Copyright 2014 Netflix, Inc.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
    http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
/**
 * Copyright 2014 Netflix, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.netflix.config;

import com.google.common.collect.Lists;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;
import java.util.concurrent.atomic.AtomicReference;

Derives a complex value from a DynamicStringProperty. 
Author:
mhawthorne/**
 * Derives a complex value from a {@link DynamicStringProperty}.
 *
 * @author mhawthorne
 */
public abstract class DerivedStringProperty<D> implements Property<D> {

    private static final Logger log = LoggerFactory.getLogger(DerivedStringProperty.class);

    private final DynamicStringProperty delegate;

    private final List<Runnable> callbackList = Lists.newArrayList();

    
Holds derived value, which may be null.
/**
     * Holds derived value, which may be null.
     */
    private final AtomicReference<D> derived = new AtomicReference<D>(null);

    public DerivedStringProperty(String name, String defaultValue) {
        delegate = DynamicPropertyFactory.getInstance().getStringProperty(name, defaultValue);
        deriveAndSet();
        Runnable callback = new Runnable() {
            public void run() {
                propertyChangedInternal();
            }
        };
        delegate.addCallback(callback);
        callbackList.add(callback);
    }

    
Fetches derived value.
/**
     * Fetches derived value.
     */
    public D get() {
        return derived.get();
    }
    
    @Override
    public D getValue() {
        return get();
    }

    @Override
    public D getDefaultValue() {
        return derive(delegate.getDefaultValue());
    }

    @Override
    public String getName() {
        return delegate.getName();
    }

    @Override
    public long getChangedTimestamp() {
        return delegate.getChangedTimestamp();
    }

    @Override
    public void addCallback(Runnable callback) {
        delegate.addCallback(callback);
        callbackList.add(callback);
    }

    
Remove all callbacks registered through this instance of property
/**
     * Remove all callbacks registered through this instance of property
     */
    @Override
    public void removeAllCallbacks() {
        for (Runnable callback: callbackList) {
            delegate.prop.removeCallback(callback);
        }
    }

    
Invoked when property is updated with a new value.  Should return the new derived value, which may be null.
/**
     * Invoked when property is updated with a new value.  Should return the new derived value, which may be null.
     */
    protected abstract D derive(String value);

    
PropertyWrapper.propertyChanged() /**
     * {@link com.netflix.config.PropertyWrapper#propertyChanged()}
     */
    protected void propertyChanged() {}

    void propertyChangedInternal() {
        deriveAndSet();
        propertyChanged();
    }

    private void deriveAndSet() {
         try {
            derived.set(derive(this.delegate.get()));
        } catch (Exception e) {
            log.error("error when deriving initial value", e);
        }
    }
}