diff --git a/FileProvider.podspec b/FileProvider.podspec
index b8344a8..7ffe1fb 100644
--- a/FileProvider.podspec
+++ b/FileProvider.podspec
@@ -16,7 +16,7 @@ Pod::Spec.new do |s|
   #
 
   s.name         = "FileProvider"
-  s.version      = "0.11.4"
+  s.version      = "0.12.0"
   s.summary      = "FileManager replacement for Local and Remote (WebDAV/Dropbox/OneDrive/SMB2) files on iOS and macOS."
 
   # This description is used to generate tags and improve search results.
diff --git a/FileProvider.xcodeproj/project.pbxproj b/FileProvider.xcodeproj/project.pbxproj
index a0c56c8..e5a4db2 100644
--- a/FileProvider.xcodeproj/project.pbxproj
+++ b/FileProvider.xcodeproj/project.pbxproj
@@ -603,7 +603,7 @@
 		799396601D48B7BF00086753 /* Debug */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
-				BUNDLE_VERSION_STRING = 0.11.4;
+				BUNDLE_VERSION_STRING = 0.12.0;
 				CLANG_WARN_BOOL_CONVERSION = YES;
 				CLANG_WARN_CONSTANT_CONVERSION = YES;
 				CLANG_WARN_EMPTY_BODY = YES;
@@ -633,7 +633,7 @@
 		799396611D48B7BF00086753 /* Release */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
-				BUNDLE_VERSION_STRING = 0.11.4;
+				BUNDLE_VERSION_STRING = 0.12.0;
 				CLANG_WARN_BOOL_CONVERSION = YES;
 				CLANG_WARN_CONSTANT_CONVERSION = YES;
 				CLANG_WARN_EMPTY_BODY = YES;
diff --git a/README.md b/README.md
index accbf62..287b172 100644
--- a/README.md
+++ b/README.md
@@ -7,14 +7,14 @@
 [![License][license-image]][license-url]
 
 [![Release versin][release-image]][release-url]
-[![CocoaPods version](https://img.shields.io/cocoapods/v/FileProvider.svg)](cocoapods)
+[![CocoaPods version](https://img.shields.io/cocoapods/v/FileProvider.svg)][cocoapods]
 [![Carthage compatible][carthage-image]](https://github.com/Carthage/Carthage)
 
 [![Build Status][travis-image]][travis-url]
 [![Codebeat Badge][codebeat-image]][codebeat-url]
 [![Cocoapods Docs][docs-image]](docs-url)
-[![Cocoapods Downloads][cocoapods-downloads]](cocoapods)
-[![Cocoapods Apps][cocoapods-apps]](cocoapods)
+[![Cocoapods Downloads][cocoapods-downloads]][cocoapods]
+[![Cocoapods Apps][cocoapods-apps]][cocoapods]
 
 <!--- 
 [![codecov](https://codecov.io/gh/amosavian/FileProvider/branch/master/graph/badge.svg)](https://codecov.io/gh/amosavian/FileProvider) 
@@ -31,13 +31,13 @@ Local and WebDAV providers are fully tested and can be used in production enviro
 - [x] **LocalFileProvider** a wrapper around `FileManager` with some additions like searching and reading a portion of file.
 - [x] **WebDAVFileProvider** WebDAV protocol is defacto file transmission standard, replaced FTP.
 - [x] **DropboxFileProvider** A wrapper around Dropbox Web API.
-* For now it has limitation in uploading files up to 150MB.
+	* For now it has limitation in uploading files up to 150MB.
 - [x] **OneDriveFileProvider** A wrapper around OneDrive Web API, works with `onedrive.com` and compatible (business) servers.
 * For now it has limitation in uploading files up to 100MB.
 - [x] **CloudFileProvider** A wrapper around app's ubiquitous container to iCloud Drive in iOS 8+ API.
 - [ ] **SMBFileProvider** SMB2/3 introduced in 2006, which is a file and printer sharing protocol originated from Microsoft Windows and now is replacing AFP protocol on MacOS.
-* Data types and some basic functions are implemented but *main interface is not implemented yet!*
-*  SMB1/CIFS is depericated and very tricky to be implemented
+	* Data types and some basic functions are implemented but *main interface is not implemented yet!*
+	*  SMB1/CIFS is depericated and very tricky to be implemented
 - [ ] **FTPFileProvider** while deprecated in 1990s, it's still in use on some Web hosts.
 - [ ] **AmazonS3FileProvider** 
 - [ ] **GoogleDriveFileProvider**
diff --git a/Sources/CloudFileProvider.swift b/Sources/CloudFileProvider.swift
index 4e879f0..683deb8 100644
--- a/Sources/CloudFileProvider.swift
+++ b/Sources/CloudFileProvider.swift
@@ -366,7 +366,9 @@ open class CloudFileProvider: LocalFileProvider {
         return file
     }
     
