为什么需要用对象包装类 Preference<T>？

Question

以下代码A来自https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/extensions/DelegatesExtensions.kt

当我使用代码 A 时，我可以使用private var zipCode: Long by DelegatesExt.preference(this, ZIP_CODE, DEFAULT_ZIP) 来调用。

我不明白为什么作者用对象 DelegatesExt 包装类 Preference(...)

我觉得Code B更简单，我在使用Code B的时候可以使用private val zipCode: Long by Preference(this, ZIP_CODE, DEFAULT_ZIP)来调用

为什么类 Preference 需要用对象包装？

代码 A

object DelegatesExt {
    fun <T> notNullSingleValue() = NotNullSingleValueVar<T>()
    fun <T> preference(context: Context, name: String,
            default: T) = Preference(context, name, default)
}

class NotNullSingleValueVar<T> {

    private var value: T? = null

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            value ?: throw IllegalStateException("${property.name} not initialized")

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        this.value = if (this.value == null) value
        else throw IllegalStateException("${property.name} already initialized")
    }
}

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

    private val prefs: SharedPreferences by lazy {
        context.getSharedPreferences("default", Context.MODE_PRIVATE)
    }

    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()
    }
}

代码 B

class NotNullSingleValueVar<T> {

    private var value: T? = null

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            value ?: throw IllegalStateException("${property.name} not initialized")

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        this.value = if (this.value == null) value
        else throw IllegalStateException("${property.name} already initialized")
    }
}

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

    private val prefs: SharedPreferences by lazy {
        context.getSharedPreferences("default", Context.MODE_PRIVATE)
    }

    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()
    }
}

Answer 1

当我使用 Code A 时，我可以使用 private var zipCode: Long by DelegatesExt.preference(this, ZIP_CODE, DEFAULT_ZIP) 来调用。

我觉得 Code B 更简单，我在使用 Code B 的时候可以使用 private val zipCode: Long by Preference(this, ZIP_CODE, DEFAULT_ZIP) 来调用

在第一种情况下，您还可以导入 DelegatesExt.* 或 DelegatesExt.preference 而不是 DelegatesExt 并写入 by preference。

为什么类 Preference 需要用对象包装？

它需要是（我不会这样做），这只是作者的偏好。