diff --git a/__tests__/async.test.js b/__tests__/async.test.js
new file mode 100644
index 0000000..e53abe3
--- /dev/null
+++ b/__tests__/async.test.js
@@ -0,0 +1,70 @@
+const { readFile, writeFileSync, unlinkSync, mkdirSync, rmdirSync, chmodSync } = require('../')
+
+function getScriptFolder(context) {
+  const parts = context.scriptPath.split('/')
+  parts.pop()
+  return parts.join('/')
+}
+
+test('failed to read', (context) => {
+  const targetFile = `${getScriptFolder(context)}/test.js`
+  const targetDir = `${getScriptFolder(context)}/test`
+  return Promise.all([
+    new Promise((resolve) => {
+      readFile(targetFile, 'utf8', (err, res) => {
+        if (err) {
+          resolve(err)
+          return
+        }
+        resolve(res)
+      })
+    }).then(err => {
+      expect(err.errno).toEqual(-2) // no such a file
+    }),
+    new Promise((resolve) => {
+      mkdirSync(targetDir)
+      readFile(targetDir, 'utf8', (err, res) => {
+        if (err) {
+          resolve(err)
+          return
+        }
+        resolve(res)
+      })
+    }).then(err => {
+      expect(err.errno).toEqual(-21) // operations on directories
+      rmdirSync(targetDir)
+    }),
+    new Promise((resolve) => {
+      writeFileSync(targetFile, 'test')
+      chmodSync(targetFile, 0o000)
+      readFile(targetFile, 'utf8', (err, res) => {
+        if (err) {
+          resolve(err)
+          return
+        }
+        resolve(res)
+      })
+    }).then(err => {
+      expect(err.errno).toEqual(-13) // permission denied
+      chmodSync(targetFile, 0o777)
+      unlinkSync(targetFile)
+    })
+  ])
+})
+
+test('should read a file', (context) => {
+  const targetFile = `${getScriptFolder(context)}/test.js`
+  writeFileSync(targetFile, 'test')
+  return new Promise((resolve, reject) => {
+    readFile(targetFile, 'utf8', (err, res) => {
+      if (err) {
+        reject(err)
+        return
+      }
+      resolve(res)
+    })
+  }).then((res) => {
+    expect(res).toBe('test')
+    unlinkSync(targetFile)
+  })
+})
diff --git a/delegate.js b/delegate.js
new file mode 100644
index 0000000..2ffa1d2
--- /dev/null
+++ b/delegate.js
@@ -0,0 +1,69 @@
+// based on https://github.com/matt-curtis/MochaJSDelegate
+
+module.exports = function(selectorHandlerDict) {
+  var uniqueClassName = 'MochaJSDelegate_DynamicClass_' + NSUUID.UUID().UUIDString()
+
+  var delegateClassDesc = MOClassDescription.allocateDescriptionForClassWithName_superclass_(uniqueClassName, NSObject)
+
+  delegateClassDesc.registerClass()
+
+  //	Handler storage
+  var handlers = {}
+
+  //	Define interface
+  this.setHandlerForSelector = function(selectorString, func) {
+    var handlerHasBeenSet = (selectorString in handlers)
+    var selector = NSSelectorFromString(selectorString)
+
+    handlers[selectorString] = func
+
+    if (!handlerHasBeenSet) {
+      /*
+        For some reason, Mocha acts weird about arguments:
+        https://github.com/logancollins/Mocha/issues/28
+        We have to basically create a dynamic handler with a likewise dynamic number of predefined arguments.
+      */
+      var dynamicHandler = function() {
+        var functionToCall = handlers[selectorString]
+
+        if(!functionToCall) return
+
+        return functionToCall.apply(delegateClassDesc, arguments)
+      }
+
+      var args = [], regex = /:/g
+      while(match = regex.exec(selectorString)) args.push('arg' + args.length)
+
+      dynamicFunction = eval('(function(' + args.join(',') + '){ return dynamicHandler.apply(this, arguments) })')
+
+      delegateClassDesc.addInstanceMethodWithSelector_function_(selector, dynamicFunction)
+    }
+  }
+
+  this.removeHandlerForSelector = function(selectorString) {
+    delete handlers[selectorString]
+  }
+
+  this.getHandlerForSelector = function(selectorString) {
+    return handlers[selectorString]
+  }
+
+  this.getAllHandlers = function() {
+    return handlers
+  }
+
+  this.getClass = function() {
+    return NSClassFromString(uniqueClassName)
+  }
+
+  this.getClassInstance = function() {
+    return NSClassFromString(uniqueClassName).new()
+  }
+
+  //	Convenience
+  if (typeof selectorHandlerDict === 'object') {
+    for (var selectorString in selectorHandlerDict) {
+      this.setHandlerForSelector(selectorString, selectorHandlerDict[selectorString])
+    }
+  }
+}
diff --git a/index.js b/index.js
index 79bbc6d..cf6d55e 100644
--- a/index.js
+++ b/index.js
@@ -1,6 +1,41 @@
 // TODO: async. Should probably be done with NSFileHandle and some notifications
 // TODO: file descriptor. Needs to be done with NSFileHandle
 var Buffer = require('buffer').Buffer
+var Delegate = require('./delegate')
+var notificationCenter = NSNotificationCenter.defaultCenter()
+
+function dispatchToNSData(dispatchData) {
+  return NSString.alloc().initWithData_encoding(dispatchData, NSISOLatin1StringEncoding)
+    .dataUsingEncoding(NSISOLatin1StringEncoding)
+}
+
+function addObserver(name, notificationType, callback) {
+  var option = {}
+  var observerInstance
+  option[name] = function(notification) {
+    // need to unregister the observer after callback
+    notificationCenter.removeObserver_name_object(observerInstance, notificationType, nil)
+    callback && callback(dispatchToNSData(notification.userInfo().NSFileHandleNotificationDataItem))
+  }
+
+  observerInstance = new Delegate(option).getClassInstance()
+  notificationCenter.addObserver_selector_name_object(observerInstance, NSSelectorFromString(name), notificationType, nil)
+}
+
+function fsError(options) {
+  var ERROR_MESSAGES = {
+    '-2': 'no such file or directory',
+    '-13': 'permission denied',
+    '-21': 'illegal operation on a directory'
+  }
+
+  return Object.assign(new Error(
+    options.code + ': '
+    + ERROR_MESSAGES[options.errno] + ', '
+    + options.syscall
+    + (options.path ? ' \'' + options.path + '\'' : '')
+  ), options)
+}
 
