how the nativescript radlist view load on demand works - nativescript

Question

This might not be the question but it was the list of doubts which comes when learning native script from scratch.
I had a 1000 or more list of data stored in data table. know i want to display it on a list view but i don't want to read all the data at once. because i have images stored in other directory and want to read that also. So, for 20 to 30 data's the performance is quite good. but for 1000 data it is taking more than 15 minutes to read the data as well as images associated with it. since i'm storing some high quality images.
Therefore i decided to read only 20 data's with their respective images. and display it on list. know when user reaches the 15th data of the list. i decided to read 10 more data from the server.
know when i search this i came across "RadListView Load on Demand".
then i just looked at the code below.
public addMoreItemsFromSource(chunkSize: number) {
let newItems = this._sourceDataItems.splice(0, chunkSize);
this.dataItems.push(newItems);
}
public onLoadMoreItemsRequested(args: LoadOnDemandListViewEventData) {
const that = new WeakRef(this);
const listView: RadListView = args.object;
if (this._sourceDataItems.length > 0) {
setTimeout(function () {
that.get().addMoreItemsFromSource(2);
listView.notifyLoadOnDemandFinished();
}, 1500);
args.returnValue = true;
} else {
args.returnValue = false;
listView.notifyLoadOnDemandFinished(true);
}
}
In nativescript if i want to access binding element xml element. i must use observables in viewmodel or exports.com_name on associated js file.
but in this example it is started with public..! how to use this in javascript.
what is new WeakRef(this) ?
why it is needed ?
how to identify user has scrolled to 15 data, as i want to load more data when he came at 15th data.
after getting data how to update array of list and show it in listview ?
Finally i just want to know how to use load on demand
i tried to create a playground sample of what i have tried but it is giving error. it cannot found module of radlistview.
Remember i'm a fresher So, kindly keep this in mind when answering. thank you,
please modify the question if you feel it is not upto standards.

Answer 1

you can check the updated answer here
https://play.nativescript.org/?template=play-js&id=1Xireo

Answer 2

TypeScript to JavaScript
You may use any TypeScript compiler to convert the source code to JavaScript. There are even online compilers like the official TypeScript Playground for instance.
In my opinion, it's hard to expect ES5 examples any more. ES6-9 introduced a lot of new features that makes JavaScript development much more easier and TypeScript takes JavaScript to next level, interpreter to compiler.
To answer your question, you will use the prototype chain to define methods on your class in ES5.
YourClass.prototype.addMoreItemsFromSource = function (chunkSize) {
var newItems = this._sourceDataItems.splice(0, chunkSize);
this.dataItems.push(newItems);
};
YourClass.prototype.onLoadMoreItemsRequested = (args) {
var that = new WeakRef(this);
var listView = args.object;
if (this._sourceDataItems.length > 0) {
setTimeout(function () {
that.get().addMoreItemsFromSource(2);
listView.notifyLoadOnDemandFinished();
}, 1500);
args.returnValue = true;
} else {
args.returnValue = false;
listView.notifyLoadOnDemandFinished(true);
}
}
If you are using fromObject syntax for your Observable, then these functions can be passed inside
addMoreItemsFromSource: function (chunkSize) {
....
};
WeakRef: It helps managing your memory effiencetly by keeping a loose reference to the target, read more on docs.
How to load more:
If you set loadOnDemandMode to Auto then loadMoreDataRequested event will be triggered whenever user reaches the end of scrolling.
loadOnDemandBufferSize decides how many items before the end of scroll the event should be triggered.
Read more on docs.
How to update the array:
That's exactly what showcased in addMoreItemsFromSource function. Use .push(item) on the ObservableArray that is linked to your list view.