Wednesday, 24 June 2015

Looking Deeper into Async and Await

In this post, we will see the concepts of async and await in more details.

async keyword
A simple async method looks like this:
public async Task MyMethodAsync()
{
  . . .
}

An async keyword does not make a method to run asynchronously. What it does, it allows the method to use one or more await keyword to call other asynchronous method(s). The compiler will not generate an error if we fail to provide at least one await, it will only give a warning. We cannot use an await in a method that does not marked with async.

Secondly, it enables the method to return a value or exception into Task or Task<T> return type which will help us a lot in doing asynchronous programming.

A method that is decorated with async will actually start running its codes synchronously like a normal method until it finds an await keyword. Then it will see, if the method called by the await needs to be awaited, only then the asynchronous process will happen. We will see this in more details below.


await Keyword
Below are two examples of using await:
public async Task MyMethodAsync()
{
  // initially this runs synchronously
  . . .

  // 'await' waits for an asynchronous method to complete. It enables the process to be asynchronous,
  //    depending on how soon the method is completing.
  await RunSomethingAsync();

  // the codes past 'await' will only be executed after the awaited method completes,
  //    however 'await' does not block the thread
  . . .
}
// this second variation, shows where 'await' is used a little bit later after some other codes
public async Task MyMethodAsync()
{
  // initially this runs synchronously
  . . .

  var result = RunSomethingAsync();

  // some other codes here still run synchronously
  . . .

  // 'await' waits for an asynchronous method to complete. It enables the process to be asynchronous,
  //    depending on how soon the method is completing.
  await result;

  // the codes past 'await' will only be executed after the awaited method completes,
  //    however 'await' does not block the thread
  . . .
}

This is the keyword that enables the execution process to become asynchronous. If we do not have any await in our method, then the method will be executed synchronously even though the method is marked with async.

The keyword calls an asynchronous method. It allows the asynchronous method to be awaited until it completes then it will continue the codes after the await. While awaiting, it does not block the current thread but returns the control to the caller (i.e. MyMethodAsync() caller). This is when the asynchronous process is happening. The diagram in 'What Happens in an Async Method' section on this page explains this clearly.

Only when the called asynchronous method needs to be awaited, the process becomes asynchronous. It waits for the method to complete but also allow the caller (i.e. MyMethodAsync() caller) to execute other codes. If the outer codes would encounter an await keyword also then similar process would happen as well.

The await keyword does not need to be put together when calling an asynchronous method, it can be called later (note in our second variation example above, the await is not called straight away).

If the awaited method completes right away before the process above happens then all of those codes will be run in synchronous mode.


Task and Task<T>
We should always use either Task or Task<T> as the return type of our asynchronous method. The earlier is used when we do not have anything to be returned. Yes, when the method does not return anything, use Task instead of void (the exception will be explained below).

// below is an example of a method that returns Task<T>
public async Task<int> CalculateAsync()
{
   . . .

   // return an 'int' not 'Task<int>' type
   return 5;
}

Task should be used because this type allows the method to be awaited using await keyword which could lift up the process to be asynchronous. The return type enables the caller as well to know the progress when executing the method. Lastly, it encapsulates any exception that could occur inside the method to allow the exception to be handled easily.

Using void is also possible but it should be used for event handler only. An async void method has to use a different way to handle exception as it does not have a wrapper for the exception. It also does not provide an easy way to notify the caller when it has completed. In addition, this will be difficult to test.


Execution Thread
Our asynchronous method will begin with the caller's thread until it finds a process that needs to be awaited by await keyword (recall that not every asynchronous method that is called by await needs to be awaited). When it happens, the context of the caller's thread will be captured. The awaited process will run on a new thread from thread pool. Then when the awaited task is completed, the control is returned back to the captured context which is using the original thread (caller's thread).

If the awaited task finishes before the signing up process above is completed then the rest of the codes will be run synchronously without opening a new thread (from thread pool). Thus all the codes will always run on the initial thread which is the caller's thread.

public async Task MyMethodAsync()
{
  // initially this runs on the caller's thread
  . . .

  // since the process inside LongOperationAsync() needs to be awaited, 
  //    the original thread is captured and signed up for continuation.
  // The process is run using a new thread from thread pool 
  await LongOperationAsync();
  // after the awaiting is complete, the control is handed back to the original thread (caller's thread)

  // the codes past 'await' will be executed under original thread
  . . .
}

The process to returning back the control back to the captured context has some overhead. Moreover, the original context can suffer more if it has already had a performance issue due to other processes that it handles.