-    /// Removes local copy of file, but spares cloud copy
+    /// Removes local copy of file, but spares cloud copy/
+    /// - Parameter path: Path of file or directory to be remoed from local
+    /// - Parameter completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
     open func evictItem(path: String, completionHandler: SimpleCompletionHandler) {
         operation_queue.addOperation {
             do {
@@ -378,6 +380,7 @@ open class CloudFileProvider: LocalFileProvider {
         }
     }
     
+    /// Returns a pulic url with expiration date, can be shared with other people.
     open func temporaryLink(to path: String, completionHandler: @escaping ((_ link: URL?, _ attribute: FileObject?, _ expiration: Date?, _ error: Error?) -> Void)) {
         operation_queue.addOperation {
             do {
diff --git a/Sources/DropboxFileProvider.swift b/Sources/DropboxFileProvider.swift
index ef9ca4e..a7c8590 100644
--- a/Sources/DropboxFileProvider.swift
+++ b/Sources/DropboxFileProvider.swift
@@ -231,6 +231,10 @@ extension DropboxFileProvider: FileProviderOperations {
 
 extension DropboxFileProvider: FileProviderReadWrite {
     public func contents(path: String, offset: Int64, length: Int, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle? {
+        if length == 0 {
+            return nil
+        }
+        
         let opType = FileOperationType.fetch(path: path)
         let url = URL(string: "files/download", relativeTo: contentURL)!
         var request = URLRequest(url: url)
diff --git a/Sources/ExtendedLocalFileProvider.swift b/Sources/ExtendedLocalFileProvider.swift
index a51c086..678116e 100644
--- a/Sources/ExtendedLocalFileProvider.swift
+++ b/Sources/ExtendedLocalFileProvider.swift
@@ -61,6 +61,7 @@ extension LocalFileProvider: ExtendedFileProvider {
     }
     
     public func thumbnailOfFile(path: String, dimension: CGSize? = nil, completionHandler: @escaping ((_ image: ImageClass?, _ error: Error?) -> Void)) {
+        let dimension = dimension ?? CGSize(width: 64, height: 64)
         (dispatch_queue).async {
             var thumbnailImage: ImageClass? = nil
             // Check cache
@@ -85,7 +86,7 @@ extension LocalFileProvider: ExtendedFileProvider {
             }
             
             if let image = thumbnailImage {
-                let scaledImage = dimension != nil ? LocalFileProvider.scaleDown(image: image, toSize: dimension!) : image
+                let scaledImage = LocalFileProvider.scaleDown(image: image, toSize: dimension)
                 completionHandler(scaledImage, nil)
             }
         }
diff --git a/Sources/FileObject.swift b/Sources/FileObject.swift
index b6638c8..b3ba257 100644
--- a/Sources/FileObject.swift
+++ b/Sources/FileObject.swift
@@ -10,6 +10,7 @@ import Foundation
 
 /// Containts path and attributes of a file or resource.
 open class FileObject {
+    /// A `Dictionary` contains file information,  using `URLResourceKey` keys.
     open internal(set) var allValues: [String: Any]
     
     internal init(allValues: [String: Any]) {
@@ -215,13 +216,19 @@ public struct FileObjectSorting {
     public static let createdAscending = FileObjectSorting(type: .creationDate, ascending: true)
     public static let createdDesceding = FileObjectSorting(type: .creationDate, ascending: false)
     
+    /// Initializes a `FileObjectSorting` allows to sort an `Array` of `FileObject`.
+    ///
+    /// - Parameters:
+    ///   - type: Determines to sort based on which file property.
+    ///   - ascending: `true` of resulting `Array` is ascending
+    ///   - isDirectoriesFirst: Puts directoris on the top of resulting `Array`.
     public init (type: SortType, ascending: Bool = true, isDirectoriesFirst: Bool = false) {
         self.sortType = type
         self.ascending = ascending
         self.isDirectoriesFirst = isDirectoriesFirst
     }
     
