Update only selected array item in *ngFor array Angular2 - arrays

Question

I have an *ngFor loop to display libraries. When you click on a library, categories of that library appear beneath in a tree-like structure. The categories are also being displayed with an *ngFor loop. When I have one library expanded and click on another one, the categories in BOTH libraries are updating to the categories in the library that was just clicked. The functionality I am looking for is to only update the selected library categories and leave the others alone. There is a post here that seems to be close to my problem but I couldn't get it to work.
Using *ngFor to loop through an array and *ngIf to select which elements from the array to list
Here is my code:
library.component.html
<div *ngFor="let messageLibrary of onHoldMessageLibraryService.data" class="library mgn-top10 ft-size14">
<a (click)="onHoldMessageLibraryService.getSelectedMessageLibrary(messageLibrary)"><i class="fa fa-folder-o mgn-right10" aria-hidden="true"></i>{{messageLibrary.Name}}</a>
<library-category *ngIf="messageLibrary.treeIsExpanded" (displayMessagesFromSelectedCategory)="getOnHoldMessages()"></library-category>
</div>
library.service
public treeIsExpanded: boolean;
public selectMessages: boolean;
public data: TreeData[];
public selectedName: string;
public selectedValue: number;
public getSelectedMessageLibrary(messageLibrary): void {
this.selectedMessagesLibrary = messageLibrary;
this.selectedMessagesLibrary.treeIsExpanded = !this.selectedMessagesLibrary.treeIsExpanded;
}
public dummyData(): void {
let myTree: TreeData[] = new Array();
myTree.push(
{
Name: 'Banking Library',
Id: 1,
Category: [{
Name: 'Credit Cards',
Id: 11,
Category: null
}]
},
{
Name: 'Automobile Library',
Id: 2,
Category: [{
Name: 'Cars',
Id: 12,
Category: null
}]
},
{
Name: 'Coffee Library',
Id: 3,
Category: [{
Name: 'Americano',
Id: 13,
Category: null
}]
}
)
this.data = myTree;
}
library-category.component.html
<ul *ngFor="let category of onHoldMessageLibraryService.selectedMessagesLibrary.Category">
<li><i class="fa fa-folder-o mgn-right10" aria-hidden="true"></i>{{category.Name}}</li>
</ul>
tree-data.ts
export class TreeData {
Name: string;
Id: number;
Category: TreeData[];
}

Answer 1

public getSelectedMessageLibrary(messageLibrary): void {
this.selectedMessagesLibrary = messageLibrary;
this.selectedMessagesLibrary.treeIsExpanded = !this.selectedMessagesLibrary.treeIsExpanded; //this line is not giving desired result
}
to be replaced with
public getSelectedMessageLibrary(messageLibrary): void {
messageLibrary.treeIsExpanded = !messageLibrary.treeIsExpanded;
}
And it will be good if you use trackBy with NgFor