Fortunately, the codes after the await keyword can be run with a different thread unless if they really need the original context to run. For example; if the method is an ASP.NET MVC controller action method, the original context is an ASP.NET context and the codes after the await need to do some controller related stuffs then they will need to be run under ASP.NET context as well.

To enable codes after await to be run under a different context, we use ConfigureAwait(continueOnCapturedContext: false).
public async Task MyMethodAsync()
{
  // initially this runs on the caller's thread
  . . .

  // since 'ConfigureAwait(false)' is used, the process of capturing original context 
  //    and signing up for continuation can be skipped
  // the process in LongOperationAsync() is run using a new thread from thread pool 
  await LongOperationAsync().ConfigureAwait(continueOnCapturedContext: false);
  // after the awaiting is complete, the control will continue under the newly created thread,
  //    the handling back process to the original context can be skipped as well

  // the codes past 'await' will be executed under the newly created thread
  . . .
}

Whenever possible, we should always set ConfigureAwait(continueOnCapturedContext: false), unless if we know that we are going to need the original thread for the codes after the await.


References and further reading:
Async and Await Post by Stephen Cleary
Best Practices in Asynchronous Programming on MSDN
Async/Await FAQ on MSDN Blogs
Asynchronous Programming with Async and Await on MSDN

Tuesday, 23 June 2015

Getting Started with Async Await in C#

I was trying to start using async await in my data layer. After reading a few articles such as this one from MSDN, I start writing some asynchronous methods from my original synchronous methods.

A few steps that need to be done to create an asynchronous method from its counterpart synchronous method:
1. make sure System.Threading.Tasks is referred in one of the using statements
2. change the method return type to be async Task<returntype> or simply async Task if the original method was decorated with void
3. call the asynchronous version of the Entity Framework method
4. most of the time, use await keyword just before the Entity Framework asynchronous method. We could assign the result to a Task variable first then call the await keyword a little bit later if we have some operation that can be executed right away and is not dependent on the asynchronous method's result.

Below are a few simple examples:
- an example of a method that does not return anything
// original synchronous method
public void Insert(Person person)
{
 _context.Person.Add(person);
 _context.SaveChanges();
}

// the async version
public async Task InsertAsync(Person person)
{
 _context.Person.Add(person);
 await _context.SaveChangesAsync();
}

- an example of a method returning a value
// original synchronous method
public void int GetTotal()
{
    return _context.Person.Count();
}

// the async version
public async Task<int> GetTotalAsync()
{
    return await _context.Person.CountAsync();
}

- an example that shows using await a little bit later:
// original synchronous method
public void int GetTotalNumber()
{
    var result = _context.Person.Count();

    DoSomeOtherOperation();

    return result;
}

// the async version
public async Task<int> GetTotalNumberAsync()
{
    var result = _context.Person.CountAsync(); // use await a little bit later

    // do non-related operation to the result above
    DoSomeOtherOperation();

    return await result;
}

On the next post we will see these concepts in more details.

Monday, 18 May 2015

Using When, All, Then and Reject functions of $q Service

We will see how to use when, all, then and reject functions of the AngularJS $q service. To learn the basic usage of $q, please see my previous post.

$q.when
$q.when() can be used to wrap an object or function if we are not sure whether it returns a promise or not. If we wrap a value object or a value returned function, the value will be passed as a result argument to the next function. When we wrap a function, the when() method will ensure that the function is executed first before going to the next one.

Below are some code examples:
$q.when('aaa').then(function(result) {
    alert(result);
})

var vm = this;
vm.chars = '';

$q.when(_test()).then(function(){
 vm.chars = vm.chars + 'END'
})

_test = function() {
 for (i = 0; i <= 10000; i++) {
   vm.chars = vm.chars + '.'
 }
}


$q.all
$q.all() is used to combine promise returned functions and only continue to progress further after all of the functions complete successfully. If one of the functions fails, immediately as soon as $q.all() gets a rejected promise from one of the functions, it will end the execution and pass forward to an error callback if there is any.
vm.numbers = '';    
_promiseFunction = function(x) {
 var deferred = $q.defer();

 if (x > 0) {
   deferred.resolve(x * 100);
 } else {
   deferred.reject(x + " is smaller than 0")
 }

 return deferred.promise;
}
 
var first = _promiseFunction(1);
var second = _promiseFunction(5);
var third = _promiseFunction(10);
var four = _promiseFunction(-1);
var five = _promiseFunction(-10)

