Virtual For Directive
In Ignite UI for Angular, igxForOf
is an alternative to ngForOf
for templating large amounts of data. The igxForOf
uses virtualization behind the scenes to optimize DOM rendering and memory consumption. Virtualization works similarly to Paging by slicing the data into smaller chucks which are swapped from a container viewport while the user scrolls the data horizontally/vertically. The difference with the Paging is that virtualization mimics the natural behavior of the scrollbar. The directive is creating scrollable containers and renders small chunks of the data. It is used inside igxGrid
and it can be used to build virtual igx-list
.
Demo
Dependencies
The igxFor
directive is exported as an NgModule
, thus all you need to do in your application is to import the IgxForOfModule
inside your AppModule
:
// app.module.ts
import { IgxForOfModule } from 'igniteui-angular';
@NgModule({
imports: [
...
IgxForOfModule,
...
]
})
export class AppModule {}
Usage
Now that we have the igxForOf
module imported, let’s get started with a basic configuration of the igxFor that binds to local data:
<span #container>
<ng-template *igxFor="data"></ng-template>
</span>
The data property is an array that provides the virtualized data.
The directive can be used to virtualize the data in vertical, horizontal or both directions.
Vertical virtualization
<table style='height: 500px; overflow: hidden; position: relative;'>
<ng-template igxFor let-item [igxForOf]="data" #virtDirVertical
[igxForScrollOrientation]="'vertical'"
[igxForContainerSize]='"500px"'
[igxForItemSize]='"50px"'
let-rowIndex="index">
<tr style='height:50px;'>{{rowIndex}} : {{item.text}}</tr>
</ng-template>
</table>
Horizontal virtualization
<table>
<tbody style='display: grid; position: relative;'>
<tr style='width:500px; height:118px; overflow: hidden;'>
<ng-template igxFor let-item [igxForOf]="data" #virtDirHorizontal
[igxForScrollOrientation]="'horizontal'"
[igxForContainerSize]='"500px"'
let-rowIndex="index">
<td [style.width.px]='item.width' style='height:100px;'>{{rowIndex}} : {{item.text}}</td>
</ng-template>
</tr>
</tbody>
</table>
Note: It is strongly advised that the parent container of the igxForOf
template for has the related dimension set (height
for vertical and width
for horizontal), overflow: hidden
and position: relative
CSS rules applied. This is because the smooth scrolling behavior is achieved through content offsets that could visually affect other parts of the page if they remain visible.
Horizontal and vertical virtualization
<table #container [style.width]='width' [style.height]='height' [style.overflow]='"hidden"' [style.position]='"relative"'>
<ng-template #scrollContainer igxFor let-rowData [igxForOf]="data"
[igxForScrollOrientation]="'vertical'"
[igxForContainerSize]='height'
[igxForItemSize]='"50px"'>
<tr [style.display]="'flex'" [style.height]="'50px'">
<ng-template #childContainer igxFor let-col [igxForOf]="cols"
[igxForScrollOrientation]="'horizontal'"
[igxForScrollContainer]="parentVirtDir"
[igxForContainerSize]='width'>
<td [style.min-width]='col.width + "px"'>{{rowData[col.field]}}</td>
</ng-template>
</tr>
</ng-template>
</table>
igxFor bound to remote service
The igxForOf
directive can be bound to remote service. You need to use Observable
property - remoteData
(in the following case). Also the chunkLoading
event should be utilized to trigger the requests to the data.
<div style='height: 500px; overflow: hidden; position: relative;'>
<ng-template igxFor let-item [igxForOf]="remoteData | async" (chunkPreload)="chunkLoading($event)"
[igxForScrollOrientation]="'vertical'"
[igxForContainerSize]='"500px"'
[igxForItemSize]='"50px"'
[igxForRemote]='true'
let-rowIndex="index" #virtDirRemote>
<div style='height:50px;'>{{item.ProductID}} : {{item.ProductName}}</div>
</ng-template>
</div>
Also there is a requirement to set the totalItemCount
property in the instance of igxForOf
.
this.virtDirRemote.totalItemCount = data["@odata.count"];
In order access the directive instance from the component it should be marked as ViewChild
:
@ViewChild("virtDirRemote", { read: IgxForOfDirective })
public virtDirRemote: IgxForOfDirective<any>;
And after the request to load the first chunk, the totalItemCount
can be set:
public ngAfterViewInit() {
this.remoteService.getData(this.virtDirRemote.state, (data) => {
this.virtDirRemote.totalItemCount = data["@odata.count"];
});
}
When requesting data you can take advantage of IgxForOfState
interface, which provides the startIndex
and chunkSize
. But note, that initialy the chunkSize will be 0, so you have to specify the size of the first loaded chunk(the best value is igxForContainerSize
initially divided by igxForItemSize
).
public getData(data?: IForOfState, cb?: (any) => void): any {
var dataState = data;
return this.http
.get(this.buildUrl(dataState))
.map((response) => response.json())
.map((response) => {
return response;
})
.subscribe((data) => {
this._remoteData.next(data.value);
if (cb) {
cb(data);
}
});
}
private buildUrl(dataState: any): string {
let qS: string = "?", requiredChunkSize: number;
if (dataState) {
const skip = dataState.startIndex;
requiredChunkSize = dataState.chunkSize === 0 ?
// Set initial chunk size, the best value is igxForContainerSize initially divided by igxForItemSize
10 : dataState.chunkSize;
const top = requiredChunkSize;
qS += `$skip=${skip}&$top=${top}&$count=true`;
}
return `${this.url}${qS}`;
}
And every time the chunkPreload
event is thrown the new chunk of data should be requested:
chunkLoading(evt) {
if(this.prevRequest){
this.prevRequest.unsubscribe();
}
this.prevRequest = this.remoteService.getData(evt, ()=> {
this.virtDirRemote.cdr.detectChanges();
});
}
Local Variables
The igxFor
directive includes the following helper properties in its context: even
, odd
, first
and last
. They are used to identify the current element position in the collection.
The following code snippet demonstrates how to use the even
property in an ng-template
.
<ng-template igxFor let-item
[igxForOf]="data"
[igxForScrollOrientation]="'vertical'"
let-isEven="even">
<div [ngClass]="{even: isEven}">
</div>
</ng-template>
In the example above, an even
class is assigned to every even div element.
Known Limitations
Limitation | Description |
---|---|
Scroll position of components that use igxForOf is not preserved in projected container |
When a component that uses igxForOf directive is in a <ng-content> , or other projected container, its scrollbar position won't be preserved, even though the data position would be. That's because the DOM elements are detached and later re-attached to the DOM tree and as a result, losing the scrollbar scroll position. The affected controls that use igxForOf internally are: igxGrid , igxHierarchicalGrid , igxTreeGrid , igxCombo . Some possible workarounds are:
|