[移动开发]Spinner （点击展示选项）使用方法

1. xml 使用spinner控件
? ? ? ? <Spinner
? ? ? ? ? ? android:id="@+id/orientation_spinner"
? ? ? ? ? ? android:layout_width="wrap_content"
? ? ? ? ? ? android:layout_height="wrap_content" />
?? ??? ??? ?
2. 代码加载spinner控件
? ? ? ? val spinner = findViewById<Spinner>(R.id.orientation_spinner)
3. 创建adapter(注意setDropDownViewResource)
? ? ? ? val adapter = ArrayAdapter(spinner.context, android.R.layout.simple_spinner_item,
? ? ? ? ? ? arrayOf(HORIZONTAL, VERTICAL))?? ?
? ? ? ? adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)
?? ??? ?spinner.adapter = adapter?? ?
4. spinner 设置选择?? ??? ?
? ? ? ? val initialPosition = adapter.getPosition(orientationToString(orientation))
? ? ? ? if (initialPosition >= 0) {
? ? ? ? ? ? spinner.setSelection(initialPosition)
? ? ? ? }
5. spinner设置点击响应
? ? ? ? spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
? ? ? ? ? ? override fun onItemSelected(
? ? ? ? ? ? ? ? parent: AdapterView<*>,
? ? ? ? ? ? ? ? view: View?,
? ? ? ? ? ? ? ? position: Int,
? ? ? ? ? ? ? ? id: Long
? ? ? ? ? ? ) {
? ? ? ? ? ? ? ? viewPager.orientation = stringToOrientation(parent.selectedItem.toString())
? ? ? ? ? ? }

? ? ? ? ? ? override fun onNothingSelected(parent: AdapterView<*>?) {
? ? ? ? ? ? ? ? TODO("Not yet implemented")
? ? ? ? ? ? }

? ? ? ? }

（此外，可以获取到spinner选择的项，如getSelectedItemPosition， getSelectedItem等方法）
2~5完整代码：

package com.example.cardwithviewpager2

import android.view.View
import android.widget.AdapterView
import android.widget.ArrayAdapter
import android.widget.Spinner
import androidx.viewpager2.widget.ViewPager2

/**
 * 配置spinner显示方向选项，并设置ViewPager2的方方向 （持有Spinner 和 ViewPager2对象，相当于委托管理）
 */
class OrientationController(private val viewPager: ViewPager2, private val spinner: Spinner) {

    fun setup(){
        val orientation = viewPager.orientation
        val adapter = ArrayAdapter(spinner.context, android.R.layout.simple_spinner_item,
            arrayOf(HORIZONTAL, VERTICAL))
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)
        spinner.adapter = adapter
        
        val initialPosition = adapter.getPosition(orientationToString(orientation))
        if (initialPosition >= 0) {
            spinner.setSelection(initialPosition)
        }

        spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
            override fun onItemSelected(
                parent: AdapterView<*>,
                view: View?,
                position: Int,
                id: Long
            ) {
                viewPager.orientation = stringToOrientation(parent.selectedItem.toString())
            }

            override fun onNothingSelected(parent: AdapterView<*>?) {
                TODO("Not yet implemented")
            }

        }
    }

    private fun stringToOrientation(orientation: String): Int {
        return when(orientation) {
            HORIZONTAL -> ViewPager2.ORIENTATION_HORIZONTAL
            VERTICAL -> ViewPager2.ORIENTATION_VERTICAL
            else -> throw  IllegalAccessException("orientation $orientation doesn't exit")
        }
    }

    private fun orientationToString(orientation: Int): String {
       return when(orientation) {
           ViewPager2.ORIENTATION_HORIZONTAL -> HORIZONTAL
           ViewPager2.ORIENTATION_VERTICAL -> VERTICAL
           else -> throw  IllegalAccessException("orientation $orientation doesn't exit")
       }
    }

    companion object{
        const val HORIZONTAL = "Horizontal"
        const val VERTICAL = "Vertical"
    }
}

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