單例包裝類

open class SingletonHolder<out T, in A>(creator: (A) -> T) {
    private var creator: ((A) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg: A): T {
        val i = instance
        if (i != null) {
            return i
        }

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg)
                instance = created
                creator = null
                created
            }
        }
    }
}

使用如下

class Manager private constructor(context: Context) {
    init {
        // Init using context argument
    }

    companion object : SingletonHolder<Manager, Context>(::Manager)
}

Manager.getInstance(context).doStuff()

無參構(gòu)造函數(shù)的單例模式

interface GitHubService {

    companion object {
        val instance: GitHubService by lazy {
            val retrofit = Retrofit.Builder()
                    .baseUrl("https://api.github.com/")
                    .build()
            retrofit.create(GitHubService::class.java)
        }
    }
}