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Kotlin委托屬性(1)

2024-01-06 09:40 ttylinux 閱讀(250) 評論(0) 收藏舉報(bào)

在Kotlin中，委托屬性（Delegated Properties）是一種強(qiáng)大的語言特性，允許你將屬性的 getter 和 setter 方法的實(shí)現(xiàn)委托給其他對象。這使得你能夠通過委托來重用代碼、將屬性的行為解耦，并實(shí)現(xiàn)一些通用的模式。下面是一些關(guān)鍵概念和用法：

class Example {
    var property: String by Delegate()
}

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        // 獲取屬性值的實(shí)際實(shí)現(xiàn)
        return "Delegated value"
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        // 設(shè)置屬性值的實(shí)際實(shí)現(xiàn)
        println("Setting value to: $value")
    }
}

在上面的代碼中，Example類中的property屬性的訪問，比如，訪問example.property,就會委托到Delegate.getValue; 屬性值的設(shè)置example.property = "str",就會委托到

Delegae.setValue。

實(shí)際例子：

   fun saveCookie(url: String?, domain: String?, cookies: String) {
        url ?: return
        var spUrl: String by Preference(url, cookies)
        @Suppress("UNUSED_VALUE")
        spUrl = cookies
        domain ?: return
        var spDomain: String by Preference(domain, cookies)
        @Suppress("UNUSED_VALUE")
        spDomain = cookies
    }

var spUrl: String by Preference(url, cookies)，定義一個(gè)委托屬性spUrl，Preference委托，執(zhí)行spUrl = cookies，會將這個(gè)setValue的動作委托到Preference類中的setValue。

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

其中value的值是cookies.如果要訪問spUrl，那么，對應(yīng)的getValue方法，會被委托到Preference中的getValue方法中去。

總結(jié)來說：

委托屬性，就是將一個(gè)屬性的getValue方法和setValue方法委托到另外一個(gè)代理類來實(shí)現(xiàn)。將屬性的獲取和設(shè)置隔離開來。

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

    companion object {
        private val file_name = "wan_android_file"

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

        /**
         * 刪除全部數(shù)據(jù)
         */
        fun clearPreference() {
            prefs.edit().clear().apply()
        }

        /**
         * 根據(jù)key刪除存儲數(shù)據(jù)
         */
        fun clearPreference(key: String) {
            prefs.edit().remove(key).apply()
        }

        /**
         * 查詢某個(gè)key是否已經(jīng)存在
         *
         * @param key
         * @return
         */
        fun contains(key: String): Boolean {
            return prefs.contains(key)
        }

        /**
         * 返回所有的鍵值對
         *
         * @param context
         * @return
         */
        fun getAll(): Map<String, *> {
            return prefs.all
        }
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
        return getSharedPreferences(name, default)
    }

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

    @SuppressLint("CommitPrefEdits")
    private fun putSharedPreferences(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 -> putString(name, serialize(value))
        }.apply()
    }

    @Suppress("UNCHECKED_CAST")
    private fun getSharedPreferences(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 -> deSerialization(getString(name, serialize(default)) ?: "")
        }
        return res as T
    }

    /**
     * 序列化對象

     * @param person
     * *
     * @return
     * *
     * @throws IOException
     */
    @Throws(IOException::class)
    private fun <A> serialize(obj: A): String {
        val byteArrayOutputStream = ByteArrayOutputStream()
        val objectOutputStream = ObjectOutputStream(
            byteArrayOutputStream
        )
        objectOutputStream.writeObject(obj)
        var serStr = byteArrayOutputStream.toString("ISO-8859-1")
        serStr = java.net.URLEncoder.encode(serStr, "UTF-8")
        objectOutputStream.close()
        byteArrayOutputStream.close()
        return serStr
    }

    /**
     * 反序列化對象

     * @param str
     * *
     * @return
     * *
     * @throws IOException
     * *
     * @throws ClassNotFoundException
     */
    @Suppress("UNCHECKED_CAST")
    @Throws(IOException::class, ClassNotFoundException::class)
    private fun <A> deSerialization(str: String): A {
        val redStr = java.net.URLDecoder.decode(str, "UTF-8")
        val byteArrayInputStream = ByteArrayInputStream(
            redStr.toByteArray(charset("ISO-8859-1"))
        )
        val objectInputStream = ObjectInputStream(
            byteArrayInputStream
        )
        val obj = objectInputStream.readObject() as A
        objectInputStream.close()
        byteArrayInputStream.close()
        return obj
    }

}

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Kotlin委托屬性(1)

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