RxJava2 complex use case - rx-java2

Question

Firstly, I have a API to download an Article object that contains a list of Attachments:
// retrofit article endpoint
#GET("/article")
Single<Article> getArticle();
// article class
public class Article {
List<? extends Attachment> attachments;
}
However, the attachment has different subclass such as Video, TextFile etc.
For TextFile, it only has an id while text content needs to be downloaded from another API by using the id:
// retrofit text content endpoint
#GET("/text/{id}")
Single<String> getTextContent(#Path("id") int textFileId);
// text file class
public class TextFile extends Attachment {
// we get id from getArticle API and it's used for download content
private int id;
// we get content from getTextContent API
private String content;
// getters, setters...
}
After I downloaded the text content string for each TextFile object in the attachment list, I need to use set them to back to the corresponding TextFile object list, and finally return the Article object to be displayed.
Currently, what I'm doing is:
public Single<Article> getArticleForView() {
return service.getArticle()
.somehowChainWithGetTextFileWithContent?
}
// method to download text content and set it to TextFile object in the attachment list
private Flowable<TextFile> getTextFileWithContent(List<? extends Attachment> attachments) {
return Flowable.fromIterable(attachments)
// filter out non TextFile attachments
.filter(attachment -> attachment instanceof TextFile)
// download text content and set to TextFile
.concatMap(attachment -> {
TextFile textFile = (TextFile) attachment;
return service.getTextContent(textFile.getId())
.map(textContent -> {
textFile.setContent(textContent);
return textFile;
})
.toFlowable();
});
}
So, I wonder is there any RxJava2 operators can chain those calls to fit in the use case I have? Any help will be appreciated.
Edited:
So this is solution I end up with:
First, I changed the Attachment list in Article class to set, So I don't have to worry about merging updated TextFiles into Article object. But obviously I need to override equals() method in Attachment Class to maintain uniqueness by id.
// article class
public class Article {
Set<? extends Attachment> attachments;
}
And I create a method to take a TextFile object (without content) and return a Single of updated TextFile object:
private Single<TextFile> getTextFileWithContent(TextFile textFile) {
return service.getTextContent(textFile.getId())
.map(content -> {
textFile.setContent(content);
return textFile;
});
}
Then another method takes an Article object and returns a Single> which I will chain to getArticle() call:
private Single<Set<TextFile>> getArticleTextFileDetails(Article article) {
Set<Attachment> attachments = article.getAttachments();
if(attachments == null || attachments.isEmpty()) {
return Single.just(Collections.emptySet());
}
// initial item for collectInto operator
Set<TextFile> updatedTextFiles = new HashSet<>();
return Flowable.fromIterable(attachments)
// filter out non TextFile attachments
.filter(attachment -> attachment instanceof TextFile)
// download content and set to TextFile
.flatMapSingle(this::getTextFileWithContent)
// collect all emits from Flowable into a Single
.collectInto(updatedTextFiles, Set::add);
}
After I got that Set of updated TextFiles, I simply chain it with getArticle() method by flatMap operator:
public Single<Article> getArticleForView() {
return service.getArticle()
.flatMap(article -> {
return getArticleTextFileDetails(article)
.map(updatedTextFileSet -> {
if(!updatedTextFileSet.isEmpty()) {
// simply add updated TextFile set to Attachment set
// since we already override equals() method in Attachment class
article.addAttachments(updatedTextFileSet);
}
return article;
})
});
}