Wednesday, April 4, 2018

android kotlin - AlertDialog setSingleChoiceItems example

AlertDialog setSingleChoiceItems()
The AlertDialog is a subclass of Dialog that can display buttons, a simple text message, a list of items, single-choice items, multiple-choice items, etc. Developers can display a more complex View on the alert dialog window using a custom View.

The following android application development tutorial will demonstrate to us how we can display a list of single-choice items in the alert dialog window as the content. And how developers can be notified when users select an item from the single choice items list via the supplied listener. Android developers can add a click listener for the alert dialog single-choice list items.

To create an alert dialog developers have to initialize an instance of AlertDialog.Builder object. This AlertDialog Builder object can define the alert dialog’s various properties such as button, message, icon, list and etc. Finally, they can create an AlertDialog using this builder instance.

So how can android developers display a list of single-choice items with a click listener inside the alert dialog? The single-choice items list means users are allowed to select only one item from the list. When they select an item from the list the previously selected item will be unchecked. They can change their selection but at a time only one item can be selected/checked.

By using the AlertDialog Builder instance developers can set its content to a list of single-choice items. The android SDK displays the list of single-choice items inside a dialog window in a good format and position.

The AlertDialog Builder object’s setSingleChoiceItems() method set a list of single-choice items to be displayed as the content of the alert dialog window. This method also allows us to set a click listener for the list items. So developers can be programmatically notified in real-time which item is selected from the list.

But commonly developers get the users selected item on the positive button click event. They do not dismiss the alert dialog on item selection, so users can change their selection and submit their choice on the button click event.

This setSingleChoiceItems() method takes three arguments which are ‘items’, ‘checkedItem’, and ‘listener’. The ‘items’ argument represents an array of items, the ‘checkedItem’ define the initially checked/selected item of the list, and the ‘listener’ argument passes a click event listener for the list items.

Practically the AlertDialog Builder setSingleChoiceItems() method renders a RadioButtons group inside its window. Developers can set an initially selected RadioButton or they can uncheck all RadioButtons on load. To fully uncheck all RadioButtons from this RadioGroup we have to pass ‘-1’ for the value of the ‘checkedItem’ argument. To check a RadioButton on the load we have to pass the index number of the item to the ‘checkedItem’ argument value. Android SDK decorates and populates the RadioGroup in a good format inside the alert dialog window, developers have to do nothing for it.

This android kotlin tutorial code is written in an android studio IDE. Copy the code and paste it into your android studio IDE and run it on an emulator to see how we create an AlertDialog with a single-choice list of items and allow users to select only an item from them in an android application.

And how we can handle the click events for the alert dialog’s single-choice items list in an application. We displayed screenshots of the emulator screen, which also help you to understand the code without running it on an android device or emulator.
MainActivity.kt

package com.cfsuman.kotlintutorials

import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AlertDialog
import androidx.appcompat.app.AppCompatActivity


class MainActivity : AppCompatActivity() {
    private lateinit var context:MainActivity
    private lateinit var textView:TextView
    var checkedItem:Int = -1

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // Get the context
        context = this;

        // Get the widgets reference from XML layout
        textView = findViewById(R.id.textView)
        val button = findViewById<Button>(R.id.button)

        // Button click listener
        button.setOnClickListener{
            // Show the single choice list items on an alert dialog
            showDialog()
        }
    }


    // Method to show an alert dialog with single choice list items
    private fun showDialog(){
        // Late initialize an alert dialog object
        lateinit var dialog:AlertDialog

        // Initialize an array of colors
        val array = arrayOf(
            "Red","Green","Yellow",
            "Black","Pink","Cyan"
        )

        // Initialize a new instance of alert dialog builder object
        val builder = AlertDialog.Builder(context)

        // Set a title for alert dialog
        builder.setTitle("Choose a color.")

        // Set the single choice items for alert dialog
        // with initial selection
        builder.setSingleChoiceItems(array,checkedItem){ _, which ->
            // Get the dialog selected item index
            checkedItem = which
        }

        // Set the positive/yes button click listener
        builder.setPositiveButton("OK") { _, _ ->
            // Show the dialog selected item to text view
            if (checkedItem!=-1){
                val selected = array[checkedItem]
                textView.text = "Selected item : $selected"
            }else{
                textView.text = "Nothing selected"
            }
        }

        // Set the dialog neutral/cancel button
        builder.setNeutralButton("Cancel"){_, _ ->
            textView.text = "Dialog cancelled"
        }

        // Initialize the AlertDialog using builder object
        dialog = builder.create()

        // Finally, display the alert dialog
        dialog.show()
    }
}
activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rootLayout"
    android:background="#FEFEFA"
    android:padding="24dp">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show Options"
        android:textAllCaps="false"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="32dp"
        android:fontFamily="sans-serif"
        android:textSize="24sp"
        app:layout_constraintStart_toStartOf="@+id/button"
        app:layout_constraintTop_toBottomOf="@+id/button" />

</androidx.constraintlayout.widget.ConstraintLayout>
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