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#region

/* -----------------------------------------------------------------------

 * <copyright file="FileSystemWatcherAdapter.cs" company="GRNet">

 * 

 * Copyright 2011-2012 GRNET S.A. All rights reserved.

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#endregion

using System.Diagnostics.Contracts;

using System.IO;

using System.Threading.Tasks;

using Pithos.Interfaces;

namespace Pithos.Core.Agents

{

    using System;

    /// <summary>

    /// Wraps a FileSystemWatcher and raises Move and child object events 

    /// </summary>

    public class FileSystemWatcherAdapter

    {

         public event FileSystemEventHandler Changed;

        public event FileSystemEventHandler Created;

        public event FileSystemEventHandler Deleted;

        public event RenamedEventHandler Renamed;

        public event MovedEventHandler Moved;

        public FileSystemWatcherAdapter(FileSystemWatcher watcher)

        {

            if (watcher==null)

                throw new ArgumentNullException("watcher");

            Contract.EndContractBlock();

            watcher.Changed += OnChangeOrCreate;

            watcher.Created += OnChangeOrCreate;

            watcher.Deleted += OnDeleted;

            watcher.Renamed += OnRename;            

        }

        private string _cachedDeletedFullPath;

        private const int PropagateDelay = 10;

        private void OnDeleted(object sender, FileSystemEventArgs e)

        {

            if (sender == null)

                throw new ArgumentNullException("sender");

            if (e.ChangeType != WatcherChangeTypes.Deleted)

                throw new ArgumentException("e");

            if (string.IsNullOrWhiteSpace(e.FullPath))

                throw new ArgumentException("e");

            Contract.Ensures(!String.IsNullOrWhiteSpace(_cachedDeletedFullPath));

            Contract.EndContractBlock();

            //Handle any previously deleted event

            PropagateCachedDeleted(sender);

            //A delete event may be an actual delete event or the first event in a move action.

            //To decide which action occured, we need to wait for the next action, so

            //we  store the file path and return .

            //A delete action will not be followed by any other event, so we need to add a watchdog 

            //that will declare a Delete action after a short amount of time

            //TODO: Moving a folder to the recycle bin results in a single delete event for the entire folder and its contents

            //      as this is actually a MOVE operation

            //Deleting by Shift+Delete results in a delete event for each file followed by the delete of the folder itself

            _cachedDeletedFullPath = e.FullPath;

            //TODO: This requires synchronization of the _cachedDeletedFullPath field

            //TODO: This creates a new task for each file even though we can cancel any existing tasks if a new event arrives

            //Maybe, use a timer instead of a task

            TaskEx.Delay(PropagateDelay).ContinueWith(t =>

                                                           {

                                                               var myPath = e.FullPath;

                                                               if (_cachedDeletedFullPath==myPath)

                                                                    PropagateCachedDeleted(sender);

                                                           });

        }

        private void OnRename(object sender, RenamedEventArgs e)

        {

            if (sender == null)

                throw new ArgumentNullException("sender");

            Contract.Ensures(_cachedDeletedFullPath == null);

            Contract.EndContractBlock();

            try

            {

                //Propagate any previous cached delete event

                PropagateCachedDeleted(sender);

                if (Renamed!=null)

                    Renamed(sender, e);

            }

            finally

            {

                _cachedDeletedFullPath = null;    

            }

        }

        private void OnChangeOrCreate(object sender, FileSystemEventArgs e)

        {

            if (sender == null)

                throw new ArgumentNullException("sender");

            if (!(e.ChangeType == WatcherChangeTypes.Created || e.ChangeType == WatcherChangeTypes.Changed))

                throw new ArgumentException("e");

            Contract.Ensures(_cachedDeletedFullPath == null);

            Contract.EndContractBlock();

            try

            {

                //A Move action results in a sequence of a Delete and a Create or Change event

                //If the actual action is a Move, raise a Move event instead of the actual event

                if (HandleMoved(sender, e))

                    return;

                //Otherwise, propagate the Delete event if it exists 

                PropagateCachedDeleted(sender);

                //and propagate the actual event

                var actualEvent = e.ChangeType == WatcherChangeTypes.Created ? Created : Changed;

                if (actualEvent != null)

                {

                    actualEvent(sender, e);

                    //For Folders, raise Created events for all children

                    RaiseCreatedForChildren(sender,e);

