I have seen multiple blog posts on this topic which make use of one method or the other. I just thought of putting everything together under one hood -
Angular Material and in the end, pure document property access. This is what is leads to:
Let's quickly talk about the setup here - I am using an Angular
CLI 12.1.0 project. I have
NgRx state management set up, with a root state and a feature state. Of course, NgRx is in no way related to the purpose of this application, but I just thought it would not hurt to implement it. We have to admit, as your application grows in size, NgRx does give you some peace of mind :-)
The UI is built using
angular material. I have used the mat-list and and the mat-spinner components. I have some hard-coded dummy data, which I have converted into an observable stream, with a delay of 1500 ms, which gives some feeling of having fetched data from a REST service.
The key logic is to calculate when the user has scrolled down to the edge of the page. I chose a simple formula derived from the document object. Of course there are other fancy approaches like using an
IntersectionObserver, but that's something I felt was too much for me!
Once the scroll event is set up, whenever the above formula is satisfied, the next set of data is called for. For my sample use case, I have used a limit and skip value, which I have used to slice off a chunk from my dummy data set. Every time a new set is emitted by my data observable, I have made use of a reducer function to append it to my previous list of data.
This is where I store the next limit and skip values for the next data load, and also make a simple calculation of whether my data load is complete, and set it on my state. Once this flag becomes true, subsequent scroll actions do not trigger any more data fetch.