Event.java

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package org.dlect.events;

import com.google.common.base.Objects;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;

import static org.dlect.helper.Conditions.checkNonNull;

/**
 *
 * @author lee
 */
public class Event {

    @Nonnull
    private final Object source;
    @Nonnull
    private final EventID eventID;
    @Nullable
    private final Object before;
    @Nullable
    private final Object after;

    /**
     * Creates a new event with the given source, eventID, before and after states.
     *
     * @param source  A non-null source that is assignable from the given {@code eventID}'s
     *                {@link EventID#getAppliedClass()}.
     * @param eventID A non-null eventID that must represent an event that is applied to a super-type of the given
     *                {@code source}.
     * @param before  The state before the change. This object is not checked for equality with {@code after}; or
     *                null-ness. It is recommended that this object be immutable as no copy is made.
     * @param after   The state after the change. This object is not checked for equality with {@code before}; or
     *                null-ness. It is recommended that this object be immutable as no copy is made.
     */
    public Event(@Nonnull Object source, @Nonnull EventID eventID, @Nullable Object before, @Nullable Object after) {
        checkNonNull(source, "Source");
        checkNonNull(eventID, "Event ID");
        if (!eventID.getAppliedClass().isAssignableFrom(source.getClass())) {
            // If the event ID represents a supertype of or equal to source class; then good. Otherwise error.
            throw new IllegalArgumentException("Source(" + source.getClass() + ") is not a sub-type of the class that the event ID(" + eventID.getAppliedClass() + ") applies to.");
        }
        this.source = source;
        this.eventID = eventID;
        this.before = before;
        this.after = after;
    }

    public String debugVars() {
        return "source=" + source + ", eventID=" + eventID + ", before=" + before + ", after=" + after;
    }

    @Override
    public String toString() {
        return "Event{" + debugVars() + '}';
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(this.getSource(),
                                this.getEventID(),
                                this.getBefore(),
                                this.getAfter());
    }

    @Override
    public boolean equals(Object obj) {
        if (obj != null && obj.getClass().equals(this.getClass())) {
            final Event other = (Event) obj;
            return equalsImpl(other);
        } else {
            return false;
        }
    }

    /**
     * Perform an implementer independent equality check. This method will only check for equality with the elements of
     * this class. This method does not perform any type checking and is safe to use in an implementation to check if
     * the root event is equal.
     *
     * This method will check equality on {@link #getBefore()}, {@link #getAfter()}, {@link #getEventID()}, and
     * {@link #getSource()}
     *
     * @param other The event to check equality on.
     *
     * @return {@code true} if this event has the same event data as {@code other}. Otherwise {@code false}
     */
    protected final boolean equalsImpl(Event other) {
        return Objects.equal(this.getSource(), other.getSource())
               && Objects.equal(this.getEventID(), other.getEventID())
               && Objects.equal(this.getBefore(), other.getBefore())
               && Objects.equal(this.getAfter(), other.getAfter());
    }

    /**
     * The source class that this event represents. This is generated by calling {@link EventID#getAppliedClass() }.
     *
     * @return The source class that this event represents.
     */
    @Nonnull
    public Class<?> getSourceClass() {
        return eventID.getAppliedClass();
    }

    /**
     * The source object of this event. Note that this source is guaranteed to be castable to, but not necessarily the
     * same as, the class returned by {@link #getSourceClass()}
     *
     * @return The source of the object.
     */
    @Nonnull
    public Object getSource() {
        return source;
    }

    /**
     * The ID of the event.
     *
     * @return The ID of event.
     */
    @Nonnull
    public EventID getEventID() {
        return eventID;
    }

    /**
     * The object before it was changed.This may be equal to {@link #getAfter() }; this class makes no assumptions on
     * this.
     *
     * @return The object before it was changed.
     */
    @Nullable
    public Object getBefore() {
        return before;
    }

    /**
     * The object after the change. This may be equal to {@link #getBefore() }; this class makes no assumptions on this.
     *
     * @return The object after the change.
     */
    @Nullable
    public Object getAfter() {
        return after;
    }

}