OkHttp client.newcall(request).enqueue ...获取空指针

Question

我正在尝试设置 var json，但最后我得到了空指针。任何想法如何解决这一问题 ？还是安卓新手

class Requester(activity: Activity) {
    ...
    var json : Info?  = null
    ...

    fun getData (counter : Int) {
        println("frist attemp"+json)// here it is null as it should be
        var show = 5*counter
        var url = "https://reqres.in/api/users?page=1&per_page=$show"
        var request = Request.Builder().url(url).build()
        val client = OkHttpClient()

        client.newCall(request).enqueue(object: Callback {
            override fun onResponse(call: Call?, response: Response?) {
                val body = response?.body()?.string()
                val gson = GsonBuilder().create()
                json = gson.fromJson(body,Info::class.java)//it should be init here

                activity.runOnUiThread {
                    dataSize = json!!.total
                    val adapter = UsersAdapter(activity ,json!!,activity)
                    if(activity is MainActivity)
                    activity.recycler_View.adapter = adapter

                }
                isLoading=false
                toast.cancel()


            }
            override fun onFailure(call: Call?, e: IOException?) {
                activity.runOnUiThread {
                    val adapter = UsersAdapter(activity ,Info(0,0,0,0,
                            listOf(Data(0,"NO CONECTION","",""))),activity)
                    activity.recycler_View.adapter = adapter
                    Toast.makeText(activity,"Failed to connect", Toast.LENGTH_SHORT).show()
                    toast.cancel()
                }

                isLoading=false


            }

        })
        println("end "+json)// here i get NULL any ideas why ?
    }

}

班级信息：

class Info(val page: Int,
           val per_page: Int,
           val total: Int,
           val total_pages: Int,
           val data: List<Data>)

类数据：

class Data(val id: Int,
           val first_name: String,
           val last_name: String,
           val avatar: String)

类用户活动：

class UserActivity : AppCompatActivity(){
var id : Int =0
val requester : Requester = Requester(this)
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_user)
     id = intent.getIntExtra("id",0)
    display()

}

private fun display() {
    textView_ID_DEFINED.setText("$id")
    requester.getData(id/3+1)
    var info : Info? = requester.json
    println("requester="+requester)
    println("requester.info= "+requester.json)

    if (info !=null){
        for (i in 0..info.data.size){
            if (id == info.data[i].id){
                println("ahoj")
            }
        }
    }
}

}

堆栈跟踪：

I/时间线：时间线：Activity_launch_request id:com.example.android.zadnie time:197846234 I/System.out:requester=com.example.android.zadanie.Requester@127852b 请求者信息=空 I/Timeline: Timeline: Activity_idle id: android.os.BinderProxy@5994e58 time:197846419 I/System.out：中 com.example.android.zadanie.Info@e380f1b

Answer 1

当 HTTP 请求完成时，你的 json 变量应该被初始化的地方（即fun onResponse）被异步调用。这意味着getData 在实际调用回调之前在结束，这就是你得到null 的原因。

为什么你的回调被异步调用？嗯，这就是enqueue 的实现方式，它的主要优点是它在单独的线程中运行HTTP 请求，这样你的UI 就不会冻结。但是，在使用异步回调时，您必须考虑到您的代码可能会在任何时间点执行，即使您的应用程序不再处于前台（想象一下用户在您的应用程序运行时接到电话的场景）