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/*
 * Copyright (c) 2022, 2023, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.javafx.binding;

import java.util.Objects;

import javafx.beans.value.ObservableValue;
import javafx.util.Subscription;

public class ConditionalBinding extends LazyObjectBinding {

    private final ObservableValue source;
    private final ObservableValue nonNullCondition;

    private Subscription subscription;

    public ConditionalBinding(ObservableValue source, ObservableValue condition) {
        this.source = Objects.requireNonNull(source, "source cannot be null");
        this.nonNullCondition = Objects.requireNonNull(condition, "condition cannot be null").orElse(false);

        // condition is always observed and never unsubscribed
        nonNullCondition.subscribe(this::conditionChanged);
    }

    private void conditionChanged(boolean active) {
        if (!active && !isValid()) {
            getValue();  // makes binding valid, which it should always be when inactive
        }
        else if (isValid() && source.getValue() != getValue()) {
            invalidate();
        }

        updateSubscription();
    }

    /**
     * This binding is valid whenever it is observed, or it is currently inactive.
     * When inactive, the binding has the value of its source at the time it became
     * inactive.
     */
    @Override
    protected boolean allowValidation() {
        return super.allowValidation() || !isActive();
    }

    @Override
    protected T computeValue() {
        updateSubscription();

        return source.getValue();
    }

    private void updateSubscription() {
        if (isObserved() && isActive()) {
            if (subscription == null) {
                subscription = source.subscribe(this::invalidate);
            }
        }
        else {
            unsubscribe();
        }
    }

    @Override
    protected Subscription observeSources() {
        return this::unsubscribe;
    }

    private boolean isActive() {
        return nonNullCondition.getValue();
    }

    private void unsubscribe() {
        if (subscription != null) {
            subscription.unsubscribe();
            subscription = null;
        }
    }
}