GWT, how to make EntryPoint class wait for AsyncCallBack

I am attempting to convert an android app to a website using GWT. This was just an idea for a future-project.

What I want to do is mimic the Cursor function. I can do that just fine creating a class and just mimicking the functions I want to use.

But here is my issue:
My UI finishes its code before the Cursor is fully initialized.

Here's how it works so far. I have the following code in my onModuleLoad, in my entrypoint:

Cursor cursor = databaseHelper.retrieveData("select * from '0-1'");
	label.setText("Column-count: "+cursor.getColumnCount());

This outputs 0, because the entrypoint does not wait for my cursor-class to fully initialize.

My retrieveData method is here:

Cursor retrieveData(String query){
    return new Cursor(query);
}

And my constructor in the Cursor-class is here

Cursor(String sql) {
	greetingService = GWT.create(GreetingService.class);
	columns = new ArrayList<>();
	rows = new ArrayList<>();
	currentPosition = -1;
	
    greetingService.greetServer(sql, new AsyncCallback<List<List<String>>>() {
		@Override
		public void onFailure(Throwable caught) {}
		
		@Override
		public void onSuccess(List<List<String>> result) {
	    	if (!result.isEmpty()) {
				columns = result.get(0);
				if (result.size() > 1) {
					rows.addAll(result.subList(1, result.size()));
				}
			}
		}
	});
}

I have tried stopping the UI-activity with a while-loop waiting for a boolean to go from true to false (set to false in the onSuccess of the AsyncCallback), but the UI-activity is just locked and the site crashes.

Does somebody have a solution here?

Use Promises.

If you are making a HTTP request to a backend service, you could use the Fetch API, which uses promises, though convenient rather than necessary.

GWT's Elemental2 library provides bindings to the promises and fetch API's. For example:

DomGlobal.fetch("http://example.com/bar?x=5")
.then(response -> response.json())
.then(jsonData -> {
    parseDataAndUpdateUI(jsonData);
    return null;
});

For doing those server calls you can also use domino-rest which already have an implementation for android domino-rest-android and to make several calls and wait for them to finish you could try using domino-aggregator

with domino-aggregator you can do something like the following

ContextAggregator.ContextWait<String> waitForString = ContextAggregator.ContextWait.create();
ContextAggregator.ContextWait<Integer> waitForInteger = ContextAggregator.ContextWait.create();

ContextAggregator.waitFor(waitForString, waitForInteger)
        .onReady(() -> {
            //will be called when both waits are resolved.
        });

waitForString.complete("some string");
waitForInteger.complete(5);

so simply you could make a waiting context for the server call, meanwhile move the initialization of the UI into the onReady method.

Both answers before point out the solution to your problem. And they show two more - and I think modern - ways to get data from the server.

Your problem with the following code is:

Cursor cursor = databaseHelper.retrieveData("select * from '0-1'");
	label.setText("Column-count: "+cursor.getColumnCount());

that 'retrieveData' calls the server but does not wait until the server returns. That's the nature of asynchronous calls.

Move 'label.setText("Column-count: "+cursor.getColumnCount());' into the 'onSuccess'-method and it will work.

Cursor(String sql) {
    greetingService = GWT.create(GreetingService.class);
    columns = new ArrayList<>();
    rows = new ArrayList<>();
    currentPosition = -1;
    
    greetingService.greetServer(sql, new AsyncCallback<List<List<String>>>() {
        @Override
        public void onFailure(Throwable caught) {}
        
        @Override
        public void onSuccess(List<List<String>> result) {
            if (!result.isEmpty()) {
                columns = result.get(0);
                if (result.size() > 1) {
                    rows.addAll(result.subList(1, result.size()));
                }
            }
            label.setText("Column-count: "+cursor.getColumnCount());
        }
    });
}