• Delegates.notNull()：可用于基本类型和引用类型，lateinit只能用于引用类型
• Delegates.observable()：监听属性变化
• Delegates.vetoable()：决定是否允许属性修改

自定义Delegates

class PreferenceDelegate<T>(
    private val context: Context,
    private val name: String,
    private val default: T,
    private val prefName: String = "default"
) : ReadWriteProperty<Any?, T> {
    private val prefs: SharedPreferences by lazy {
    context.getSharedPreferences(prefName, Context.MODE_PRIVATE)
    }

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        println("setValue from delegate")
        return getPreference(key = name)
    }

    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        println("setValue from delegate")
        putPreference(key = name, value = value)
    }

    private fun getPreference(key: String): T {
        return when (default) {
            is String -> prefs.getString(key, default)
            is Long -> prefs.getLong(key, default)
            is Boolean -> prefs.getBoolean(key, default)
            is Float -> prefs.getFloat(key, default)
            is Int -> prefs.getInt(key, default)
            else -> throw IllegalArgumentException("Unknown Type.")
        } as T
    }

    private fun putPreference(key: String, value: T) = with(prefs.edit()) {
        when (value) {
            is String -> putString(key, value)
            is Long -> putLong(key, value)
            is Boolean -> putBoolean(key, value)
            is Float -> putFloat(key, value)
            is Int -> putInt(key, value)
            else -> throw IllegalArgumentException("Unknown Type.")
        }
    }.apply()

}