-    /// Sorts array of FileObjects by criterias set in properties
+    /// Sorts array of `FileObject`s by criterias set in properties
     public func sort(_ files: [FileObject]) -> [FileObject] {
         return files.sorted {
             if isDirectoriesFirst {
@@ -259,11 +266,13 @@ public struct FileObjectSorting {
 }
 
 extension Array where Element: FileObject {
+    /// Returns a sorted array of `FileObject`s by criterias set in properties.
     public func sorted(by type: FileObjectSorting.SortType, ascending: Bool = true, isDirectoriesFirst: Bool = false) -> [Element] {
         let sorting = FileObjectSorting(type: type, ascending: ascending, isDirectoriesFirst: isDirectoriesFirst)
         return sorting.sort(self) as! [Element]
     }
     
+    /// Sorts array of `FileObject`s by criterias set in properties
     public mutating func sorted(by type: FileObjectSorting.SortType, ascending: Bool = true, isDirectoriesFirst: Bool = false) {
         self = self.sorted(by: type, ascending: ascending, isDirectoriesFirst: isDirectoriesFirst)
     }
diff --git a/Sources/FileProvider.swift b/Sources/FileProvider.swift
index e83b8f0..a0dc4e7 100644
--- a/Sources/FileProvider.swift
+++ b/Sources/FileProvider.swift
@@ -18,25 +18,89 @@ public typealias ImageClass = NSImage
 public typealias SimpleCompletionHandler = ((_ error: Error?) -> Void)?
 
 public protocol FileProviderBasic: class {
+    /// An string to identify type of provider.
     static var type: String { get }
+    
+    /// An string to identify type of provider.
     var type: String { get }
+    
+    /// The paths in arguments should resolved against base url.
     var isPathRelative: Bool { get }
+    
+    /// The url of which paths should resolve against.
     var baseURL: URL? { get }
+    
+    /// Current active path used in `contentsOfDirectory(path:completionHandler:)` method.
     var currentPath: String { get set }
+    
+    /**
+     Dispatch queue usually used in query methods.
+     Set it to a new object to switch between cuncurrent and serial queues.
+     - **Default:** Cuncurrent `DispatchQueue` object.
+    */
     var dispatch_queue: DispatchQueue { get set }
+    
+    /// Operation queue ususlly used in file operation methods.
+    /// use `maximumOperationTasks` property of provider to manage operation queue.
     var operation_queue: OperationQueue { get set }
+    
+    /// Delegate to update UI after finishing file operations.
     var delegate: FileProviderDelegate? { get set }
+    
+    /**
+     login credential for provider. Should be set in `init` method.
+     
+     **Example initialization:**
+     ````
+     let credential = URLCredential(user: "user", password: "password", persistence: .forSeession)
+     ````
+     
+     - Note: In OAuth based providers like `DropboxFileProvider` and `OneDriveFileProvider`, password is Token.
+     use [OAuthSwift](https://github.com/OAuthSwift/OAuthSwift) library to fetch clientId and Token of user.
+     */
     var credential: URLCredential? { get }
     
+    /**
+     Returns an Array of `FileObject`s identifying the the directory entries via asynchronous completion handler.
+     
+     If the directory contains no entries or an error is occured, this method will return the empty array.
+     
+     - Parameter path: path to target directory. If empty, `currentPath` value will be used.
+     - Parameter completionHandler: a block with result of directory entries or error.
+        `contents`: An array of `FileObject` identifying the the directory entries.
+        `error`: Error returned by system.
+     */
     func contentsOfDirectory(path: String, completionHandler: @escaping ((_ contents: [FileObject], _ error: Error?) -> Void))
+    
+    /**
+     Returns a `FileObject` containing the attributes of the item (file, directory, symlink, etc.) at the path in question via asynchronous completion handler.
+     
+     If the directory contains no entries or an error is occured, this method will return the empty `FileObject`.
+     
+     - Parameter path: path to target directory. If empty, `currentPath` value will be used.
+     - Parameter completionHandler: a block with result of directory entries or error.
+        `attributes`: A `FileObject` containing the attributes of the item.
+        `error`: Error returned by system.
+     */
     func attributesOfItem(path: String, completionHandler: @escaping ((_ attributes: FileObject?, _ error: Error?) -> Void))
     