$q.all([first, second, third /*, four, five*/]).
then(function(results){
  console.log(results.length);
  for(i=0; i < results.length; i++) {
    vm.numbers = vm.numbers + ' ' + results[i];
  }
 },
 function(errors){
  // if there is any error in the sequence, the success callback will not be called
  console.log(errors);
 })


$q.then
$q.then() is useful for executing asynchronous functions (or blocks of codes) sequentially in a chain. If the function inside a then() does not return a promise, it will be wrapped as a resolved promise. Otherwise the resolved or rejected promise will be passed forward.

_promiseFunction(5).then(
 function() { alert('aaa')}
).then(
 function() { alert('bbb');}
).then(
 function() {alert('ccc');}
);

_promiseFunction(5).then(
 function() { return $q.reject('failed')}
).then(
 function(){ alert('result success')},
 function(){ alert('result fail')}
)

Note also that if we return a non-promise value in the error callback inside then(), this will be treated as a resolved promise. So we need to be careful when handling error if our intention is to pass the error forward to the next function.
$q.reject().then(
 // success
 function() {},
 // error
 function(error) {
   return 'i got the error'
 }
).then(
 //success
 function() {
   alert('success');
 },
 // error
 function() {
   alert('error')
 }
)

$q.reject
Last thing, we have $q.reject() that is useful as a quick way to create then send a rejected promise (with a value optionally) to the next function in a chain. For example, in a chain of multiple then(). We can see this in the last two examples.

See the examples in Plunker.

Wednesday, 29 April 2015

AngularJS Custom Validator Example - Comparing Two Dates

Below is an example of AngularJS custom validator to compare an end date with a start date to make sure that the end date is equal or bigger than the start date. To see basic usage of AngularJS validation, please see my previous post.

HTML codes:
<!DOCTYPE html>
<html>

  <head>
    <script src="https://code.angularjs.org/1.4.0-beta.6/angular.js"></script>
    <script src="https://code.angularjs.org/1.3.15/angular-messages.js"></script>
    <script src="script.js"></script>
  </head>

  <body ng-app="validationExample">
    <div ng-controller="MyCtrl as vm">
      <form name="myForm" novalidate="" ng-submit="vm.submitted(myForm, vm.input)">
        <span class="input-label">Start Date</span> 
        <input type="date" name="startDate" ng-model="vm.input.startDate" required />
        <span ng-messages="myForm.startDate.$error" ng-messages-include="errors-template.html"></span>
        <br />       
        <span class="input-label">End Date</span> 
        
        <input type="date" name="endDate" ng-model="vm.input.endDate" start-date="{{vm.input.startDate.toDateString()}}" compare-with-start-date />
        <span ng-messages="myForm.endDate.$error" ng-messages-include="errors-template.html"></span>
        <br />             
        <button>submit</button>
      </form>
    </div>
    
    <!-- error messages template -->
    <script id="errors-template.html" type="text/ng-template">
        <span ng-message="required">* required!</span>
        <span class="form-error" ng-message="checkEndDate">* end date cannot be earlier than start date</span>
    </script>
  </body>

</html>

JavaScript codes:
var myApp = angular.module('validationExample', ['ngMessages'])

myApp.controller('MyCtrl', [function () {
    var vm = this;
    vm.submitted = function(form, input) {
      if(form.$valid) {
        alert('submitted');
      }
    }
} ]);

myApp.directive("compareWithStartDate", function () {
    return {
        restrict: "A",
        require: "?ngModel",
        link: function (scope, element, attributes, ngModel) {
            validateEndDate = function (endDate, startDate) {
                if (endDate && startDate) {
                    return endDate >= startDate;
                }
                else {
                    return true;
                }
            }

            // use $validators.validation_name to do the validation
            ngModel.$validators.checkEndDate = function (modelValue) {
                var startdate = Date.parse(attributes.startDate);
                var enddate = Date.parse(modelValue);
                return validateEndDate(enddate, startdate);
            };
            
            // use $observe if we need to keep an eye for changes on a passed value
            attributes.$observe('startDate', function (value) {
                var startdate = Date.parse(value);
                var enddate = Date.parse(ngModel.$viewValue);
                
                // use $setValidity method to determine the validation result 
                // the first parameter is the validation name, this name is the same in ng-message template as well
                // the second parameter sets the validity (true or false), we can pass a function returning a boolean
                ngModel.$setValidity("checkEndDate", validateEndDate(enddate, startdate));
            });
        }
    };
});

See the example in Plunker.

