In this tutorial, we’ll be discussing and implementing Spinners in our Android Application using Kotlin. Android Spinner is used to create a drop-down list on the screen.

在本教程中，我们将使用Kotlin在Android应用程序中讨论和实现Spinners。 Android Spinner用于在屏幕上创建一个下拉列表。

什么是Android Spinner？ (What is Android Spinner?)

Spinners are like a drop-down menu that contains a list of items to select from. Once a value is selected the Spinner returns to its default state with that selected value.

微调框就像一个下拉菜单，其中包含要选择的项目列表。 选择一个值后，微调框将使用所选值返回其默认状态。

After Android 3.0, it’s not possible to display a prompt in a Spinner as the default state in the Spinner. Instead, the first item is displayed.

在Android 3.0之后，无法在Spinner中显示提示作为Spinner中的默认状态。 而是显示第一项。

Data inside a spinner is loaded with an Adapter.

微调器内部的数据已加载有适配器。

Take the following scenario:

请采取以下情况：

Imagine you need to charge your phone. For that, you must connect your phone charger to the electricity board using a pin (adapter). Then the adapter provides your phone with electricity. In Android, the Spinner is like your phone which is loaded with data using an Adapter. The adapter sets the data as well as the layout for the items to be loaded in the Spinner.

假设您需要为手机充电。 为此，必须使用引脚（适配器）将手机充电器连接到配电板上。 然后，适配器为您的手机供电。 在Android中，Spinner就像您的手机一样，它使用Adapter加载了数据。 适配器设置数据以及要在微调器中加载的项目的布局。

微调器回调事件 (Spinner Callback Events)

AdapterView.onItemSelectedListener interface is used to trigger the Spinner click event callbacks.

AdapterView.onItemSelectedListener接口用于触发Spinner点击事件回调。

It consists of two methods:

它包含两种方法：

• onItemSelectedonItemSelected
• onNothingSelectedonNothingSelected

In the following section, we’ll create a new Android Studio Project and implement Spinners in our Application. We’ll customize the layouts and learn how to handle different scenarios.

在下一节中，我们将创建一个新的Android Studio项目并在应用程序中实现Spinners。 我们将自定义布局，并学习如何处理不同的场景。

Android Spinner Kotlin项目 (Android Spinner Kotlin Project)

1. XML布局代码 (1. XML Layout Code)

The code for the activity_main.xml layout file is given below.

下面给出了activity_main.xml布局文件的代码。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical"android:id="@+id/linearLayout"android:gravity="center"tools:context=".MainActivity"><Spinnerandroid:id="@+id/mySpinner"android:layout_width="match_parent"android:spinnerMode="dialog"android:layout_height="wrap_content" /></LinearLayout>

It hosts a single Spinner at the moment android:spinnerMode can be either dialog or dropdown.

当android：spinnerMode可以是dialog或dropdown它会托管一个Spinner。

2.微调器XML代码 (2. Spinner XML Code)

The code for spinner_right_aligned.xml is given below.

下面给出了spinner_right_aligned.xml的代码。

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"android:id="@+id/textView"android:layout_width="match_parent"android:layout_height="wrap_content"android:gravity="end"android:padding="15dp"android:textAlignment="gravity"android:textColor="@color/colorPrimary"android:textSize="16sp"/>

3. MainActivity Kotlin代码 (3. MainActivity Kotlin Code)

The code for the MainActivity.kt class is given below.

MainActivity.kt类的代码如下。

package net.androidly.androidspinnerkotlinimport android.content.Context
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.Gravity
import android.view.View
import android.widget.*
import kotlinx.android.synthetic.main.activity_main.*class MainActivity : AppCompatActivity(), AdapterView.OnItemSelectedListener {var languages = arrayOf("Java", "PHP", "Kotlin", "Javascript", "Python", "Swift")val NEW_SPINNER_ID = 1override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)var aa = ArrayAdapter(this, android.R.layout.simple_spinner_item, languages)aa.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)with(mySpinner){adapter = aasetSelection(0, false)onItemSelectedListener = this@MainActivityprompt = "Select your favourite language"gravity = Gravity.CENTER}val spinner = Spinner(this)spinner.id = NEW_SPINNER_IDval ll = LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT)ll.setMargins(10, 40, 10, 10)linearLayout.addView(spinner)aa = ArrayAdapter(this, R.layout.spinner_right_aligned, languages)aa.setDropDownViewResource(R.layout.spinner_right_aligned)with(spinner){adapter = aasetSelection(0, false)onItemSelectedListener = this@MainActivitylayoutParams = llprompt = "Select your favourite language"setPopupBackgroundResource(R.color.material_grey_600)}}override fun onNothingSelected(parent: AdapterView<*>?) {showToast(message = "Nothing selected")}override fun onItemSelected(parent: AdapterView<*>?, view: View?, position: Int, id: Long) {when (view?.id) {1 -> showToast(message = "Spinner 2 Position:${position} and language: ${languages[position]}")else -> {showToast(message = "Spinner 1 Position:${position} and language: ${languages[position]}")}}}private fun showToast(context: Context = applicationContext, message: String, duration: Int = Toast.LENGTH_LONG) {Toast.makeText(context, message, duration).show()}
}

Important Points:

重要事项 ：

• Thanks to Kotlin Android extensions, the XML Spinner widget is automatically available in our Kotlin Activity class.感谢Kotlin Android扩展，XML Spinner小部件可在我们的Kotlin Activity类中自动提供。
• We’ve created an arrayOf strings that consist of programming languages. These are filled in the adapter using the ArrayAdapter.我们创建了一个由编程语言组成的arrayOf字符串。 这些使用ArrayAdapter填充到适配器中。
• The setDropDownViewResource is used to set the layout for the selected state and the spinner list rows.setDropDownViewResource用于设置选定状态和微调setDropDownViewResource列表行的布局。
• The android.R.layout.simple_spinner_item is used to set the default android SDK layout. By default, the TextView is left aligned in this type of layout.android.R.layout.simple_spinner_item用于设置默认的android SDK布局。 默认情况下， TextView在这种布局中保持对齐。

We’ve created a second Spinner programmatically that loads the layouts from the spinner_right_aligned.xml file.

我们已经以编程方式创建了第二个Spinner，它从spinner_right_aligned.xml文件中加载布局。

How does it work?
The setSelection() method tells the Activity that the first spinner item was already selected. We must place this statement before onItemSelectedListener = this.

它是如何工作的？
setSelection（）方法告诉Activity已经选择了第一个微调器项目。 我们必须将此语句onItemSelectedListener = this之前。

The setPopupBackgroundResource is used to set the background color on the dropdown list.

setPopupBackgroundResource用于在下拉列表上设置背景颜色。

Inside the onItemSelected function, we use the when statement to trigger a Toast for the respective Spinner item.

在onItemSelected函数内部，我们使用when语句为相应的Spinner项触发Toast。

Thanks to Kotlin and functions with default values, we’ve reduced the verbose call to the Toast.

感谢Kotlin和具有默认值的函数，我们减少了对Toast的冗长调用。

4. Spinner Kotlin应用程序输出 (4. Spinner Kotlin App Output)

The following is the output when the above application was run on an emulator.

以下是在模拟器上运行上述应用程序时的输出。

You can download the source code of the above project from the link below.
AndroidSpinnerKotlin

您可以从下面的链接下载上述项目的源代码。
AndroidSpinnerKotlin

翻译自: https://www.journaldev.com/214/android-spinner-using-kotlin