+    
+    /// Returns total and used space in provider container asynchronously.
     func storageProperties(completionHandler: @escaping ((_ total: Int64, _ used: Int64) -> Void))
     
+    /**
+     Returns an independent url to access the file. Some providers like `Dropbox` due to their nature.
+     don't return an absolute url to be used to access file directly.
+     - Parameter path: Relative path of file or directory.
+     - Returns: An url, can be used to access to file directly.
+    */
     func url(of path: String?) -> URL
 }
 
 public extension FileProviderBasic {
+    /// The maximum number of queued operations that can execute at the same time.
+    ///
+    /// The default value of this property is `OperationQueue.defaultMaxConcurrentOperationCount`.
     public var maximumOperationTasks: Int {
         get {
             return operation_queue.maxConcurrentOperationCount
@@ -48,9 +112,19 @@ public extension FileProviderBasic {
 }
 
 public protocol FileProviderBasicRemote: FileProviderBasic {
+    ///
     var session: URLSession { get }
+    
+    /// A `URLCache` to cache downloaded files and contents.
+    ///
+    /// If set to nil, `URLCache.shared` object will be used.
+    /// - Note: It has no effect unless setting `useCache` property to `true`.
     var cache: URLCache? { get }
+    
+    /// Determine to use `cache` property to cache downloaded file objects. Doesn't have effect on query type methods.
     var useCache: Bool { get set }
+    
+    /// Validating cached data using E-Tag or Revision identifier if possible.
     var validatingCache: Bool { get set }
 }
 
@@ -105,24 +179,140 @@ internal extension FileProviderBasicRemote {
 public protocol FileProviderOperations: FileProviderBasic {
     var fileOperationDelegate : FileOperationDelegate? { get set }
     
+    /**
+     Creates a new directory at the specified path asynchronously. 
+     This will create any necessary intermediate directories.
+     
+     - Parameters:
+       - folder: Directory name.
+       - at: Parent path of new directory.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     */
     @discardableResult
     func create(folder: String, at: String, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Creates an new file with data passed to method asynchronously. 
+     Returns error via completionHandler if file is already exists.
+     
+     - Parameters:
+       - file: New file name with extension separated by period.
+       - at: Parent path of new file.
+       - data: Data of new files. Pass nil or `Data()` to create empty file.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
+    
     func create(file: String, at: String, contents data: Data?, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Moves a file or directory from `path` to designated path asynchronously.
+     When you want move a file, destination path should also consists of file name.
+     Either a new name or the old one. If file is already exist, an error will be returned via completionHandler.
+     
+     - Parameters:
+       - path: original file or directory path.
+       - to: destination path of file or directory, including file/directory name.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func moveItem(path: String, to: String, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Moves a file or directory from `path` to designated path asynchronously.
+     When you want move a file, destination path should also consists of file name.
+     Either a new name or the old one.
+     
+     - Parameters:
+       - path: original file or directory path.
+       - to: destination path of file or directory, including file/directory name.
+       - overwrite: Destination file should be overwritten if file is already exists. **Default** is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func moveItem(path: String, to: String, overwrite: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Copies a file or directory from `path` to designated path asynchronously.
+     When want copy a file, destination path should also consists of file name.
+     Either a new name or the old one. If file is already exist, an error will be returned via completionHandler.
+     
+     - Parameters:
+       - path: original file or directory path.
+       - to: destination path of file or directory, including file/directory name.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func copyItem(path: String, to: String, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Copies a file or directory from `path` to designated path asynchronously.
+     When want copy a file, destination path should also consists of file name.
+     Either a new name or the old one.
+     
+     - Parameters:
+       - path: original file or directory path.
+       - to: destination path of file or directory, including file/directory name.
+       - overwrite: Destination file should be overwritten if file is already exists. **Default** is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func copyItem(path: String, to: String, overwrite: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Removes the file or directory at the specified path.
+     
+     - Parameters:
+       - path: file or directory path.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     
+     */
     @discardableResult
     func removeItem(path: String, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Uploads a file from local file url to designated path asynchronously.
+     Method will fail if source is not a local url with `file://` scheme.
+     
+     - Parameters:
+       - localFile: a file url to file.
+       - to: destination path of file, including file/directory name.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress.
+     */
     @discardableResult
     func copyItem(localFile: URL, to: String, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Uploads a file from local file url to designated path asynchronously.
+     Method will fail if source is not a local url with `file://` scheme.
+     
+     - Parameters:
+       - localFile: a file url to file.
+       - to: destination path of file, including file/directory name.
+       - overwrite: Destination file should be overwritten if file is already exists. **Default** is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress.
+     */
     @discardableResult
     func copyItem(localFile: URL, to: String, overwrite: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Download a file from `path` to designated local file url asynchronously.
+     Method will fail if destination is not a local url with `file://` scheme.
+     
+     - Parameters:
+       - path: original file or directory path.
+       - toLocalURL: destination local url of file, including file/directory name.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func copyItem(path: String, toLocalURL: URL, completionHandler: SimpleCompletionHandler) -> OperationHandle?
 }
@@ -145,20 +335,104 @@ extension FileProviderOperations {
 }
 