                }

            }

            finally

            {

                //Finally, make sure the cached path is cleared

                _cachedDeletedFullPath = null;

            }

        }

        private void RaiseCreatedForChildren(object sender, FileSystemEventArgs e)

        {

            Contract.Requires(sender!=null);

            Contract.Requires(e!=null);

            if (e.ChangeType != WatcherChangeTypes.Created)

                return;

            var dir= new DirectoryInfo(e.FullPath);

            //Skip if this is not a folder

            if (!dir.Exists)

                return;

            foreach (var info in dir.EnumerateFileSystemInfos("*",SearchOption.AllDirectories))

            {

                var path = Path.GetDirectoryName(info.FullName);

                Created(sender,new FileSystemEventArgs(WatcherChangeTypes.Created,path,info.Name));

            }            

        }

        private bool HandleMoved(object sender, FileSystemEventArgs e)

        {

            if (sender == null)

                throw new ArgumentNullException("sender");

            if (!(e.ChangeType == WatcherChangeTypes.Created ||

                                                  e.ChangeType == WatcherChangeTypes.Changed))

                throw new ArgumentException("e");

            Contract.EndContractBlock();

            //TODO: If a file is deleted and another file with the same name is created, it will be detected as a MOVE

            //instead of a sequence of independent actions

            //One way to detect this would be to request that the full paths are NOT the same

            var oldName = Path.GetFileName(_cachedDeletedFullPath);

            //NOTE: e.Name is a path relative to the watched path. We MUST call Path.GetFileName to get the actual path

            var newName = Path.GetFileName(e.Name);

            //If the last deleted filename is equal to the current and the action is create, we have a MOVE operation

            var hasMoved = (_cachedDeletedFullPath != e.FullPath && oldName == newName);

            if (!hasMoved)

                return false;

            try

            {

                //If the actual action is a Move, raise a Move event instead of the actual event

                var newDirectory = Path.GetDirectoryName(e.FullPath);

                var oldDirectory = Path.GetDirectoryName(_cachedDeletedFullPath);

                if (Moved != null)

                {

                    Moved(sender, new MovedEventArgs(newDirectory, newName, oldDirectory, oldName));

                    //If the moved item is a dictionary, we need to raise a change event for each child item

                    //When a directory is moved within the same volume, Windows raises events only for the directory object,

                    //not its children. This happens because the move actually changes a single directory entry. It doesn't

                    //affect the entries of the children.

                    var directory = new DirectoryInfo(e.FullPath);

                    if (directory.Exists)

                    {

                        foreach (var child in directory.EnumerateFileSystemInfos("*", SearchOption.AllDirectories))

                        {

                            var newChildDirectory = Path.GetDirectoryName(child.FullName);

                            var relativePath=child.AsRelativeTo(newDirectory);

                            var relativeFolder = Path.GetDirectoryName(relativePath);

                            var oldChildDirectory = Path.Combine(oldDirectory, relativeFolder);

                            Moved(sender,new MovedEventArgs(newChildDirectory,child.Name,oldChildDirectory,child.Name));

                        }

                    }

                }

            }

            finally

            {

                _cachedDeletedFullPath = null;

            }

            return true;

        }

        private void PropagateCachedDeleted(object sender)

        {

            if (sender == null)

                throw new ArgumentNullException("sender");

            Contract.Ensures(_cachedDeletedFullPath == null);

            Contract.EndContractBlock();

            //Nothing to handle if there is no cached deleted file

            if (String.IsNullOrWhiteSpace(_cachedDeletedFullPath))

                return;

            var deletedFileName = Path.GetFileName(_cachedDeletedFullPath);

            var deletedFileDirectory = Path.GetDirectoryName(_cachedDeletedFullPath);

            //Only a single file Delete event is raised when moving a file to the Recycle Bin, as this is actually a MOVE operation

            //In this case we need to raise the proper events for all child objects of the deleted directory.

            //UNFORTUNATELY, this can't be detected here, eg. by retrieving the child objects, because they are already deleted

            //This should be done at a higher level, eg by checking the stored state

            if (Deleted != null)            

                Deleted(sender,new FileSystemEventArgs(WatcherChangeTypes.Deleted, deletedFileDirectory, deletedFileName));

            _cachedDeletedFullPath = null;

        }

    }

}