How to use

DiscardSourceCost

in

mage.abilities.costs.common

  @Override
  public DiscardSourceCost copy() {
    return new DiscardSourceCost(this);
  }
}

public ChannelAbility(String manaString, Effect effect, TimingRule timing) {
  super(Zone.HAND, effect, new ManaCostsImpl(manaString));
  this.addCost(new DiscardSourceCost());
  this.timing = timing;
}

public TransmuteAbility(String manaCost) {
  super(Zone.HAND, new TransmuteEffect(), new ManaCostsImpl(manaCost));
  this.setTiming(TimingRule.SORCERY);
  this.addCost(new DiscardSourceCost());
}

public BloodrushAbility(String manaString, Effect effect) {
  super(Zone.HAND, effect, new ManaCostsImpl(manaString));
  this.addCost(new DiscardSourceCost());
  this.addTarget(new TargetAttackingCreature());
}

public ReinforceAbility(DynamicValue count, Cost cost) {
  super(Zone.HAND, new AddCountersTargetEffect(CounterType.P1P1.createInstance(0), count), cost);
  this.addCost(new DiscardSourceCost());
  this.addTarget(new TargetCreaturePermanent());
  this.cost = cost.copy();
  this.count = count;
}

public FaerieMacabre(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId, setInfo, new CardType[]{CardType.CREATURE}, "{1}{B}{B}");
  this.subtype.add(SubType.FAERIE);
  this.subtype.add(SubType.ROGUE);
  this.power = new MageInt(2);
  this.toughness = new MageInt(2);
  // Flying
  this.addAbility(FlyingAbility.getInstance());
  // Discard Faerie Macabre: Exile up to two target cards from graveyards.
  Ability ability = new SimpleActivatedAbility(Zone.HAND, new ExileTargetEffect(), new DiscardSourceCost());
  ability.addTarget(new TargetCardInGraveyard(0, 2, new FilterCard("cards from graveyards")));
  this.addAbility(ability);
}