How to kill a GWT RPC which has not yet completed - gwt

Question

My code is for sending Emails to multiple users.User will click on send button,and rpc will be called. Now if user clicks on Cancel button .Ongoing rpc should be cancelled. . Can anyone help ?
I googled a lot, they have introduced the concept of Request Builder. But I am not getting any perfect idea.

Answer 1

Make your async method return a Request instead of void so you can call cancel() on it.
For the same reason, asynchronous methods do not have return types; they generally return void. Should you wish to have more control over the state of a pending request, return Request instead.
— Source: https://developers.google.com/web-toolkit/doc/latest/DevGuideServerCommunication#DevGuideCreatingServices
FYI, you can also use RequestBuilder as the return type, you'll then have to call the send() method by yourself (after possibly customizing the request, e.g. adding headers) to actually make the request to the server.
And of course, if you need to tell the server to abort the processing, you'll have to make another RPC call.

Answer 2

The request is asynch, so the client side can do anything it wants.
All you need to do is add a flag to indicate that the request should be cancelled, and then change the onSuccess method to check the flag and do nothing if it is set.
You should clear the requestCancelled flag each time you make a request - or else after the first request is cancelled, you won't be able to make another one...
e.g.
boolean requestCancelled = false;
void onSuccess(...)
{
if (!requestCancelled) {
// actual response handing code
}
}
If you really want to cancel the request on the server side, it is a lot more complicated. You could do this by sending a second request - one where the fuinctionality is to cancel a request.
To make this work, the "cancel request" has to set a field somewhere the "email request" can read. The "email request" needs to check if the "cancel field" has been set.
// server side Impl
void cancelRequest()
{
// You need to implement this class and ensure it really is a singleton
// and thread safe.
RequestStatusSingleton.setCancelled(true);
}
void serverSideEmailFunc()
{
while(modeEmailAddrs && ! RequestStatusSingleton.getCancelled()) {
// get next address and send email
}
}
Obviously this is a lot of work. Have you considered:
Not having a cancel button on your GUI?
Getting the server to process emails a few at a time (i.e. client sends multiple requests until server tells the client all emails are done).

Answer 3

I totally understand your user. No one wants to wait for 15 seconds.
There is no standard way to "kill" the request, because there is no way to know where your server/datastore is in implementing it. Unless you deal with a process that can be put in a single transaction that can be rolled back, you will have to implement your own logic. For example, if you asked the server to save an entity, you will have to tell the server to save this entity again, but this time without the changes.
Also, think again about your use case. Why a user wants to kill the request? May be he simply wants to go to another place in the app. Then there is no need to kill the request: when the response arrives, check if the user is still in the same place patiently waiting. If not, do not execute onSuccess().