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/*
 * #%L
 * fujion
 * %%
 * Copyright (C) 2018 Fujion Framework
 * %%
 * 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.
 *
 * #L%
 */
package org.fujion.client;

import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;
import java.util.concurrent.atomic.AtomicInteger;

import org.fujion.common.Logger;
import org.fujion.ancillary.IResponseCallback;
import org.fujion.component.Page;
import org.fujion.event.CallbackEvent;
import org.fujion.event.ITypedEventListener;

/**
 * A registry for managing pending callbacks. There is timestamp-based eviction logic for removing
 * orphaned entries.
 */
public class CallbackRegistry {
    
    private static final Logger log = Logger.create(CallbackRegistry.class);
    
    private static final long HALF_HOUR = 1000 * 60 * 30;

    private final Map> callbacks = Collections.synchronizedMap(new HashMap<>());
    
    private final Map timestamps = Collections.synchronizedSortedMap(new TreeMap<>());
    
    private final AtomicInteger callbackId = new AtomicInteger();
    
    private final long evictionInterval;
    
    private final ITypedEventListener callbackListener = (event) -> {
        invokeCallback(event.getHandle(), event.getData());
    };
    
    /**
     * Creates a callback registry for the specified page using the default eviction interval (30
     * minutes).
     *
     * @param page The owning page.
     */
    public CallbackRegistry(Page page) {
        this(page, HALF_HOUR);
    }
    
    /**
     * Creates a callback registry for the specified page using the specified eviction interval.
     *
     * @param page The owning page.
     * @param evictionInterval The time in milliseconds after which an entry will be evicted.
     */
    public CallbackRegistry(Page page, long evictionInterval) {
        this.evictionInterval = evictionInterval;
        page.addEventListener(CallbackEvent.class, callbackListener);
    }
    
    /**
     * Registers a callback.
     *
     * @param callback Callback to register.
     * @return A unique handle.
     */
    public int registerCallback(IResponseCallback callback) {
        int handle = callbackId.incrementAndGet();
        callbacks.put(handle, callback);
        timestamps.put(handle, System.currentTimeMillis());
        return handle;
    }
    
    /**
     * Unregisters a callback.
     *
     * @param handle The callback's handle.
     * @return The callback that was unregistered (may be null).
     */
    private IResponseCallback unregisterCallback(int handle) {
        timestamps.remove(handle);
        IResponseCallback callback = callbacks.remove(handle);
        evict();
        return callback;
    }

    /**
     * Unregisters and invokes a callback.
     *
     * @param handle The callback's handle.
     * @param response The response to pass to the callback.
     */
    private void invokeCallback(int handle, Object response) {
        @SuppressWarnings("unchecked")
        IResponseCallback