Modern Android App Development with Kotlin/

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Data Validation and Error Handling

Data Validation and Error Handling

Learn and practice how to validate user input text and show error messages in this lesson.

We'll cover the following...

View binding

Because we are going to use view references across different methods, we need to declare binding as a global variable (1).

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class LoginActivity : AppCompatActivity() {



    private lateinit var binding: ActivityLoginBinding // 1



    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)



        binding = ActivityLoginBinding.inflate(layoutInflater)

        setContentView(binding.root)

    }

}

Validation & error message

To perform validation, we need to set a click listener on the login button via the setOnClickListener method, which accepts the OnClickListener interface as a parameter. When the login button is clicked, we execute our onLoginClicked method.

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class LoginActivity : AppCompatActivity() {



    private lateinit var binding: ActivityLoginBinding



    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)



        binding = ActivityLoginBinding.inflate(layoutInflater)

        setContentView(binding.root)



        binding.loginButton.setOnClickListener(object : View.OnClickListener {

            override fun onClick(v: View?) {

                onLoginClicked()

            }

        })

    }

}

Click listener anonymous object implementation (1) can be slightly simplified to anonymous function (2) or lambda (3).

In Kotlin, if lambda is the last parameter it can be moved outside function parentheses (4). ...