How To Do Recursive Observable Call in RxJava?
I am quite new to RxJava (and Reactive paradigm in general), so please bear with me.
Suppose I have this News and this nested Comment data structure:
public class News {
public int id;
public int[] commentIds; //only top level comments
public News(int id, int[] commentIds) {
this.id = id;
this.commentIds = commentIds;
}
}
public class Comment {
public int id;
public int parentId; //ID of parent News or parent comment
public int[] childIds;
public Comment(int id, int parentId, int[] childIds) {
this.id = id;
this.parentId = parentId;
this.childIds = childIds;
}
}
and suppose I have this API endpoint:
getComments(int commentId) //return Observable<Comment> for Comment with ID commentId
Now, let's assume:
getComments(1); //will return Comment(1, 99, [3,4])
getComments(2); //will return Comment(2, 99, [5,6])
getComments(3); //will return Comment(3, 1, [])
getComments(4); //will return Comment(4, 1, [])
getComments(5); //will return Comment(5, 2, [])
getComments(6); //will return Comment(6, 2, [])
**
Now, if I have News n = News(99, [1,2]) , how do I get all of its children comment recursively? ie to get comments with ID [1,2,3,4,5,6]?
**
I have searched and stumbled upon this: https://jkschneider.github.io/blog/2014/recursive-observables-with-rxjava.html
This is the recursion function:
public class FileRecursion {
static Observable<File> listFiles(File f) {
if(f.isDirectory())
return Observable.from(f.listFiles()).flatMap(FileRecursion::listFiles);
return Observable.just(f);
}
public static void main(String[] args) {
Observable.just(new File("/Users/joschneider/Desktop"))
.flatMap(FileRecursion::listFiles)
.subscribe(f -> System.out.println(f.getAbsolutePath()));
}
}
It shows an example on how to do recursive observable calls, but the inner function ( f.listFiles() ) is a blocking operation (doesn't return another Observable). In my case, the inner function ( getComments ) is a non-blocking function that returns another Observables. How do I do that?
Any help will be much appreciated.
This does practically the same thing described in the article:
Observable<Comment> getInnerComments(Comment comment) {
if (comment.childIds.length > 0)
return Observable.merge(
Observable.just(comment),
Observable.from(comment.childIds)
.flatMap(id -> getComments(id))
.flatMap(this::getInnerComments));
return Observable.just(comment);
}
public static void main(String[] args) {
getComments(1)
.flatMap(this::getInnerComments)
.subscribe(c -> System.out.println(comment.toString()));
}
I start with the comment with id = 1, then I pass it to getInnerComments() . The getInnerComments() checks if the comment has children. If it does, it iterates over every child id ( Observable#from ) and loads every child with your getComments(int) API. Then every child is passed to the getInnerComments() to do the same procedure. If a comment doesn't have children, it is immediately returned using Observable#just .
This is pseudo-code and it wasn't tested, but you should get the idea.
Below is an example of how to get all comments and then aggregate them to one List<Comment> .
getNews(99)
.flatMap(news -> Observable.from(news.commentIds))
.flatMap(commentId -> getComments(commentId))
.flatMap(comment -> getInnerComments(comment))
.toList()
.subscribe(commentList -> { });