src/main/java/com/android/apkzlib/zip/ZFileExtension.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * 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.android.apkzlib.zip;

import com.android.apkzlib.utils.IOExceptionRunnable;
import java.io.IOException;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;

/**
 * An extension of a {@link ZFile}. Extensions are notified when files are open, updated, closed and
 * when files are added or removed from the zip. These notifications are received after the zip
 * has been updated in memory for open, when files are added or removed and when the zip has been
 * updated on disk or closed.
 * <p>
 * An extension is also notified before the file is updated, allowing it to modify the file before
 * the update happens. If it does, then all extensions are notified of the changes on the zip file.
 * Because the order of the notifications is preserved, all extensions are notified in the same
 * order. For example, if two extensions E1 and E2 are registered and they both add a file at
 * update time, this would be the flow:
 * <ul>
 *     <li>E1 receives {@code beforeUpdate} notification.</li>
 *     <li>E1 adds file F1 to the zip (notifying the addition is suspended because another
 *     notification is in progress).</li>
 *     <li>E2 receives {@code beforeUpdate} notification.</li>
 *     <li>E2 adds file F2 to the zip (notifying the addition is suspended because another
 *     notification is in progress).</li>
 *     <li>E1 is notified F1 was added.</li>
 *     <li>E2 is notified F1 was added.</li>
 *     <li>E1 is notified F2 was added.</li>
 *     <li>E2 is notified F2 was added.</li>
 *     <li>(zip file is updated on disk)</li>
 *     <li>E1 is notified the zip was updated.</li>
 *     <li>E2 is notified the zip was updated.</li>
 * </ul>
 * <p>
 * An extension should not modify the zip file when notified of changes. If allowed, this would
 * break event notification order in case multiple extensions are registered with the zip file.
 * To allow performing changes to the zip file, all notification method return a
 * {@code IOExceptionRunnable} that is invoked when {@link ZFile} has finished notifying all
 * extensions.
 */
public abstract class ZFileExtension {

    /**
     * The zip file has been open and the zip's contents have been read. The default implementation
     * does nothing and returns {@code null}.
     *
     * @return an optional runnable to run when notification of all listeners has ended
     * @throws IOException failed to process the event
     */
    @Nullable
    public IOExceptionRunnable open() throws IOException {
        return null;
    }

    /**
     * The zip will be updated. This method allows the extension to register changes to the zip
     * file before the file is written. The default implementation does nothing and returns
     * {@code null}.
     * <p>
     * After this notification is received, the extension will receive further
     * {@link #added(StoredEntry, StoredEntry)} and {@link #removed(StoredEntry)} notifications if
     * it or other extensions add or remove files before update.
     * <p>
     * When no more files are updated, the {@link #entriesWritten()} notification is sent.
     *
     * @return an optional runnable to run when notification of all listeners has ended
     * @throws IOException failed to process the event
     */
    @Nullable
    public IOExceptionRunnable beforeUpdate() throws IOException {
        return null;
    }

    /**
     * This notification is sent when all entries have been written in the file but the central
     * directory and the EOCD have not yet been written. No entries should be added, removed or
     * updated during this notification. If this method forces an update of either the central
     * directory or EOCD, then this method will be invoked again for all extensions with the new
     * central directory and EOCD.
     * <p>
     * After this notification, {@link #updated()} is sent.
     *
     * @throws IOException failed to process the event
     */
    public void entriesWritten() throws IOException {
    }

    /**
     * The zip file has been updated on disk. The default implementation does nothing.
     *
     * @throws IOException failed to perform update tasks
     */
    public void updated() throws IOException {
    }

    /**
     * The zip file has been closed. Note that if {@link ZFile#close()} requires that the zip file
     * be updated (because it had in-memory changes), {@link #updated()} will be called before
     * this method. The default implementation does nothing.
     */
    public void closed() {
    }

    /**
     * A new entry has been added to the zip, possibly replacing an entry in there. The
     * default implementation does nothing and returns {@code null}.
     *
     * @param entry the entry that was added
     * @param replaced the entry that was replaced, if any
     * @return an optional runnable to run when notification of all listeners has ended
     */
    @Nullable
    public IOExceptionRunnable added(@Nonnull StoredEntry entry, @Nullable StoredEntry replaced) {
        return null;
    }

    /**
     * An entry has been removed from the zip. This method is not invoked for entries that have
     * been replaced. Those entries are notified using <em>replaced</em> in
     * {@link #added(StoredEntry, StoredEntry)}. The default implementation does nothing and
     * returns {@code null}.
     *
     * @param entry the entry that was deleted
     * @return an optional runnable to run when notification of all listeners has ended
     */
    @Nullable
    public IOExceptionRunnable removed(@Nonnull StoredEntry entry) {
        return null;
    }
}