android - 为什么每次从 Kotlin 的 ListPreference 中选择一个项目后，val 变量总是会改变？

Question

ListPreference 绑定(bind)键 "@string/WindowScaleName"，val oldValue 来自同一个键 "@string/WindowScaleName"

变量oldValue是val，我以为它会保持不变，但实际上，它总是在我从ListPreference中选择一个项目后发生变化，我没有为什么。你可以如下图。

顺便说一句，val other 保持不变!

XML

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
    android:key="AppPreference"
    android:summary="@string/PreferenceSummary"
    android:title="@string/PreferenceTitle" >

    <ListPreference
            android:key="@string/WindowScaleName"
            android:entries="@array/WindowsScale"
            android:entryValues="@array/WindowsScale_value"
            android:defaultValue="@string/WindowScaleDefaultValue"
            android:title="@string/PreferenceWindowScaleTitle"
    />

</PreferenceScreen>

代码A

class UIPreference : AppCompatPreferenceActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.layout_preference)
        fragmentManager.beginTransaction().replace(R.id.content, MyPreferenceFragment()).commit()

    }

    class MyPreferenceFragment : PreferenceFragment() {
        private lateinit var mContext: Context

        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            addPreferencesFromResource(R.xml.mypreference)
            mContext=this.activity
            val mActivity=this.activity

            bindListPreferenceSummaryToValue( findPreference(mContext.getString(R.string.WindowScaleName)) )          

            val windowScaleListPreferenec = preferenceManager.findPreference(getString(R.string.WindowScaleName)) as ListPreference
            val oldValue: String by PreferenceTool(mContext, getString(R.string.WindowScaleName), getString(R.string.WindowScaleDefaultValue) )
            val other=oldValue

            logError("Me :$oldValue other:$other")

            windowScaleListPreferenec.onPreferenceChangeListener = Preference.OnPreferenceChangeListener { preference, newValue ->

               var isRestartApp:Boolean by PreferenceTool(mContext, getString(R.string.IsRestartAppName), false)
               isRestartApp = !(oldValue==newValue.toString())

               logError("old: $oldValue other:$other  new: $newValue istrue: $isRestartApp")
               true
            }
        }
    }
}

代码B

fun bindListPreferenceSummaryToValue(preference: Preference) {
    preference.onPreferenceChangeListener = sBindListPreferenceSummaryToValueListener

    sBindListPreferenceSummaryToValueListener.onPreferenceChange(preference,
            PreferenceManager
                    .getDefaultSharedPreferences(preference.context)
                    .getString(preference.key, ""))
}

private val sBindListPreferenceSummaryToValueListener = Preference.OnPreferenceChangeListener { preference, value ->
    val stringValue = value.toString()
    if (preference is ListPreference) {
        val listPreference = preference
        val index = listPreference.findIndexOfValue(stringValue)

        preference.setSummary(
                if (index >= 0)
                    listPreference.entries[index]
                else
                    preference.summary)

    }
    true
}

代码 C

class PreferenceTool<T>(private val context: Context, private val name: String,  private val default: T) {

    private val prefs: SharedPreferences by lazy {       
        context.defaultSharedPreferences 
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T = findPreference(name, default)

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        putPreference(name, value)
    }

    @Suppress("UNCHECKED_CAST")
    private fun findPreference(name: String, default: T): T = with(prefs) {
        val res: Any = when (default) {
            is Long -> getLong(name, default)
            is String -> getString(name, default)
            is Int -> getInt(name, default)
            is Boolean -> getBoolean(name, default)
            is Float -> getFloat(name, default)
            else -> throw IllegalArgumentException("This type can be saved into Preferences")
        }

        res as T
    }

    @SuppressLint("CommitPrefEdits")
    private fun putPreference(name: String, value: T) = with(prefs.edit()) {
        when (value) {
            is Long -> putLong(name, value)
            is String -> putString(name, value)
            is Int -> putInt(name, value)
            is Boolean -> putBoolean(name, value)
            is Float -> putFloat(name, value)
            else -> throw IllegalArgumentException("This type can't be saved into Preferences")
        }.apply()
    }
}

图片

Answer 1

oldValue 是一个 local delegated property ，不仅仅是一个简单的只读变量。

这意味着每次访问它时都会调用一个委托(delegate)。没有什么能阻止它每次返回不同的值。但是您不能自己设置它的值(因为它是 val)。