Contract，开发者和 Kotlin 编译器之间的契约 - 技术小黑屋

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data class News(val publisherId: Int, val title: String)

//检查标题是否合法，如果title为null或者内容为空返回false
fun News?.isTitleValid(): Boolean {
    return this != null && title.isNotEmpty()
}

fun testNewsTitleValid(news: News?) {
    if (news.isTitleValid()) {   
        news.title //编译失败 并报错  //Only safe (?.) or non-null asserted (!!.) calls 
        //are allowed on a nullable receiver of type News?
    }
}

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@ExperimentalContracts
fun News?.isTitleValid(): Boolean {
  //contract 开始
    contract {
        returns(true) implies (this@isTitleValid is News)
    }
    //contract 结束
    return this != null && title.isNotEmpty()
}

@ExperimentalContracts
fun testNewsTitleValid(news: News?) {
    if (news?.isTitleValid() == true) {
        news.title
    }
}

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@ExperimentalContracts
fun News?.isFake(): Boolean {
    contract {
        returns(false) implies (this@isFake is News)
    }
    return this == null || this.publisherId == 1980
}

@ExperimentalContracts
fun testNewsIsFake(news: News?) {
    if (news.isFake()) {
        news?.title
    } else {
        news.title
    }
}

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@ExperimentalContracts
fun News?.copy(): Any? {
    contract {
        returns(null) implies (this@copy is News)
    }

    return if (this == null) {
        "EMPTY"
    } else {
        null
    }
}

@ExperimentalContracts
fun testNewsCopy(news: News?) {
    if (news.copy() == null) {
        news.title
    } else {
        news?.title
    }
}

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@ExperimentalContracts
fun News?.validate() {
    contract {
        returns() implies (this@validate is News)
    }

    if (this == null) {
        throw IllegalStateException("null instance")
    }

    if (publisherId < 0) {
        throw IllegalStateException("publisherId is less than 0")
    }

    if (title.isEmpty()) {
        throw IllegalStateException("title is empty")
    }
}

@ExperimentalContracts
fun testNewsValidate(news: News?) {
    news.validate()
    news.title
}

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@ExperimentalContracts
fun News?.getTitleHashCode(): Int? {
    contract {
        returnsNotNull() implies (this@getTitleHashCode is News)
    }
    return this?.title?.hashCode()
}

@ExperimentalContracts
fun testNewsGetTitleHashCode(news: News?) {
    if (news.getTitleHashCode() != null) {
        news.title
    } else {
        news?.title
    }
}

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package com.example.androidcontractsample

fun getAppVersion() {
    val appVersion: Int
    safeRun {
        appVersion = 50
    }
}

//安全运行runFunction,捕获异常
inline fun safeRun(runFunction: () -> Unit) {
    try {
        runFunction.invoke()
    } catch(t: Throwable) {
        t.printStackTrace()
    }
}

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import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.InvocationKind
import kotlin.contracts.contract

@ExperimentalContracts
fun getAppVersion() {
    val appVersion: Int
    safeRun {
        appVersion = 50
    }
}

@ExperimentalContracts
fun safeRun(runFunction: () -> Unit) {
    contract {
      //使用EXACTLY_ONCE
        callsInPlace(runFunction, InvocationKind.EXACTLY_ONCE)
    }
    try {
        runFunction()
    } catch (t: Throwable) {
        t.printStackTrace()
    }
}

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@UseExperimental(ExperimentalContracts::class)
fun String?.isOK(): Boolean {
    contract {
        returns(true) implies(this@isOK is String)
    }
    return this != null && this.isNotEmpty()
}


@ExperimentalContracts
fun String?.isGood(): Boolean {
    contract {
        returns(true) implies(this@isGood is String)
    }
    return this != null && this.isNotEmpty()
}

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<module type="JAVA_MODULE" version="4">
  <component name="FacetManager">
    <facet type="kotlin-language" name="Kotlin">
      <configuration version="3" platform="JVM 1.8" useProjectSettings="false">
        <compilerSettings>
          <option name="additionalArguments" value="-version -Xuse-experimental=kotlin.contracts.ExperimentalContracts" />
        </compilerSettings>
        <compilerArguments>
          <option name="jvmTarget" value="1.8" />
          <option name="languageVersion" value="1.3" />
          <option name="apiVersion" value="1.3" />
        </compilerArguments>
      </configuration>
    </facet>
  </component>
  <component name="NewModuleRootManager" inherit-compiler-output="true">
    <exclude-output />
    <content url="file://$MODULE_DIR$">
      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
    </content>
    <orderEntry type="inheritedJdk" />
    <orderEntry type="sourceFolder" forTests="false" />
    <orderEntry type="library" name="KotlinJavaRuntime" level="project" />
  </component>
</module

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@ExperimentalContracts
fun validateByMistake(news: News?): Boolean {
    contract {
        returns(true) implies (news is News)
    }
    return true
}

@ExperimentalContracts
fun testValidateByMistake(news: News?) {
    if (validateByMistake(news)) {
        news.title
    }
}

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java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.androidcontractsample.News.getTitle()' on a null object reference
 at com.example.androidcontractsample.NewsKt.testValidateByMistake(News.kt:91)
 at com.example.androidcontractsample.MainActivity.onCreate(MainActivity.kt:13)
 at android.app.Activity.performCreate(Activity.java:7698)
 at android.app.Activity.performCreate(Activity.java:7687)
 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3096)
 ... 11 more