Thursday, 16 April 2015

Form Validation in Ionic or AngularJS

On this post we will see how to do form validation in AngularJS or Ionic framework. At the moment, Ionic framework does not have its own validation features and relies on the underlying framework.

Basic Usage with Built-In Validators
To begin, first we need to make sure that we have a reference to angular.js file. Then we need to do the following steps:
- give a name to our form
- put novalidate="" attribute on the form
So we should have something like this on our form:
<form name="myForm" novalidate="" ng-submit="" >
 . . .
</form>
- add a unique name to each input field
- add validation attributes that we need on the input fields. AngularJS provides some validation directives such as these; required, pattern, minlength, maxlength, min and max. It also has basic built-in validators for these input types; text, number, url, email and date.
An example on an input field:
<input type="text" name="name" ng-model="vm.input.name" required />
- add an error message to be displayed for each validator checking the corresponding property name on input field's $error object in conditional statement like ng-if or ng-show. Properties under $error object with the same names as the validators' names exist for us to use. The properties are on this format; formName.fieldName.$error.builtInValidatorName. For example:
<span ng-show="myForm.name.$error.required == true">* required</span>
- on a method that is called in ng-submit, check whether the form is valid using formName.$valid before doing any action

Below is a complete example:
<!-- HTML page -->
<body ng-app="validationExample">
    <div ng-controller="MyCtrl as vm">
      <form name="myForm" novalidate="" ng-submit="vm.submitted(myForm, vm.input)">
        <div>
          <input type="text" name="name" ng-model="vm.input.name" required>
          <span ng-show="myForm.name.$error.required == true">test: * required</span>
        </div>
        <button>submit</button>
      </form>
    </div>
</body>

// JavaScript codes
var myApp = angular.module('validationExample', [])

myApp.controller('MyCtrl', [function () {
    var vm = this;
    vm.submitted = function(form, input) {
      if(form.$valid) {
        // post the form
        . . .
      }
    }
} ]);
See these codes on Plunker.


Creating Custom Validator
If we would like to create a custom validator, we would need to do these in addition of the above steps:
- create a directive
- on the directive's link function, use the new AngularJS $validators object on ngModel controller to do the validation. We need to create a function property on ngModel.$validators. By doing so, we register the function as a new validation rule. Each time the model changes, the validation rule is invoked. For example:
link: function(scope, element, attributes, ngModel) {
 ngModel.$validators.wrongInitial = function(modelValue) {
   . . .
 };
}
A corresponding property on input field's $error object is also created that can be used for checking validity.
- similar like in using built-in validator, we need to add an error message to be displayed by checking the newly created corresponding property on the input field's $error object:
<span ng-show="myForm.name.$error.wrongInitial == true">* must start with letter r</span>

Additional codes to be added from the first example:
<!-- HTML page -->
<input type="text" name="name" ng-model="vm.input.name" start-with-r />
<span ng-show="myForm.name.$error.wrongInitial == true">* must start with letter r</span>

// JavaScript codes
myApp.directive("startWithR", function() {
    return {
      restrict: "A",
      require: "?ngModel",
      link: function(scope, element, attributes, ngModel) {
        ngModel.$validators.wrongInitial = function(modelValue) {
          if (modelValue) {
            return modelValue.charAt(0).toUpperCase() == 'R';
          }
          else {
            return true;
          }
        };
      }
    };
});
See a complete example on Plunker.


Using ngMessages to Display Error Messages
In the recent version of AngularJS, started with version 1.3, there is a new feature that we could use to display messages including form error messages. To use this, we need to:
- include a reference to angular-messages.js file in our page
- inject ngMessages module to our AngularJS app
var myApp = angular.module('validationExample', ['ngMessages'])
- if there are only a few fields to be validated, we could specify each error message for each field like this:
<!-- use ng-messages with each field's $error object in this format; formName.fieldName.$error -->
<div ng-messages="myForm.name.$error" >  
        <!-- specify an error message for each validator -->
 <span ng-message="required">Name is required!</span>
 <span ng-message="wrongInitial">Name must start with letter r</span>
</div>
- however if we know that we are going to repeat this multiple times, we could use a template instead:
<!-- create a template by using script tag with type="text/ng-template" and give an Id -->
<script id="errors-template.html" type="text/ng-template" >  
        <!-- specify an error message for each validator -->
 <span ng-message="required">* required!</span>
 <span ng-message="wrongInitial">* must start with letter r!</span>
</script>
Then use the template to display error message(s) on each field:
<!-- use ng-messages with each field's $error object and ng-messages-include referring to the template Id -->
<span ng-messages="myForm.name.$error" ng-messages-include="errors-template.html" />
See the complete example on Plunker.


