Kotlin骚气写法 二

List 增删改

fun <T> List<T>.upsert(value: T, finder: (T) -> Boolean) = indexOfFirst(finder).let { index ->
    if (index >= 0) copy(index, value) else this + value
}

fun <T> List<T>.copy(i: Int, value: T): List<T> = toMutableList().apply { set(i, value) }

inline fun <T> List<T>.delete(filter: (T) -> Boolean): List<T> = toMutableList().apply { removeAt(indexOfFirst(filter)) }

Kotlin 带参回调

fun withState(block: (state: S) -> Unit) {
     block.invoke(state)
     //block(state)
}
// 调用
withState { state-> 
          
}

kotlin操作符重载

class Test {
    /**
     * Can be invoked as an operator like: `yourProp()`
     */
    open operator fun invoke(): String? = null
}
调用
Test().invoke()?:"MaQi"
Test()()?:"MaQi"

当然也可以被重写

class Son : Test() {
    override fun invoke(): String = "Son"
}

data copy函数

data class User(val name:String)
User("").copy(name = "马齐")

Array joinToString 扩展

private fun demonstrates(vararg items: String) = 
	arrayOf("Demonstrates:", *items).joinToString("\n\t\t• ")

navigateTo 函数

  protected fun navigateTo(@IdRes actionId: Int, arg: Parcelable? = null) {
        val bundle = arg?.let { Bundle().apply { putParcelable("KEY_ARG", it) } }
        findNavController().navigate(actionId, bundle)
    }

Fragment.kt 扩展函数

fun Fragment.findNavController(): NavController =
        NavHostFragment.findNavController(this)

Unit 无返回调用

override fun onOptionsItemSelected(item: MenuItem?) = when (item?.itemId ?: 0) {
    R.id.menu_refresh -> viewModel.refreshTasks().andTrue()
    R.id.menu_filter -> showFilteringPopUpMenu().andTrue()
    R.id.menu_clear -> viewModel.clearCompletedTasks().andTrue()
    else -> super.onOptionsItemSelected(item)
}
/**
//	翻译：Unit相当于Java的Void类型
 * The type with only one value: the `Unit` object. This type corresponds to the `void` type in Java.
 */
@Suppress("Detekt.FunctionOnlyReturningConstant")
private fun Unit.andTrue() = true

反编译Java代码

public boolean onOptionsItemSelected(@Nullable MenuItem item) {
  boolean andTrue;
   switch (item != null ? item.getItemId() : 0) {
       case R.id.menu_clear:
           getViewModel().clearCompletedTasks();
           andTrue = andTrue(Unit.INSTANCE);
           break;
       case R.id.menu_filter:
           showFilteringPopUpMenu();
           andTrue = andTrue(Unit.INSTANCE);
           break;
       case R.id.menu_refresh:
           getViewModel().refreshTasks();
           andTrue = andTrue(Unit.INSTANCE);
           break;
       default:
           andTrue = super.onOptionsItemSelected(item);
           break;
   }
   return andTrue;
}
private final boolean andTrue(@NotNull Unit $receiver) {
	return true;
}

可以发现Java中完全无关的两个函数被Kotllin巧妙的关联起来

PopupMenu ToolBar点击弹窗

//使用方法
PopupMenu(requireContext(), requireActivity().findViewById<View>(R.id.menu_filter)).run {
            menuInflater.inflate(R.menu.filter_tasks, menu)
            setOnMenuItemClickListener { item ->
              	 //doSomething
                true
            }
            show()
        }
        
//menu
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_filter"
        android:title="@string/xxx"
        android:icon="@drawable/xxx"
        app:showAsAction="always" />
</menu>

//填充Menu
override fun onCreateOptionsMenu(menu: Menu?, inflater: MenuInflater?) {
  	 inflater?.inflate(R.menu.tasks_fragment_menu, menu)
}

JvmOverloads 优化自定义View构造函数

class FullScreenMessageView : FrameLayout {
    @JvmOverloads
    constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0) : super(context, attrs, defStyleAttr) {
        inflate(context, R.layout.full_screen_message, this)
    }
}

get()= ，get() ， =的区别

// 1
private val navController: NavController
    get() = findNavController(R.id.nav_host)
// 2
private val navController: NavController
	get() {
	   return findNavController(R.id.nav_host)
	}
// 3
private val navController = findNavController(R.id.nav_host)

Java代码

// 1
private final NavController getNavController() {
	return ActivityKt.findNavController(this, R.id.nav_host);
}
//2
private final NavController getNavController() {
	return ActivityKt.findNavController(this, R.id.nav_host);
}
// 3
private final NavController navController = ActivityKt.findNavController(this, R.id.nav_host);

可以发现
1.get()= 和get()没区别
2.get() 和 = 加载时机不同

typealias 别名的使用

申明

//Tasks.kt
typealias Tasks = List<Task>
fun Tasks.findTask(id: String?) = firstOrNull { it.id == id }

调用

//在申明的时候不用暴露类型？
private var oldTasks: Tasks? = null
oldTasks?.findTask(lastId)

Java代码

TasksKt.java

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import java.util.List;
import kotlin.Metadata;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

/* compiled from: Tasks.kt */
public final class TasksKt {
    @Nullable
    public static final Task findTask(@NotNull List<Task> $receiver, @Nullable String id) {
        Intrinsics.checkParameterIsNotNull($receiver, "receiver$0");
        int $i$f$firstOrNull = 0;
        for (Object next : $receiver) {
            if (Intrinsics.areEqual(((Task) next).getId(), (Object) id) != null) {
                break;
            }
        }
        Object next2 = null;
        return (Task) next2;
    }
}

调用

List list = this.oldTasks;
TasksKt.findTask(list , lastId)

不要纠结这个Task啊 他可以是Any类型

其他:
Intrinsics.areEqual()相当于 Java 的 ‘==’