 public protocol FileProviderReadWrite: FileProviderBasic {
+    /**
+     Retreives a `Data` object with the contents of the file asynchronously vis contents argument of completion handler.
+     If path specifies a directory, or if some other error occurs, data will be nil.
+     
+     - Parameters:
+       - path: Path of file.
+       - completionHandler: a block with result of file contents or error.
+         `contents`: contents of file in a `Data` object.
+         `error`: Error returned by system.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+    */
     @discardableResult
     func contents(path: String, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle?
+    
+    /**
+     Retreives a `Data` object with a portion contents of the file asynchronously vis contents argument of completion handler.
+     If path specifies a directory, or if some other error occurs, data will be nil.
+     
+     - Parameters:
+       - path: Path of file.
+       - offset: First byte index which should be read. **Starts from 0.**
+       - length: Bytes count of data. Pass `-1` to read until the end of file.
+       - completionHandler: a block with result of file contents or error.
+         `contents`: contents of file in a `Data` object.
+         `error`: Error returned by system.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func contents(path: String, offset: Int64, length: Int, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle?
     
+    /**
+     Write the contents of the `Data` to a location asynchronously.
+     It will return error if file is already exists.
+     Not attomically by default, unless the provider enforces it.
+     
+     - Parameters:
+       - path: Path of target file.
+       - contents: Data to be written into file.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func writeContents(path: String, contents: Data, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Write the contents of the `Data` to a location asynchronously.
+     It will return error if file is already exists.
+     
+     - Parameters:
+       - path: Path of target file.
+       - contents: Data to be written into file.
+       - atomically: data will be written to a temporary file before writing to final location. Default is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func writeContents(path: String, contents: Data, atomically: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Write the contents of the `Data` to a location asynchronously.
+     Not attomically by default, unless the provider enforces it.
+     
+     - Parameters:
+     - path: Path of target file.
+       - contents: Data to be written into file.
+       - overwrite: Destination file should be overwritten if file is already exists. Default is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func writeContents(path: String, contents: Data, overwrite: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
+    
+    /**
+     Write the contents of the `Data` to a location asynchronously.
+     
+     - Parameters:
+       - path: Path of target file.
+       - contents: Data to be written into file.
+       - overwrite: Destination file should be overwritten if file is already exists. Default is `false`.
+       - atomically: data will be written to a temporary file before writing to final location. Default is `false`.
+       - completionHandler: If an error parameter was provided, a presentable `Error` will be returned.
+     - Returns: An `OperationHandle` to get progress or cancel progress. Doesn't work on `LocalFileProvider`.
+     */
     @discardableResult
     func writeContents(path: String, contents: Data, atomically: Bool, overwrite: Bool, completionHandler: SimpleCompletionHandler) -> OperationHandle?
     
