Every webpage can be considered as object and it exists inside a browser window. We can access the webpage using the web browser and it needed connected to the internet. The DOM is the acronyms of Document object model. A Document object denotes the HTML document that is displayed in that window. The document object model consists of several properties that refer to the other objects which give the facility to modify the document content. The process that a content of document is accessed is called the Document Object Model, or DOM. The Objects are organized in a hierarchy. The hierarchical structure uses to the organization of objects in a web document. Below is the hierarchy of the few important DOM objects. We can manipulate the objects and element in the DOM by using the dart:html library. Console-based application cannot use the dart:html library. In work with the HTML library in the web application, we need to import the dart:html. Let’s understand the DOM operation in the following section. Finding DOM Elements A document can contain many attributes sometime we need to search particular attribute. The dart:html library provides the querySelector function to search element in the DOM. The querySelector() function returns the first element that matches the specified group of the selector. Let’s understand the. following syntax. Let’s understand the following example. Example – We create a HTML file name index.html and also create a dart file. Main.dart Event Handling The dart:html library provides the onClick event for DOM Elements. The syntax shows how an element could handle a stream of click events. The querySelector() function returns the element from the given DOM and onClick.listen() will take an eventHandler method which will be invoked when a click event is raised. The syntax of eventHandler is given below ? Let us now take an example to understand the concept of Event Handling in Dart. TestEvent.html TestEvent.dart

Unit testing is a part of the software development process where particular unit/components of an application are tested. Unit testing of each component is necessary to enhance the application performance. We can say that a unit is the smallest testable chunk of a program that can be logically isolated in a system. In various programming languages, the individual program, a subroutine, a function, a method, or a class can be represented as a Unit. There can be many individual units within the module. In object-oriented programming, the method that belongs to base class/superclass, an abstract class can be represented as smaller units. Below image displays the type of testing. Unit Testing Task The unit testing task is given below. Advantage of Unit Testing Few advantages of unit testing are given below. Dart Unit Testing In Dart, we required to include an external library named “test”, which facilitates a standard way of writing and running individual unit test. Unit testing can be achieved using the following steps. Step – 1: Installing “test” package To include the unit testing in our project, we must install a third-party packages “test” in our current working project. Let’s open the pubspec.yaml in the our project and type the following statement. Now, right-click on pubspec.yaml file and click on Pub: get dependencies. This will install the “test” package in our project. We can also install it by using the following command. Step – 2: Importing “test” package Type the following line to import the “test” package in your project. Step – 3: Writing Test Cases The top-level test() function is added by the Test cases. In the test() function, we make the test assertion using the expect() function. The expert() function takes two arguments actualValue and MatchValue. Syntax: Group of Test Cases We can create a group of multiple test cases using the group() function. It helps grouping of test cases based on some criteria. The description of each group is specified in the beginning of its test’s description. Syntax: The syntax is given below. Example – 1(Passing Test) Here is the example where we define an add() method for unit test. It accepts two parameters as an integer values and returns an integer representing the sum. To test the add() method, understand the following example – Step – 1: We import the test package. Step -2: We define the test using the test() function and it uses the expert() function to impose an assertion. Output: Example – 2 A unsuccessful Test We define the sub() method which has a logical mistake. Let’s have a following example. Output: In the above example, the add() function is tested successfully but the sub() function failed the unit test due to logical error. Grouping Test Cases We can write the multiple test cases in the form of group. The group() is used to grouped together these methods. It helps to write much cleaner code. In the following example, we are writing a test case for the split() function and the trim() function. We grouped together these function and name it String. Let’s understand the following example – Example – Output:

The Dart concurrency allows us to run multiple programs or multiple parts of a program simultaneously. It executes the several instructions at the same time. Dart provides the Isolates as a tool for doing works for parallel. The concurrency makes the program highly effective and throughput by utilizing the unused capabilities of essential operating system and machine hardware. How to achieve concurrency? In Dart, we can achieve concurrency by using the Isolates. We have discussed Dart isolates in previous tutorial. Here we will understand the brief introduction of it. Dart isolate is a version of the thread. But there is key difference between the common implementation of “Thread” or “Isolates”. The isolate works differently in comparison of Thread. The isolates are independent workers that do not share memory, but instead interconnect by passing message over channels. Since isolates completes its task by passing message thus it need a way to serialize a message. The communication between the isolates is done by the message passing as a client and server. It helps the program to take advantage of multicore microprocessor out of the box. Dart provides the dart:isolate package to apply the isolate in our program. It provides the solution to taking single-threaded Dart code and allowing application to make greater use of the hardware available. Let’s understand the following example – Example – Output: Output 2: The concept of the above code is similar to the dart isolates. If we run the above program multiple times, then output will be varied every time.

