Adding Slices to an Firefly Semantics Slice Entity Store

Ole Ersoy
3 min readMay 3, 2022


Image by Tamanna Rumee from Pixabay


We have a Firefly Semantics Slice Entity Store ( EStore<Todo> ) containing Todo Entity instances.

We want to add 2 slices, or live filtered observable collections, to the entity store. One for complete Todo instances and one for incomplete Todo instances.

We also want to observe the filters as we change the stores state.


In this demo we will create a Todo interface for our entities and two Todo objects that we will use for our initial entity store state and to display how the Slice instances track modified state.

interface Todo {
gid?: string;
id?: string;
complete: boolean;
title: string;
const TODO_ID_1 = '1';
const TODO_ID_2 = '2';
const TODO1: Todo = {
id: TODO_ID_1,
complete: false,
title: 'You complete me!'
const TODO2: Todo = {
id: TODO_ID_2,
complete: true,
title: 'You completed me!'
let todos: Todo[] = [TODO1, TODO2];

Next we will initialize our store with the todos array we created and obtain references of the added Todo instances from the store:

// Constructor Initialize the Store
let store: EStore<Todo> = new EStore<Todo>(todos);
// Retrieve Entities from the Store by ID
let t1 = store.findOneByID(TODO_ID_1);
let t2 = store.findOneByID(TODO_ID_2);

Now we are going to add our slices for tracking complete and incomplete Todo instances.

// Add Slices
// Create an enum to key slices
export const enum TodoSlices {
COMPLETE = 'Complete',
INCOMPLETE = 'Incomplete'
store.addSlice((todo) => todo.complete, TodoSlices.COMPLETE);
store.addSlice((todo) => !todo.complete, TodoSlices.INCOMPLETE);

We first create an enum TodoSlices containing the keys for the slices we will be adding to the store.

We then call store.addSlice passing in the predicate function and the key.

As can be seen for the complete slice we pass in (todo)=>todo.complete as the predicate function. And this function will filter out all of the complete Todo instances. We pass in TodoSlices.complete as the key.

Next we will take a snapshot of each slice to see what is in it.

const completeTodos = store.getSlice(TodoSlices.COMPLETE).allSnapshot();//console.log(JSON.stringify(completeTodos));const incompletedTodos = store.getSlice(TodoSlices.INCOMPLETE).allSnapshot();//console.log(JSON.stringify(incompletedTodos));

And if we log each snapshot we will see that the completeTodos one contains the complete Todo instance, and the incompleteTodos one contains the incomplete instance.

Next we will observe each the complete Todos slice:

const completeTodos$ = store.getSlice(TodoSlices.COMPLETE).obs;
completeTodos$.subscribe((todos) => {
//console.log('COMPLETED TODOS');

If we log the todos array emitted in our subscription function we will see that the array contains the completed Todo instance.

We will now post a third Todo object to the store.{ id: '3', title: 'Adding a Complete Todo', complete: true });

And after doing this we notice that the subscription to the completed Todos slice now logs a new array with two completed items.

If we change the complete property value to false and do a put on the entity store the subscription only logs one array.

T3.complete = false;

Now the subscription to the completed Todos slice only logs the first completed todo we added. The third Todo instance has been dynamically removed from the slice.




Ole Ersoy

Founder of Firefly Semantics Corporation