+    /**
+     Search files inside directory using query asynchronously.
+     
+     - Note: For now only it's limited to file names. `query` parameter may take `NSPredicate` format in near future.
+     
+     - Parameters:
+       - path: location of directory to start search
+       - recursive: Searching subdirectories of path
+       - query: Simple string of file name to be search (for now).
+       - foundItemHandler: Block which is called when a file is found
+       - completionHandler: Block which will be called after finishing search. Returns an arry of `FileObject` or error if occured.
+     */
     func searchFiles(path: String, recursive: Bool, query: String, foundItemHandler: ((FileObject) -> Void)?, completionHandler: @escaping ((_ files: [FileObject], _ error: Error?) -> Void))
 }
 
@@ -185,8 +459,33 @@ extension FileProviderReadWrite {
 }
 
 public protocol FileProviderMonitor: FileProviderBasic {
+    
+    /**
+     Starts monitoring a path and its subpaths, including files and folders, for any change,
+     including copy, move/rename, content changes, etc.
+     To avoid thread congestion, `evetHandler` will be triggered with 0.2 seconds interval,
+     and has a 0.25 second delay, to ensure it's called after updates.
+     
+     - Note: this functionality is available only in `LocalFileProvider` and `CloudFileProvider`.
+     - Note: `eventHandler` is not called on main thread, for updating UI. dispatch routine to main thread.
+     - Important: `eventHandler` may be called if file is changed in recursive subpaths of registered path.
+       This may cause negative impact on performance if a root path is being monitored.
+    
+     - Parameters:
+       - path: path of directory.
+       - eventHandler: Block executed after change, on a secondary thread.
+     */
     func registerNotifcation(path: String, eventHandler: @escaping (() -> Void))
+    
+    /// Stops monitoring the path.
+    ///
+    /// - Parameter path: path of directory.
     func unregisterNotifcation(path: String)
+    
+    /// Investigate either the path is registered for change notification or not.
+    ///
+    /// - Parameter path: path of directory.
+    /// - Returns: Directory is being monitored or not.
     func isRegisteredForNotification(path: String) -> Bool
 }
 
@@ -199,6 +498,7 @@ extension FileProviderBasic {
         return Self.type
     }
     
+    /// path without heading and trailing slash
     public var bareCurrentPath: String {
         return currentPath.trimmingCharacters(in: pathTrimSet)
     }
@@ -248,6 +548,8 @@ extension FileProviderBasic {
         return p
     }
     
+    /// Returns a file name supposed to be unique with adding numbers to end of file.
+    /// - Important: It's a synchronous method. Don't use it on matin thread.
     public func fileByUniqueName(_ filePath: String) -> String {
         let fileUrl = URL(fileURLWithPath: filePath)
         let dirPath = fileUrl.deletingLastPathComponent().path 
@@ -302,10 +604,61 @@ extension FileProviderBasic {
 }
 
 public protocol ExtendedFileProvider: FileProviderBasic {
+    /// Returuns true if thumbnail preview is supported by provider and file type accordingly.
+    ///
+    /// - Parameter path: path of file.
+    /// - Returns: A `Bool` idicates path can have thumbnail.
     func thumbnailOfFileSupported(path: String) -> Bool
+    
+    /// Returns true if provider supports fetching properties of file like dimensions, duration, etc.
+    /// Usually media or document files support these meta-infotmations.
+    ///
+    /// - Parameter path: path of file.
+    /// - Returns: A `Bool` idicates path can have properties.
     func propertiesOfFileSupported(path: String) -> Bool
+    
+    /**
+     Generates ans returns a thumbnail preview of document asynchronously. The defualt dimension of returned image is different
+     regarding provider type, usually 64x64 pixels.
+     
+     - Parameters:
+       - path: path of file.
+       - completionHandler: a block with result of preview image or error.
+         `image`: `NSImage`/`UIImage` object contains preview.
+         `error`: Error returned by system.
+    */
     func thumbnailOfFile(path: String, completionHandler: @escaping ((_ image: ImageClass?, _ error: Error?) -> Void))
+    
+    /**
+     Generates ans returns a thumbnail preview of document asynchronously. The defualt dimension of returned image is different
+     regarding provider type, usually 64x64 pixels. Default value used when `dimenstion` is `nil`.
+     
+     - Note: `LocalFileInformationGenerator` variables can be set to change default behavior of
+             thumbnail and properties generator of `LocalFileProvider`.
+     
+     - Parameters:
+       - path: path of file.
+       - dimension: width and height of result preview image.
+       - completionHandler: a block with result of preview image or error.
+         `image`: `NSImage`/`UIImage` object contains preview.
+     `error`: Error returned by system.
+     */
     func thumbnailOfFile(path: String, dimension: CGSize?, completionHandler: @escaping ((_ image: ImageClass?, _ error: Error?) -> Void))
+    
+    /**
+     Fetching properties of file like dimensions, duration, etc. It's variant depending on file type.
+     Images, videos and audio files meta-information will be returned.
+     
+     - Note: `LocalFileInformationGenerator` variables can be set to change default behavior of 
+             thumbnail and properties generator of `LocalFileProvider`.
+     
+     - Parameters:
+       - path: path of file.
+       - completionHandler: a block with result of preview image or error.
+         `propertiesDictionary`: A `Dictionary` of proprty keys and values.
+         `keys`: An `Array` contains ordering of keys.
+         `error`: Error returned by system.
+     */
     func propertiesOfFile(path: String, completionHandler: @escaping ((_ propertiesDictionary: [String: Any], _ keys: [String], _ error: Error?) -> Void))
 }
 
