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CSS integration through fernice-flare in Java Swing as a LookAndFeel

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/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

package org.fernice.reflare.util

import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty

object Observables {

    inline fun  observable(initialValue: T, crossinline onChange: (property: KProperty<*>, oldValue: T, newValue: T) -> Unit): ReadWriteProperty {
        return object : ObservableProperty(initialValue) {
            override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
                onChange(property, oldValue, newValue)
            }
        }
    }
}

abstract class ObservableProperty(initialValue: T) : ReadWriteProperty {
    private var value = initialValue

    /**
     *  The callback which is called before a change to the property value is attempted.
     *  The value of the property hasn't been changed yet, when this callback is invoked.
     *  If the callback returns `true` the value of the property is being set to the new value,
     *  and if the callback returns `false` the new value is discarded and the property remains its old value.
     */
    protected open fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T): Boolean = true

    /**
     * The callback which is called after the change of the property is made. The value of the property
     * has already been changed when this callback is invoked.
     */
    protected open fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {}

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        return value
    }

    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        val oldValue = this.value
        if (oldValue != value) {
            if (!beforeChange(property, oldValue, value)) {
                return
            }
            this.value = value
            afterChange(property, oldValue, value)
        }
    }
}