GWT Fileupload, what event or handler when user click the choose file button - google-app-engine

Question

I am new to GWT/GAE.
To use GAE Blobstore service, a upload url must be returned from server side, as the tutorial in this blog .
But it seems this get upload url service can be called while user is browsing file, thus I need to find the event to handle when user press the choose file button, and set the formpanel's action attribute there.
fileupload itself has attach/detach or change event, but it seems not what i am looking for.
My code which can do upload to blobstore is as follow:
The upload form is:
import java.util.Arrays;
import java.util.Date;
import java.util.List;
import com.google.gwt.cell.client.DateCell;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.FileUpload;
import com.google.gwt.user.client.ui.FormPanel;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.VerticalPanel;
import com.google.gwt.user.client.ui.FormPanel.SubmitCompleteHandler;
import com.google.gwt.user.client.ui.FormPanel.SubmitCompleteEvent;
import com.google.gwt.user.client.ui.FlowPanel;
import com.google.gwt.user.client.ui.TextArea;
public class UploadFilePanel implements EntryPoint {
private FileServiceAsync upls = GWT.create(FileService.class);
AsyncCallback<String> callback;
FormPanel form;
String upload_url;
public void onModuleLoad() {
get_upload_url();
RootPanel rootPanel = RootPanel.get();
FlowPanel flowPanel = new FlowPanel();
rootPanel.add(flowPanel, 0, 0);
add_submit_form(flowPanel);
}
private void add_submit_form(FlowPanel flowPanel) {
VerticalPanel panel = new VerticalPanel();
flowPanel.add(panel);
// Create a FormPanel and point it at a service.
form = new FormPanel();
panel.add(form);
form.setEncoding(FormPanel.ENCODING_MULTIPART);
form.setMethod(FormPanel.METHOD_POST);
form.addSubmitCompleteHandler(new SubmitCompleteHandler() {
#Override
public void onSubmitComplete(SubmitCompleteEvent event) {
RootPanel.get().clear();
RootPanel.get().getElement()
.setInnerHTML("upload complete " + event.toString());
}
});
form.setSize("222px", "52px");
// Create a FileUpload widget.
FileUpload upload = new FileUpload();
form.setWidget(upload);
upload.setSize("100%", "100%");
upload.setName("myFile");
TextArea textArea = new TextArea();
panel.add(textArea);
textArea.setWidth("100%");
// Add a 'submit' button.
Button submitbtn = new Button("Submit");
panel.add(submitbtn);
submitbtn.addClickHandler(new ClickHandler() {
#Override
public void onClick(ClickEvent event) {
upls.get_upload_url("/stmtgen/upload", callback);
}
});
}
private void get_upload_url() {
callback = new AsyncCallback<String>() {
#Override
public void onSuccess(String result) {
form.setAction(result);
upload_url = result;
form.submit();
}
#Override
public void onFailure(Throwable caught) {
RootPanel.get().clear();
RootPanel.get().getElement()
.setInnerHTML("Can't get upload url");
}
};
if (upls == null)
upls = GWT.create(FileService.class);
}
}
The Async service I used to get upload url is this:
import com.google.gwt.user.client.rpc.AsyncCallback;
public interface FileServiceAsync {
void get_upload_url(String up, AsyncCallback<String> callback);
}
The problem is when user hit submit button, there is two services invocation which slow down the response.
I tried addHandler(Handler, Type) as below, it doesn't work.
FileUpload upload = new FileUpload();
upload.addHandler(new ClickHandler() {
#Override
public void onClick(ClickEvent event) {
//get upload url here
}
}, ClickEvent.getType()
);