@@ -432,13 +785,21 @@ extension ExtendedFileProvider {
     }
 }
 
+/// Operation type description of file operation, included files path in associated values.
 public enum FileOperationType: CustomStringConvertible {
+    /// Creating a file or directory in path.
     case create (path: String)
+    /// Copying a file or directory from source to destination.
     case copy   (source: String, destination: String)
+    /// Moving a file or directory from source to destination.
     case move   (source: String, destination: String)
+    /// Modifying data of a file o in path by writing new data.
     case modify (path: String)
+    /// Deleting file or directory in path.
     case remove (path: String)
+    /// Creating a symbolic link or alias to target.
     case link   (link: String, target: String)
+    /// Fetching data in file located in path.
     case fetch  (path: String)
     
     public var description: String {
@@ -453,16 +814,24 @@ public enum FileOperationType: CustomStringConvertible {
         }
     }
     
+    /// present participle of action, like 'Copying`.
     public var actionDescription: String {
         return description.trimmingCharacters(in: CharacterSet(charactersIn: "e")) + "ing"
     }
     
+    /// Path of subjecting file.
     public var source: String? {
         guard let reflect = Mirror(reflecting: self).children.first?.value else { return nil }
         let mirror = Mirror(reflecting: reflect)
         return reflect as? String ?? mirror.children.first?.value as? String
     }
     
+    /// Path of subjecting file.
+    public var path: String? {
+        return source
+    }
+    
+    /// Path of destination file.
     public var destination: String? {
         guard let reflect = Mirror(reflecting: self).children.first?.value else { return nil }
         let mirror = Mirror(reflecting: reflect)
@@ -479,11 +848,22 @@ public enum FileOperationType: CustomStringConvertible {
 
 
 public protocol OperationHandle {
+    /// Operation supposed to be done on files. Contains file paths as associated value.
     var operationType: FileOperationType { get }
+    
+    /// Bytes written/read by operation so far.
     var bytesSoFar: Int64 { get }
+    
+    /// Total bytes of operation.
     var totalBytes: Int64 { get }
+    
+    /// Operation is progress or not, Returns false if operation is done or not initiated yet.
     var inProgress: Bool { get }
+    
+    /// Progress of operation, usually equals with `bytesSoFar/totalBytes`. or NaN if not available.
     var progress: Float { get }
+    
+    /// Cancels operation while in progress, or cancels data/download/upload url session task.
     func cancel() -> Bool
 }
 
@@ -496,8 +876,15 @@ public extension OperationHandle {
 }
 
 public protocol FileProviderDelegate: class {
+    /// fileproviderSucceed(_:operation:) gives delegate a notification when an operation finished with success.
+    /// This method is called in main thread to avoids UI bugs.
     func fileproviderSucceed(_ fileProvider: FileProviderOperations, operation: FileOperationType)
+    /// fileproviderSucceed(_:operation:) gives delegate a notification when an operation finished with failure.
+    /// This method is called in main thread to avoids UI bugs.
     func fileproviderFailed(_ fileProvider: FileProviderOperations, operation: FileOperationType)
+    /// fileproviderSucceed(_:operation:) gives delegate a notification when an operation progess.
+    /// Supported by some providers, especially remote ones.
+    /// This method is called in main thread to avoids UI bugs.
     func fileproviderProgress(_ fileProvider: FileProviderOperations, operation: FileOperationType, progress: Float)
 }
 
