Kotlin基础知识_09-高阶函数的应用

Kotlin基础知识_09-高阶函数的应用 vararg&Any

1. 简化 SharedPreferences API

日常写法：

val sp = getSharedPreferences("data", MODE_PRIVATE)
val editor = sp.edit()
editor.putBoolean("is_switch", true)
editor.putInt("last_value", 20)
editor.putFloat("stu_grade", 20.5f)
editor.apply()

使用高阶函数简化：

首先为SharedPreferences 定义一个扩展函数：

fun SharedPreferences.save(block: SharedPreferences.Editor.() -> Unit) {
    val editor = edit()
    editor.block()
    editor.apply()
}

调用：

getSharedPreferences("data", MODE_PRIVATE).save {
    putBoolean("is_switch", true)
    putInt("last_value", 20)
    putFloat("stu_grade", 20.5f)
}

直接使用Android core-ktx的lib

Google 已为 SharedPreferences类提供了类似的扩展函数，包含在下面的lib中：

SharedPreferences.kt

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package androidx.core.content

import android.annotation.SuppressLint
import android.content.SharedPreferences

/**
 * Allows editing of this preference instance with a call to [apply][SharedPreferences.Editor.apply]
 * or [commit][SharedPreferences.Editor.commit] to persist the changes.
 * Default behaviour is [apply][SharedPreferences.Editor.apply].
 * ```
 * prefs.edit {
 *     putString("key", value)
 * }
 * ```
 * To [commit][SharedPreferences.Editor.commit] changes:
 * ```
 * prefs.edit(commit = true) {
 *     putString("key", value)
 * }
 * ```
 */
@SuppressLint("ApplySharedPref")
public inline fun SharedPreferences.edit(
    commit: Boolean = false,
    action: SharedPreferences.Editor.() -> Unit
) {
    val editor = edit()
    action(editor)
    if (commit) {
        editor.commit()
    } else {
        editor.apply()
    }
}

所以可以直接以下面的方式去调用，效果是一样的：

getSharedPreferences("data", MODE_PRIVATE).edit {
    putBoolean("is_switch", true)
    putInt("last_value", 20)
    putFloat("stu_grade", 20.5f)
}

2. 简化 ContentValues API

日常写法

val contentValues = ContentValues()
contentValues.put("name", "ana")
contentValues.put("age", "20")
contentValues.put("sex", "boy")
contentValues.put("address", "beijing")

定义一个函数简化

    fun buildContentValues(vararg pairs: Pair<String, Any?>): ContentValues {
        val cv = ContentValues()
        for (pair in pairs) {
            val key = pair.first
            val value = pair.second
            when (value) {
                is Int -> cv.put(key, value)
                is Long -> cv.put(key, value)
                is Short -> cv.put(key, value)
                is Float -> cv.put(key, value)
                is Double -> cv.put(key, value)
                is Boolean -> cv.put(key, value)
                is String -> cv.put(key, value)
                is Byte -> cv.put(key, value)
                is ByteArray -> cv.put(key, value)
                null -> cv.putNull(key)
            }
        }
        return cv
    }

 val values = buildContentValues("name" to "ana", "age" to 20, "sex" to "boy", "address" to "beijing")

使用高阶函数继续简化

    fun buildContentValues(vararg pairs: Pair<String, Any?>): ContentValues =
        ContentValues().apply {
            for (pair in pairs) {
                val key = pair.first
                val value = pair.second
                when (value) {
                    is Int -> put(key, value)
                    is Long -> put(key, value)
                    is Short -> put(key, value)
                    is Float -> put(key, value)
                    is Double -> put(key, value)
                    is Boolean -> put(key, value)
                    is String -> put(key, value)
                    is Byte -> put(key, value)
                    is ByteArray -> put(key, value)
                    null -> putNull(key)
                }
            }
        }

直接使用Android core-ktx的lib

Google 已为 ContentValues类提供了类似的封装函数，同样也包含在 androidx.core:core-ktx lib中，名为：contentValuesOf：

val values = contentValuesOf("name" to "ana", "age" to 20, "sex" to "boy", "address" to "beijing")

ContentValues.kt

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package androidx.core.content

import android.content.ContentValues

/**
 * Returns a new [ContentValues] with the given key/value pairs as elements.
 *
 * @throws IllegalArgumentException When a value is not a supported type of [ContentValues].
 */
public fun contentValuesOf(
    vararg pairs: Pair<String, Any?>
): ContentValues = ContentValues(pairs.size).apply {
    for ((key, value) in pairs) {
        when (value) {
            null -> putNull(key)
            is String -> put(key, value)
            is Int -> put(key, value)
            is Long -> put(key, value)
            is Boolean -> put(key, value)
            is Float -> put(key, value)
            is Double -> put(key, value)
            is ByteArray -> put(key, value)
            is Byte -> put(key, value)
            is Short -> put(key, value)
            else -> {
                val valueType = value.javaClass.canonicalName
                throw IllegalArgumentException("Illegal value type $valueType for key \"$key\"")
            }
        }
    }
}

Note:

1. vararg 是kotlin中的关键字，用于表示可变参数；
2. Any 是kotlin中的关键字，代表所有类型的基类，类似于java中的 Object类；
3. "name" to "ana" 这种写法会自动转换为Pair 键值对。

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