Dart Async is related to asynchronous programming. It executes the asynchronous operation in a thread. It ensures that the critical functions to be executed until completion. The asynchronous operation is executed, separately the main application thread. In Dart, one operation cannot interrupt the other operation; it means one operation can execute at a time no other part of the program can avert it. Let’s understand the following example – Example – Output Explanation: In the above code, we used the readLineSync(), which is synchronous method. It means; the execution of all instructions that follow the readLineSync() method call will be blocked until the readLineSync() method doesn’t complete its execution. The stdin.readLineSync () executes nothing until it gets the input from the user. It waits for the user input for further execution. Difference between Asynchronous and Synchronous Let’s understand the difference between synchronous and asynchronous. In computer science, if we say a particular program is synchronous, that means it waits for an event to execute further. This approach is driven with a demerit, if a part of the code takes much time to execute, the succeeding blocks through an unrelated block will be blocked from executing. This is the main problem of the synchronous approach. A part of the program may require executing before the current part, but the synchronous approach doesn’t allow it. This is not suitable for the webservers, where request must be independent of the others. It means, the webserver does not wait to finish the execution of the current request, it responds to the request from the other users. The web server should accept the request from the other user before executing the previous requests. This approach is called asynchronous programming. The asynchronous programming generally focuses on no waiting or non-blocking programming model. The dart: async is facilitated to implement the asynchronous programming block in a Dart script. Example – We create a file with the few names and save this file as names.txt and write a program to read this file without delaying the other part of the code. Consider the following code. Output Dart Future The Dart Future is defined as getting a result sometime in the future. The Future object uses to facilitate asynchronous programming. Future objects are a tool to denote values returned by an expression whose execution will complete at a later point in time (In Future). In order to work with the future, we can use either async and await or the Future API. Dart async and await The async and await keywords are allowed to implement asynchronous programming without using the Future API. The async keyword is necessary to run function asynchronously; we need to add async after the function name. The syntax is given below: Syntax: When an async function is invoked, the Future object instantly returns and that indicates the async function will execute later. Once the body of the async function is executed, the function call returned the Future object. The function call will be completed with its result. Dart await Keyword The await keyword is also used to execute function asynchronously. It suspends the currently running function until the result is ready. When it returns the result, then it continues on to the next line of code. The await keyword can only be used with async functions. The syntax is given below. Syntax: Here, e is an asynchronous expression, and it is expected to evaluate to a Future. The await expression evaluates e, and then suspends the currently running function until the result is ready. Let’s understand the following example – Example – 1 Output Explanation: Here, we have declared the main() function asynchronous using the async keyword because we call the hii() method asynchronously. Then, we used await modifier to call hii() that executed asynchronously.

Dart allows us to asynchronous programming which runs our program without getting blocked. The asynchronous programming is used to achieve concurrency. Dart isolate is a version of the thread. But there is key difference between the common implementation of “Thread” or “Isolates”. The isolate works differently in comparison of Thread. The isolates are independent workers that do not share memory, but instead interconnect by passing message over channels. Since isolates completes its task by passing message thus it need a way to serialize a message. The communication between the isolates is done by the message passing as a client and server. It helps the program to take advantage of multicore microprocessor out of the box. Dart provides the dart:isolate package to apply the isolate in our program. It provides the solution to taking single-threaded Dart code and allowing application to make greater use of the hardware available. Create and Start an Isolate Dart provides the spawn() method to create an isolate. It must be declared with an ‘entry point’ with a single parameter. This parameter displays a port which isolate use to refer back notification message. Let’s understand the following example – Example – Output: Output 2: Explanation: In the above program, the spawn method of the isolate class executed a function sayhii in parallel of remaining code. It takes two parameters. We have two functions sayhii() and main() function might not run in the same order each time. If you run the above program, then the output will be different each time as we can see in second output. Note – We can also pass NULL value if there is no object to pass in spawned function. Let’s understand the another example – Example – 2 Explanation: In the above code, we created an asynchronous method start() which creates a port and spawn an isolate. We signified the start method as async because of wan can await the response from the spawning of the isolates and to store a reference to the new isolate. It is essential when we want to kill the running isolates. We passed the two parameters in the spawn() method, the first parameter runTimer method, that is a callback function to execute runTimer() and second parameter sendPort which is a callback function and it will used to send message back to the caller. The start() method starts listening the receiverPort for message from isolate. Once it will receive the message then it will print as a console output. The runTimer() method begins a timer that fires every second in order to update a counter. It sends a notification message via the port which it received when the isolate was spawned. Stop an Isolate The dart: isolates package provides the kill() method which is used to stop a running isolate. Let’s understand the following example. Example – Explanation: In the above example, we have declared a stop() method that will destroy the running isolate and sets its reference to null. We defined the priority of isolate: immediate that will terminate the isolate as soon as. Complete Program Output:

Dart provides the facility to call class instances like a function. To make callable class, we need to implement a call() method in it. Let’s understand the following example – Example – 1 Output Explanation: In the above code, We defined a call() function in the class Student that takes two arguments String name, integer age and return a message with this information. Then, we have created the object of class Student and called it like a function. Let’s have a look at another example – Example – 2 Multiple callable class Output Explanation: In the above code, we defined two callable functions in class Student and class Employee. The Employee class call() function accepts two parameters as String empid and int age. We called instances of both classes as callable functions.