Question

The following Code A is from https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/extensions/DelegatesExtensions.kt

I can use private var zipCode: Long by DelegatesExt.preference(this, ZIP_CODE, DEFAULT_ZIP) to invoke when I use Code A.

I don't understand why the author wrap class Preference(...) with object DelegatesExt

I think Code B is more simple, I can use private val zipCode: Long by Preference(this, ZIP_CODE, DEFAULT_ZIP) to invoke when I use Code B

Why need the class Preference be wrapped with object ?

Code 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()
    }
}

Code 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

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

我认为代码B更简单，我可以使用私有val zipCode：Long by Preference（this，ZIP_CODE，DEFAULT_ZIP）在我使用Code B时调用

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

为什么需要将类Preference包裹在对象中？

不需要（我不会这样做），这只是作者的偏好。

Why need the class Preference<t> be wrapped with object?

1 个答案: