box2d - Libgdx body passing through block then fall on it

I'm using libgdx and its physic engine Box2d. My question is : how could I make my box2d body go through a block , ignore the first collison than make the block active so the ball can fall onto it. A good example is doodle jump.

Answer

I finally made it :)

Here's my code :

public void preSolve(Contact contact, Manifold oldManifold) {
        fixtureA = contact.getFixtureA();
        fixtureB = contact.getFixtureB();
          if(fixtureA.getBody().getUserData() == "platform" && fixtureB.getBody().getUserData() == "ball"|| fixtureA.getBody().getUserData() == "ball" && fixtureB.getBody().getUserData() == "platform"){
              if (fixtureA.getBody().getUserData() == "platform") { 
                  platform_y = fixtureA.getBody().getPosition().y;
                  ball_y = fixtureB.getBody().getPosition().y;
              } else if(fixtureA.getBody().getUserData() == "ball") {
                  ball_y = fixtureA.getBody().getPosition().y;

                  platform_y = fixtureB.getBody().getPosition().y;
              }
                 if(ball_y < platform_y + 1.5F) {             //the ball is below
                     contact.setEnabled(false);
                 }
          }

    }