vaadin-framework/documentation/articles/DynamicallyUpdatingStateBeforeSendingChangesToClient.asciidoc

---
title: Dynamically Updating State Before Sending Changes To Client
order: 75
layout: page
---

[[dynamically-updating-state-before-sending-changes-to-client]]
Dynamically updating state before sending changes to client
-----------------------------------------------------------

There are some cases where a server-side implementation must delay some
work until right before data is about to be sent to the client. Some
examples of this:

* An expensive operation that should be done only once and not every
time some input for the calculation changes.
* Anything that depends on the component (or extension) being attached
to the component hierarchy.

Vaadin provides the `ClientConnector.beforeClientResponse(boolean
initial)` method, which a server-side component or extension can override
if it wants to make some final adjustments to its shared state or send
some RPC right before data is being sent to the client. Because the
method is called just before the data will be sent, there are some
special considerations:

* You should remember to call `super.beforeClientResponse(initial)`
because e.g. `AbstractComponent` relies on the method for performing its
own last minute changes to the state.
* The component hierarchy may not be modified in the
`beforeClientResponse` method, doing so might cause undesirable side
effects.
* `markAsDirty()` has no effect - changes will only be sent for connectors
that were marked as dirty before `beforeClientResponse` was called.

Please note that `beforeClientResponse` will only be called for components
that the framework thinks might have changes, e.g. because they have
recently been attached, their `getState()` method has been called or they
have been marked as dirty using `markAsDirty()`.

This shows a simple example where two terms are summed together only
once even if the terms are changed multiple times before a response is
sent to the client.

[source,java]
....
public class Addition extends AbstractComponent {
  private int term1;
  private int term2;
  private boolean needsRecalculation = false;

  public void setTerm1(int value1) {
    this.term1 = value1;
    needsRecalculation = true;

    //Mark the component as dirty to ensure beforeClientResponse will be invoked
    markAsDirty();
  }

  public void setTerm2(int value2) {
    this.term2 = value2;
    needsRecalculation = true;

    //Mark the component as dirty to ensure beforeClientResponse will be invoked
    markAsDirty();
  }

  private int calculateSum() {
    return term1 + term2;
  }

  @Override
  public void beforeClientResponse(boolean initial) {
    super.beforeClientResponse(initial);
    if (needsRecalculation) {
      needsRecalculation = false;
      // This could be an expensive operation that we don't want to do every time setTerm1 or setTerm2 is invoked.
      getState().sum = calculateSum();
    }
  }

  @Override
  protected AddResultState getState() {
    return (AddResultState) super.getState();
  }
}

class AddResultState extends ComponentState {
  public int sum;
}
....