Angular 2 - display one component as drop down in another and grabbing the selected value in parent component - angular2-template

Question

This is my very first Angular 2 project. I created a component (category) that would pull a list of categories from a service stack API and show as a dropdown like this:
<select (change)="onSelect($event.target.value)" [(ngModel)]="selectedCategory.Id">
<option *ngFor="let category of categoriesToDisplay"
value={{category.Id}}>{{category.Name}}
</option>
</select>
Selected Category is: {{selectedCategory.Id}} and Name is {{selectedCategory.Name}}
I have another component (AddExpense) which is a form, where the user can add in the amount, category and hit submit that would POST to another endpoint. For AddExpense component, this is how the .html looks
<form [formGroup]="expense" (ngSubmit)="fileExpense($event)">
Spent <input type="number" formControlName="amt" />
for <input type="text" formControlName="name" />
on <input type="date" formControlName="transdate" />
and file it under category <show-category></show-category>
<button type="submit">Add Expense</button>
</form>
My question is how do I figure out which category from the drop down was selected in the add expense form, when the drop down itself is rendered via the show-category component and pass it on as a form control item for add-expense component's .ts to use?
I might be needing to use the Input and output decorator, but not sure how to nab that particular item thats selected in the dropdown and pass it on as input to the add expense component.

Answer 1

Sounds like you're displaying your first component (ShowCategory) in the second (AddExpense).
Now there is a couple way to do this,
The most obvious is to use Output, ShowCategory will then emit (See EventEmitter and Output example) whenever the selected value is changed.
In the template of AddExpense, you'd simply have to write something like this,
<show-category (change)="category = $event.category"></show-category>
But that's ugly, and frankly it would've been cool if we can do this and let Angular handle the two-way binding,
<show-category [(ngModel)]="category"></show-category>
Turns out it's doable, you can check out documentation for NgModel here, but what's actually useful is an example implementation for a real control like a checkbox
You might've noticed in the example that NgModel is not mentioned anywhere, and that's because we only have to create the mechanism for NgModel to write/read value from your component.
If you search on Google for NgModel ValueAccessor, there's a bunch of blog posts to help you out.
Lastly, I personally doesn't suggest doing what you're doing now if the ShowCategory component is that simple, that increases complexity a little bit, and ShowCategory isn't doing much for now. But what you provided might very well be a minimum working example, so what do I know.
Happy coding!