如何在kotlin中定义应用程序类和静态变量

Question

如何在kotlin中编写等价代码，我需要使用静态变量定义的

public class ThisForThatApplication extends Application {
static ThisForThatApplication appInstance;
public static ThisForThatApplication getAppInstance() {
if (appInstance == null) {
appInstance = new ThisForThatApplication();
}
return appInstance;
}
}

Answer 1

尝试这种方式

class ThisForThatApplication : Application() {
companion object {
@JvmField
var appInstance: ThisForThatApplication? = null

@JvmStatic fun getAppInstance(): ThisForThatApplication {
return appInstance as ThisForThatApplication
}
}
override fun onCreate() {
super.onCreate()
appInstance=this;
}
}

有关详细信息，请阅读Static Fields&Static Methods

Answer 2

Kotlin中没有static概念。但是，使用配套对象也可以实现同样的效果。查看Kotlin对象表达式和声明以获得更多解释。

由于在您的示例中，您只想创建一个Singleton，因此可以执行以下操作：

class ThisForThatApplication: Application() {
companion object {
val instance = ThisForThatApplication()
}
}

然而，当您正在创建Android应用程序类时，就Android而言，最好在onCreate((方法中初始化实例：

class ThisForThatApplication : Application() {
companion object {
lateinit var instance: ThisForThatApplication
private set
}
override fun onCreate() {
super.onCreate()
ThisForThatApplication.instance = this
}
}

伴随对象底部的private set将只允许ThisForThatApplication类设置该值。

Answer 3

Kotlin中的应用程序类和静态变量

class App : Application() {
init {
instance = this
}
companion object {
private var instance: App? = null
fun applicationContext(): Context {
return instance!!.applicationContext
}
}
override fun onCreate() {
super.onCreate()
}
}