Android Studio Kotlin 应用 "keeps stopping" 当我运行它

Question

我只是想设置一个配置为下拉菜单的微调器，但我什至无法测试我的应用程序以查看它是否有效，因为每次我运行它都会立即崩溃。我知道我的问题与 MainActivity.kt 文件中第 21 行的空对象引用有关。这是问题代码： val spinner: Spinner = findViewById<Spinner>(R.id.locations) 微调器的 id 是位置，所以我不确定为什么它会作为空值返回。这也是该文件的完整代码：


import android.app.Activity
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Spinner
import android.widget.ArrayAdapter
import android.widget.AdapterView

private var userLocation: Any = ""
private var userDestination: Any = ""


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

    val spinner: Spinner = findViewById<Spinner>(R.id.locations)
    val locationsAdapter: ArrayAdapter<CharSequence> = ArrayAdapter.createFromResource(
        this,
        R.array.rooms,
        android.R.layout.simple_spinner_item
    ).also { adapter ->
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)
        spinner.adapter = adapter
    }
}

class SpinnerActivity : Activity(), AdapterView.OnItemSelectedListener {
    override fun onItemSelected(parent: AdapterView<*>, view: View?, pos: Int, id:Long) {
        userLocation = parent.getItemAtPosition(pos)
        val spinner: Spinner = findViewById(R.id.locations)
        spinner.onItemSelectedListener = this
    }
    override fun onNothingSelected(parent: AdapterView<*>) = Unit
}

Answer 1

为 MainActivity 声明全局微调器：

private lateinit var spinner: Spinner

在 onCreate 方法中初始化微调器并设置其适配器，我建议您按如下方式设置数组适配器：

spinner = findViewById<Spinner>(R.id.locations)

spinner.adapter = ArrayAdapter(this, R.layout.drop_down_generic_item, resources.getStringArray(R.array.rooms))

我不太明白你在问题中展示的SpinnerActivity，抱歉。但是在 MainActivity 中你可以设置 spinner.onItemSelectedLister() 如下：

spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
            override fun onItemSelected(adapterView: AdapterView<*>?, view: View?, position: Int, p3: Long) {
                val selectedRoom = adapterView?.getItemAtPosition(position)
            }

            override fun onNothingSelected(p0: AdapterView<*>?) {
                
            }
}

我不想让您感到困惑，但请看一下带有 AutoCompleteTextView 的 TextInputLayout，以作为提示。 :D

Android Studio Kotlin 应用 "keeps stopping" 当我运行它

Android Studio Kotlin app "keeps stopping" when I run it

null

kotlin

android-studio

Android Studio Kotlin 应用 "keeps stopping" 当我 运行 它

Android Studio Kotlin app "keeps stopping" when I run it

null

kotlin

android-studio

Android Studio Kotlin 应用 "keeps stopping" 当我运行它