 function encodingFromOptions(options, defaultValue) {
   return options && options.encoding
@@ -145,21 +180,76 @@ module.exports.readdirSync = function(path) {
   return arr
 }
 
-module.exports.readFileSync = function(path, options) {
-  var encoding = encodingFromOptions(options, 'buffer')
+var fns = ['readFile', 'readFileSync']
+var fn = function(sync, path, options, callback) {
+  // handle data
+  var handler = function(data, options) {
+    var encoding = encodingFromOptions(options, 'buffer')
+    var buffer = Buffer.from(data)
+
+    if (encoding === 'buffer') {
+      return buffer
+    } else if (encoding === 'NSData') {
+      return buffer.toNSData()
+    } else {
+      return buffer.toString(encoding)
+    }
+  }
+  // read data
   var fileManager = NSFileManager.defaultManager()
-  var data = fileManager.contentsAtPath(path)
-  var buffer = Buffer.from(data)
-
-  if (encoding === 'buffer') {
-    return buffer
-  } else if (encoding === 'NSData') {
-    return buffer.toNSData()
+  if (sync) {
+    return handler(fileManager.contentsAtPath(path), options)
   } else {
-    return buffer.toString(encoding)
+    var fileInstance = NSFileHandle.fileHandleForReadingAtPath(path)
+    if (fileInstance) {
+      addObserver('onReadCompleted:', NSFileHandleReadCompletionNotification, function (data) {
+        callback && callback(null, handler(data, options))
+      })
+      fileInstance.readInBackgroundAndNotify()
+    } else {
+      var isExisted = fileManager.fileExistsAtPath(path)
+      var isDirectory = fileManager.fileExistsAtPath_isDirectory(path, true)
+      var isReadable = fileManager.isReadableFileAtPath(path)
+      // need to use NSFileManager to detect whether it is existed or readable
+      if (callback) {
+        if (!isExisted) {
+          // not existed
+          callback(fsError({
+            errno: -2,
+            code: 'ENOENT',
+            syscall: 'open',
+            path: path
+          }))
+        } else if (!isReadable) {
+          // permission denied
+          callback(fsError({
+            errno: -13,
+            code: 'EACCES',
+            syscall: 'open',
+            path: path
+          }))
+        } else if (isDirectory) {
+          // directory
+          callback(fsError({
+            errno: -21,
+            code: 'EISDIR',
+            syscall: 'read'
+          }))
+        }
+      }
+    }
   }
 }
 
+for (var i = 0; i < fns.length; i++) {
+  var name = fns[i]
+  var isSync = /sync$/i.test(name)
+
+  module.exports[name] = eval('(function(' + ['path', 'options'].concat(isSync ? [] : ['callback']).join(',') + '){'
+      + 'return fn.apply(this, [' + isSync + '].concat([].slice.call(arguments)))'
+    + '})')
+}
+
 module.exports.readlinkSync = function(path) {
   var err = MOPointer.alloc().init()
   var fileManager = NSFileManager.defaultManager()