diff --git a/Sources/LocalFileProvider.swift b/Sources/LocalFileProvider.swift
index fe9bdf4..215aec1 100644
--- a/Sources/LocalFileProvider.swift
+++ b/Sources/LocalFileProvider.swift
@@ -359,8 +359,12 @@ open class LocalFileProvider: FileProvider, FileProviderMonitor {
         let url = self.url(of: path)
         
         let operationHandler: (URL) -> Void = { url in
-            let data = self.fileManager.contents(atPath: url.path)
-            completionHandler(data, nil)
+            do {
+                let data = try Data(contentsOf: url)
+                completionHandler(data, nil)
+            } catch let e {
+                completionHandler(nil, e)
+            }
         }
         
         if isCoorinating {
@@ -382,7 +386,11 @@ open class LocalFileProvider: FileProvider, FileProviderMonitor {
     
     @discardableResult
     open func contents(path: String, offset: Int64, length: Int, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle? {
-        if length < 0 {
+        if length == 0 {
+            return nil
+        }
+        
+        if offset == 0 && length < 0 {
             return self.contents(path: path, completionHandler: completionHandler)
         }
         
@@ -391,18 +399,30 @@ open class LocalFileProvider: FileProvider, FileProviderMonitor {
         
         let operationHandler: (URL) -> Void = { url in
             guard self.fileManager.fileExists(atPath: url.path) && !url.fileIsDirectory else {
-                completionHandler(nil, self.throwError(path, code: URLError.cannotOpenFile as FoundationErrorEnum))
+                completionHandler(nil, self.throwError(path, code: URLError.fileDoesNotExist as FoundationErrorEnum))
                 return
             }
             guard let handle = FileHandle(forReadingAtPath: url.path) else {
                 completionHandler(nil, self.throwError(path, code: URLError.cannotOpenFile as FoundationErrorEnum))
                 return
             }
+            
             defer {
                 handle.closeFile()
             }
+            
             handle.seek(toFileOffset: UInt64(offset))
+            guard Int64(handle.offsetInFile) == offset else {
+                completionHandler(nil, self.throwError(path, code: CocoaError.fileReadUnknown as FoundationErrorEnum))
+                return
+            }
+            
             let data = handle.readData(ofLength: length)
+            guard length > 0 && data.count == length else {
+                completionHandler(nil, self.throwError(path, code: CocoaError.fileReadTooLarge as FoundationErrorEnum))
+                return
+            }
+            
             completionHandler(data, nil)
         }
         
diff --git a/Sources/OneDriveFileProvide.swift b/Sources/OneDriveFileProvide.swift
index 04e3966..1d24f7d 100644
--- a/Sources/OneDriveFileProvide.swift
+++ b/Sources/OneDriveFileProvide.swift
@@ -222,6 +222,10 @@ extension OneDriveFileProvider: FileProviderOperations {
 
 extension OneDriveFileProvider: FileProviderReadWrite {
     public func contents(path: String, offset: Int64, length: Int, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle? {
+        if length == 0 {
+            return nil
+        }
+        
         let opType = FileOperationType.fetch(path: path)
         let url =  URL(string: escaped(path: path) + ":/content", relativeTo: driveURL)!
         var request = URLRequest(url: url)
diff --git a/Sources/WebDAVFileProvider.swift b/Sources/WebDAVFileProvider.swift
index 87dee93..51f7d1e 100644
--- a/Sources/WebDAVFileProvider.swift
+++ b/Sources/WebDAVFileProvider.swift
@@ -319,6 +319,10 @@ extension WebDAVFileProvider: FileProviderOperations {
 extension WebDAVFileProvider: FileProviderReadWrite {
     @discardableResult
     public func contents(path: String, offset: Int64, length: Int, completionHandler: @escaping ((_ contents: Data?, _ error: Error?) -> Void)) -> OperationHandle? {
+        if length == 0 {
+            return nil
+        }
+        
         let opType = FileOperationType.fetch(path: path)
         let url = self.url(of: path)
         var request = URLRequest(url: url)
@@ -412,9 +416,10 @@ extension WebDAVFileProvider: FileProviderReadWrite {
          * A messy approach is listing a directory with an interval period and compare
          * with previous results
          */
+        NotImplemented()
     }
     fileprivate func unregisterNotifcation(path: String) {
-        
+        NotImplemented()
     }
     // TODO: implements methods for lock mechanism
 }