Kotlin 扩展函数与高阶函数

package com.example.ext

fun String.isValidPhone(): Boolean = length == 11 && all { it.isDigit() }
fun Int.toDp(): Float = this * 3f   // this 指接收者

// 调用："13800138000".isValidPhone() 得 true；16.toDp() 得 48.0

open class Animal
class Dog : Animal()

fun Animal.speak() = "animal"
fun Dog.speak() = "dog"

val a: Animal = Dog()        // 静态类型是 Animal
println(a.speak())           // animal，而不是 dog

class Logger {
    fun log(msg: String) = println("member: $msg")
}

fun Logger.log(msg: String) = println("extension: $msg")

fun main() {
    Logger().log("hi")           // member: hi
}

val String.firstChar: Char?
    get() = firstOrNull()

var TextView.textOrEmpty: String
    get() = text?.toString().orEmpty()
    set(value) { text = value }

// 编译错误：扩展属性不能初始化
// val String.x = 1

fun String?.orDash(): String = this?.takeIf { it.isNotBlank() } ?: "—"

fun main() {
    val a: String? = null
    println(a.orDash())          // —
    println("ok".orDash())       // ok
}

val add: (Int, Int) -> Int = { a, b -> a + b }      // Function2
val noArg: () -> Unit = { println("run") }          // Function0
val withReceiver: String.() -> Int = { length }     // Function1<String, Int>

fun applyTwice(x: Int, f: (Int) -> Int): Int = f(f(x))

// Java 接口：public interface OnClickListener { void onClick(View v); }
button.setOnClickListener { v -> v.alpha = 0.5f }

// 自定义 fun interface（Kotlin 1.4+）
fun interface Predicate<T> { fun test(value: T): Boolean }

fun <T> filterAll(items: List<T>, p: Predicate<T>): List<T> = items.filter { p.test(it) }
// 调用：filterAll(listOf(1, 2, 3, 4)) { it % 2 == 0 } 触发 SAM 转换

data class User(var name: String = "", var age: Int = 0)
val u = User()

val len = u.let { it.name = "leeting"; it.name.length }   // it，返回 lambda 结果
val age = u.run { age = 30; age }                         // this，返回 lambda 结果
val same = with(u) { name = "yan"; this }                 // this，返回 lambda 结果
val applied = u.apply { age = 31 }                        // this，返回接收者
val logged = u.also { println(it) }                       // it，返回接收者

val intent = Intent(context, DetailActivity::class.java).apply {
    putExtra("id", 42L)
    flags = Intent.FLAG_ACTIVITY_NEW_TASK
}

inline fun measure(tag: String, block: () -> Unit) {
    val start = System.nanoTime()
    try { block() } finally {
        println("$tag 耗时 ${(System.nanoTime() - start) / 1_000} us")
    }
}

inline fun <reified T> Gson.fromJsonSafe(json: String): T? =
    runCatching { fromJson(json, T::class.java) }.getOrNull()

inline fun <reified T : Activity> Context.startActivity() {
    startActivity(Intent(this, T::class.java))
}

inline fun <T> List<T>.forEachChunked(
    size: Int,
    noinline onChunk: (List<T>) -> Unit,   // 需要作为对象传递
    crossinline body: (T) -> Unit,         // 允许非局部返回被禁止
) {
    var i = 0
    while (i < this.size) {
        val chunk = subList(i, minOf(i + size, this.size))
        onChunk(chunk)
        chunk.forEach { body(it) }
        i += size
    }
}

// 编译错误：JVM 签名冲突（都是 List）
// fun List<String>.joinAll(): String = joinToString()
// fun List<Int>.joinAll(): String = joinToString()

// 正确做法：用 @JvmName 区分
@JvmName("joinAllStrings")
fun List<String>.joinAll(): String = joinToString()

@JvmName("joinAllInts")
fun List<Int>.joinAll(): String = joinToString("|")

val <T> List<T>.lastIndexOrNull: Int?
    @JvmName("lastIndexOrNullOfList")
    get() = if (isEmpty()) null else lastIndex

fun main() {
    val list = (1..1_000_000).toList()
    // 立即求值：产生 2 个中间集合，每个元素被遍历多次
    val eager = list.filter { it % 2 == 0 }.map { it * it }.take(3).toList()
    // 惰性求值：无中间集合，找到 3 个即停止
    val lazyResult = list.asSequence().filter { it % 2 == 0 }.map { it * it }.take(3).toList()
    println("$eager / $lazyResult")
}

fun buildQuery(params: Map<String, String>): String = buildString {
    append("?")
    params.entries.forEachIndexed { index, (k, v) ->
        if (index > 0) append("&")
        append(k).append("=").append(v)
    }
}

class FormBuilder {
    private val fields = mutableListOf<String>()
    fun text(name: String) { fields += name }
    fun number(name: String) { fields += "$name#num" }
    fun build(): List<String> = fields.toList()
}

fun form(block: FormBuilder.() -> Unit): List<String> = FormBuilder().apply(block).build()

// 调用：form { text("email"); number("age") } 得到 [email, age#num]

import androidx.compose.foundation.layout.padding
import androidx.compose.material3.Text
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

// 自定义 Modifier 扩展：接收者与返回值都是 Modifier
fun Modifier.cardStyle(): Modifier = this.padding(16.dp)

@Composable
fun Title(text: String) {
    Text(text = text, modifier = Modifier.padding(8.dp).cardStyle())
}