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https://issues.apache.org/jira/browse/SOLR-388?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536773
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Yonik Seeley commented on SOLR-388:
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A better way might be to add a mechanism to add fields to a document.
> Refactor ResponseWriters and Friends.
> -------------------------------------
>
> Key: SOLR-388
> URL: https://issues.apache.org/jira/browse/SOLR-388
> Project: Solr
> Issue Type: Improvement
> Components: search
> Affects Versions: 1.2
> Reporter: Luke Lu
>
> When developing custom request handlers, it's often necessary to create
> corresponding response writers that extends existing ones. In our case, we
> want to augment the result list (more attributes other than numFound,
> maxScore, on the fly per doc attributes that are not fields etc.) , only to
> find JSONWriter and friends are private to the package. We could copy the
> whole thing and modify it, but it wouldn't take advantage of recent fixes
> like Yonik's FastWriter changes without tedious manual intervention. I hope
> that we can can *at least* extends it and overrides writeVal() to add a new
> result type to call writeMyType.
> Ideally the ResponseWriter hierarchy could be rewritten to take advantage of
> a double dispatching trick to get rid of the ugly if something is instance of
> someclass else ... list, as it clearly doesn't scale well with number of
> types (_n_) and depth (_d_) of the writer hierarchy, as the complexity would
> be O(_nd_), which worse than the O(1) double dispatching mechanism. Some
> pseudo code here:
> {code:title=SomeResponseWriter.java}
> // a list of overloaded write method
> public void write(SomeType t) {
> // implementation
> }
> {code}
> {code:title=ResponseWritable.java}
> // an interface for objects that support the scheme
> public interface ResponseWritable {
> public abstract void write(ResponseWriter writer);
> }
> {code}
> {code:title=SomeType.java}
> // Sometype needs to implement the ResponseWritable interface
> // to facilitate double dispatching
> public void write(ResponseWriter writer) {
> writer.write(this);
> }
> {code}
> So when adding a new MyType and MySomeResponseWriter, we only need to add
> these two files without having to muck with the writeVal if-then-else list.
> Note, you still need to use the if else list for builtin types and any types
> that you can't modify in the write(Object) method.
> {code:title=MyType.java}
> // implements the ResponseWritable interface
> public write(ResponseWriter writer) {
> writer.write(this);
> }
> {code}
> {code:title=MySomeResponseWriter.java}
> // only need to implement this method
> public void write(MyType t) {
> // implementation
> }
> {code}
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