如何在Android Studio中将我的Java库&库设置为Kotlin库

您可以更新build.gradle文件以将模块正确配置为Kotlin库，方法是替换id("java-library") with id("org.jetbrains.kotlin.jvm") version "<kotlin_version>"，并包括Kotlin标准库和任何其他特定于Kotlin的依赖项:

plugins {
    id("org.jetbrains.kotlin.jvm") version "<kotlin_version>"
}

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")
}

kotlin {
    sourceSets {
        main {
            kotlin.srcDirs("src/main/kotlin")
        }
        test {
            kotlin.srcDirs("src/test/kotlin")
        }
    }
}

java {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}

There is an optional kotlin block to set source directories. The java block is for compatibility settings.
Sync your Gradle files after making these changes.

关于LiveData lint警告，try 修改您的ViewModel以处理可为空的MediaContent:

private val _cropRequiredLiveData = SingleLiveEvent<MediaContent?>()

fun crop(): LiveData<MediaContent?> = _cropRequiredLiveData

private fun onDownloadComplete(content: MediaContent?) {
    if (request.isCropEnabled && content != null) {
        _cropRequiredLiveData.value = content
    }
    // 
}

_cropRequiredLiveData被声明为SingleLiveEvent<MediaContent?>以接受可为空的值.这一更改要求在观察到_cropRequiredLiveData的代码部分中处理为空性.

或者:如果将content赋值给_cropRequiredLiveData时确定content非空，则在if条件中勾选content != null后可以为use !!:

private fun onDownloadComplete(content: MediaContent?) {
    if (request.isCropEnabled && content != null) {
        _cropRequiredLiveData.value = content!!
    }
    // 
}

Although, I updated the projects as you said, it didn't help me resolve both the issues.
But, to be fair, I found out that "Java Enum Nullability" issue only happens when I observe any enum using LiveData (which is a java class), and not because the enum class was declared in another module.

因此，当观察LiveData个枚举时，"Java Enum Nullability"问题浮出水面，这确实是一个Java类.这可能是因为Kotlin和Java处理空性的方式不同，以及Kotlin与Java代码交互的方式.

在Java中，所有非基元类型都可以为空，包括枚举.当Kotlin与Java类型交互时，为it often treats them as nullable以维护Java的可空性契约.这可能就是为什么在将LiveData与枚举一起使用时，您会看到与枚举为空性相关的警告.

为了解决这个问题，在观察枚举到LiveData时，显式判断null值，即使这在Kotlin中似乎是不必要的.这种方法确保了在处理来自Java类的潜在空值时的安全性.

myEnumLiveData.observe(this, Observer { enumValue ->
    if (enumValue != null) {
        // Handle non-null enum case
    } else {
        // Handle null case, even if it is unexpected in Kotlin
    }
})

另一种可能的方法是:为LiveData创建一个强制非空值的定制包装器.这可以帮助弥合Kotlin的非空类型和LiveData的基于Java的空性之间的差距.

class NonNullLiveData<T : Any>(private val liveData: LiveData<T?>) : LiveData<T>() {
    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        liveData.observe(owner, Observer { it?.let(observer::onChanged) })
    }
}

使用此包装器观察枚举，隐式过滤掉空值.

或者，在处理可能为空的LiveData值时，也可以使用Kotlin的safe calls (?.)和Elvis operator (?:).

myEnumLiveData.observe(this, Observer { enumValue ->
    val safeEnumValue = enumValue ?: return@Observer // Handle or return on null
    // Use safeEnumValue which is guaranteed non-null here
})