GWT with AsyncCallback and SuggestOracle - asynchronous

Question

I use 'AsyncCallbacks' to the server (Tomcat) to query a database for 'resultsets', which should be shown as the result of the 'SuggestOracle'.
My problem is that, while typing in the input field, several 'AsyncCallbacks' are fired, but they are not fired in the order as they are entered.
For example:
If I enter the string "user1" there should be 5 callbacks be fired off. Now look at the server log:
Dez 30, 2017 9:27:59 AM com.test.server.dispatch.actionhandlers.SuggestUsersActionHandler execute
INFORMATION: Giving suggestinos for query u searching for contacts of user 136141
Dez 30, 2017 9:27:59 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: suggesting ... with query:u
Dez 30, 2017 9:28:00 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: query resulted in: 4 elements
Dez 30, 2017 9:28:00 AM com.test.server.dispatch.ActionExecutor execute
INFORMATION: com.test.entities.User(Id: 136141, Email: h#h.com) finished executing action com.test.services.dispatch.actions.SuggestUsersAction
Dez 30, 2017 9:28:01 AM com.test.server.dispatch.actionhandlers.SuggestUsersActionHandler execute
INFORMATION: Giving suggestinos for query user1 searching for contacts of user 136141
Dez 30, 2017 9:28:01 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: suggesting ... with query:user1
Dez 30, 2017 9:28:02 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: query resulted in: 1 elements
Dez 30, 2017 9:28:02 AM com.test.server.dispatch.ActionExecutor execute
INFORMATION: com.test.entities.User(Id: 136141, Email: h#h.com) finished executing action com.test.services.dispatch.actions.SuggestUsersAction
Dez 30, 2017 9:28:02 AM com.test.server.dispatch.actionhandlers.SuggestUsersActionHandler execute
INFORMATION: Giving suggestinos for query user searching for contacts of user 136141
Dez 30, 2017 9:28:02 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: suggesting ... with query:user
Dez 30, 2017 9:28:03 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: query resulted in: 4 elements
Dez 30, 2017 9:28:03 AM com.test.server.dispatch.ActionExecutor execute
INFORMATION: com.test.entities.User(Id: 136141, Email: h#h.com) finished executing action com.test.services.dispatch.actions.SuggestUsersAction
Dez 30, 2017 9:28:03 AM com.test.server.dispatch.actionhandlers.SuggestUsersActionHandler execute
INFORMATION: Giving suggestinos for query us searching for contacts of user 136141
Dez 30, 2017 9:28:03 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: suggesting ... with query:us
Dez 30, 2017 9:28:04 AM com.test.server.dao.UserDAO getSuggestions
INFORMATION: query resulted in: 4 elements
Dez 30, 2017 9:28:04 AM com.test.server.dispatch.ActionExecutor execute
INFORMATION: com.test.entities.User(Id: 136141, Email: h#h.com) finished executing action com.test.services.dispatch.actions.SuggestUsersAction
It first queries for "u" then for "user1" then for "user" then for "us" ..
so the requests are not sent in the right order.
Do you know any tip of how to send those async queries in the right order?
I fear that using 'AsyncCallbacks' in combination with 'SuggestOracles' are not a good idea.

Answer 1

You should never assume that asynchronous callbacks would be fired in any special order. There are many factors that can cause some calls to be delayed and thus the order can be random.
My favorite method is to just keep the last callback. Please, note, that while you are typing, let's say user1 you get 5 calls to the server, but you are only interested in the result for user1 results. Previous (like us, use etc.) can (or even should) be omitted.
So I've extended AsyncCallback to CancelableAsyncCallback:
public abstract class CancelableAsyncCallback<T> implements AsyncCallback<T> {
private boolean cancelled = false;
public void cancel() {
cancelled = true;
}
public boolean isCancelled() {
return cancelled;
}
}
I keep track of the last callback and cancel the unwanted previous results like this:
if(lastCallback != null)
lastCallback.cancel();
lastCallback = new CancelableAsyncCallback<Type>() {
#Override
public void onFailure(Throwable caught) {
if(!isCancelled()) {
// do the stuff
}
}
#Override
public void onSuccess(Type result) {
if(!isCancelled()) {
// do the stuff
}
}
};

Answer 2

Adams answer will probably work in a lot of situations. He is absolutely right that you can never assume AsyncCallbacks will be handled in any order that you expect.
I handle this in a slightly different way: in my response object, I also return the original query. I can then check to see if the response still applies to the state that the client application is now in.
This would be something like:
callback = new AsyncCallback<Type>() {
#Override
public void onFailure(Throwable caught) {
// handle error
}
#Override
public void onSuccess(Type result) {
String originalQuery = result.getOriginalQuery();
if (originalQuery.equals(oracleBox.getValue())) {
// handle return values
}
}
};