References:
Working with Validators and Messages in AngularJS
Validation in Ionic Framework Apps with ngMessages

Friday, 27 March 2015

How to Pull File from an Android App

The other day when I tested my Apache Cordova based app, I tried to get a file from my Android phone to test that the file was created correctly by the app. I wanted to copy the file to a location somewhere in my Windows based desktop. Below are the steps of how I do this:
- open a command prompt
- enter 'adb shell', you might need to run this inside a folder that has adb.exe if that is not accessible globally
- enter 'run-as    your.application.name'
- enter 'chmod 777    targeted_File_Path' in order to give full access to the file
So after entering these commands, the command prompt would look like this:
C:\Users\Me>adb shell
shell@zara:/ $ run-as your.application.name
run-as your.application.name
shell@zara:/data/data/your.application.name $ chmod 777 /data/data/your.application.name/files/filename.jpg
data/data/your.application.name/files/filename.jpg                   <
shell@zara:/data/data/your.application.name $ 
- quit the adb shell or open a new command prompt window
- enter 'adb pull    targeted_File_Path    destination_Desktop_Folder'
On command prompt, it will be something like:
C:\Users\Me>adb pull /data/data/your.application.name/files/filename.jpg c:/DestinationDirectory
2097 KB/s (19335 bytes in 0.009s)


References:
http://stackoverflow.com/questions/13006315/how-to-access-data-data-folder-in-android-device
http://www.codeproject.com/Articles/825304/Accessing-internal-data-on-Android-device

Thursday, 19 March 2015

Example of Writing Image to File in Apache Cordova

Below is an example of how to writing image to a file in Apache Cordova. To have more understanding of basic file operations, please see my previous post.
window.resolveLocalFileSystemURL(my_Directory_Path,
function (dirEntry) {
 dirEntry.getFile(path_Of_File_To_Be_Written, { create: true }, 
  // getFile() success
  function (fileEntry) {
   fileEntry.createWriter(
    // createWriter() success
    function (fileWriter) {
     fileWriter.onwriteend = function (e) {
      . . .
     };

     fileWriter.onerror = function (e) {
      . . .
     };

     fileWriter.write(dataURIToBlob(getBase64Image(image_To_Be_Written, "image/jpeg")));
     // or if we already have a canvas
     //fileWriter.write(dataURIToBlob(canvas.toDataURL("image/jpeg")));
    }, 
    // createWriter() error
    function (error) {
     . . .
    }
   );
  },
  // getFile() error
  function (error) {
   . . .
  }
 );
});

A function to convert an image's base-64 encoded data to Blob type (this is taken from http://stackoverflow.com/questions/12391628/how-can-i-upload-an-embedded-image-with-javascript):
function dataURIToBlob(dataURI) {
 // serialize the base64/URLEncoded data
 var byteString;
 if (dataURI.split(',')[0].indexOf('base64') >= 0) {
  byteString = atob(dataURI.split(',')[1]);
 }
 else {
  byteString = unescape(dataURI.split(',')[1]);
 }

 // parse the mime type
 var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]

 // construct a Blob of the image data
 var array = [];
 for (var i = 0; i < byteString.length; i++) {
  array.push(byteString.charCodeAt(i));
 }
 return new Blob(
  [new Uint8Array(array)],
  { type: mimeString }
 );
}
Also a function to get base-64 encoded data from an image:
// type is either "image/png" or "image/jpeg"
function getBase64Image(img, type) {
    // create an empty canvas element
    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;

    // copy the image contents to the canvas
    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);

    // get the base-64 encoded data
    var dataURL = canvas.toDataURL(type);
 // toDataURL() actually has two optional parameters:
 // - type. The default value is 'image/png'.
 // - jpegQuality. A decimal value ranging from 0 to 1 to determine the quality of the data to be generated from a jpeg type image.
 //   The default value is browser dependant.

    return dataURL;
}