property, you can specifically define the get/set behaviour of a property (with a backing field).
In this way you can then trigger actions/events whenever the value of that property changes. For example ...

private Status _myStatus;
public Status MyStatus
{
    get { return _myStatus; }
    set 
      {
        if (value == _myStatus) return;
        _myStatus = value;
       // RAISE YOUR EVENT HERE
      }
}

This example will allow an event to be raised whenever the value of MyStatus changes. Obviously I've made an assumption that the Type of MyStatus is Status so just change that to string or whatever the type is.

In javascript, how to trigger event when a variable's value is changed?

There is no event which is raised when a given value is changed in Javascript. What you can do is provide a set of functions that wrap the specific values and generate events when they are called to modify the values.

function Create(callback) {
  var isGreen = false;
  var isRunning = false;
  return { 
    getIsGreen   : function()  { return isGreen; },
    setIsGreen   : function(p) { isGreen = p; callback(isGreen, isRunning); },
    getIsRunning : function()  { return isRunning; },
    setIsRunning : function(p) { isRunning = p; callback(isGreen, isRunning); }
  };
}

Now you could call this function and link the callback to execute go():

var traffic = Create(function(isGreen, isRunning) {
  if (isGreen && !isRunning) {
    go();
  }
});

traffic.setIsGreen(true);

Change in variable triggers an event

Yes this is possible. However, you'll need an Object Oriented approach.
You first need to define a class that raises the events you wish to hook into. Secondly you'll need a class that actually handles the event, since you can not use event handlers in a regular module.
Thirdly, in your regular module you can then just use these classes.

Here's a simple example:
Create a class module named "ClassWithEvent". Place the following code:

Public Event VariableChange(value As Integer)
Private p_int As Integer
Public Property Get value() As Integer
    value = p_int
End Property
Public Property Let value(value As Integer)
    If p_int <> value Then RaiseEvent VariableChange(value) 'Only raise on actual change.
    p_int = value
End Property

Next, create the class that can handle the events raised by this class.
Name this Class Module "ClassHandlesEvent". Place the following code in it:

Private WithEvents SomeVar As ClassWithEvent
Private Sub SomeVar_VariableChange(value As Integer) 'This is the event handler.
    Select Case value
        Case 1:
            MsgBox "here, 1!"
        Case 2:
            MsgBox "here, 2!"
        Case Default:
            'Do Nothing
    End Select
End Sub
Public Property Get EventVariable() As ClassWithEvent
    Set EventVariable = SomeVar
End Property
Public Property Let EventVariable(value As ClassWithEvent)
    Set SomeVar = value
End Property

Next, in a regular module, instantiate your ClassWithEvent and pass this one as a property to the class that handles them.

Sub test()
    Dim var As ClassHandlesEvent
    Dim tst As ClassWithEvent

    Set var = New ClassHandlesEvent
    Set tst = New ClassWithEvent
    var.EventVariable = tst

    tst.value = 2 'A messagebox saying "Here, 2!" will pop-up
End Sub

Angular - trigger an event based on a variable change

You can use the EventEmitter and @Output() to achieve your goal.

Dropdown menu component:

import { Component,  Output, EventEmitter, NgModule } from '@angular/core';
@Component({
  selector: 'app-dropdown',
  template: `
    <div>
      <select [(ngModel)]="selected" (ngModelChange)="onChange($event)">
        <option *ngFor="let category of categories" [value]="category">{{category}}</option>
      </select>
    </div>`
})
export class DropdownComponent {
  @Output() selectedCategory: EventEmitter<string> = new EventEmitter<string>();
  categories = ["All categories", "A", "B", "C"]; //your categories list
  selected: string = "All categories"; //default value 
  constructor() {
  }

  onChange(newvalue): void {
    this.selectedCategory.emit(newvalue);
  }

}  

Here is the other component that will receive the selected value whenever it changes from the dropdown component

import {Component, NgModule} from '@angular/core'
@Component({
  selector: 'app-main',
  template: `
    <div>
      <app-dropdown (selectedCategory)='onChange($event)'></app-dropdown>
    </div>
  `,
})
export class MainComponent {
  selected: string;
  constructor() {
  }

  onChange(category):void {
    this.selected = category;
    //get your items list based on the selected category
  }
}  

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