Fragment中调用ProgressBar.setVisibility出现空指针异常如何解决

• Fragment生命周期执行顺序为 onCreate → onCreateView → onViewCreated，你在onCreate阶段就调用fetchdata触发控件操作，此时布局还未加载完成，generalBar未被实例化，调用其setVisibility方法就会触发空指针异常。
• 代码中没有提前声明并初始化generalBar变量，直接通过ID访问控件必须要等视图挂载完成后才能生效。

调整请求触发时机，在视图初始化完成的onViewCreated方法中发起网络请求，同时手动初始化ProgressBar控件，修改后的完整代码如下：

class GeneralFragment : Fragment() {

    private lateinit var mAdapter: NewsListAdapter
    // 提前声明ProgressBar变量
    private lateinit var generalBar: ProgressBar

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // 移除此处的fetchdata调用
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_general, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val recyclerView = view.findViewById<RecyclerView>(R.id.recyclerView)
        // 初始化ProgressBar
        generalBar = view.findViewById(R.id.generalBar)
        mAdapter = NewsListAdapter(this)
        recyclerView.adapter = mAdapter
        // 视图加载完成后再发起网络请求
        fetchdata("https://news-api-don.herokuapp.com/api/v1?apiKey=20d14506791144cc8b424549c42068c0")
    }


    private fun fetchdata(url: String) {
        generalBar.visibility = View.VISIBLE
        val jsonObjectRequest = JsonObjectRequest(
            Request.Method.GET, url, null,
            {
                val newsJsonArray = it.getJSONArray("articles")
                val newsArray = ArrayList<News>()
                for (i in 0 until newsJsonArray.length()) {
                    val newsJsonObject = newsJsonArray.getJSONObject(i)
                    val news = News(
                        newsJsonObject.getString("title"),
                        newsJsonObject.getString("author"),
                        newsJsonObject.getString("url"),
                        newsJsonObject.getString("urlToImage")
                    )
                    newsArray.add(news)
                }
                mAdapter.updateNews(newsArray)
                generalBar.visibility = View.GONE
            },
            {
                Toast.makeText(activity, "Something went wrong", Toast.LENGTH_LONG).show()
                generalBar.visibility = View.GONE
            }
        )
        // 此处替换this为requireContext()，避免Fragment上下文获取异常
        MySingleton.getInstance(requireContext()).addToRequestQueue(jsonObjectRequest)
    }

    fun onItemClicked(item: News) {
        val builder = CustomTabsIntent.Builder()
        val customTabsIntent = builder.build()
        activity?.let { customTabsIntent.launchUrl(it, Uri.parse(item.url)) }
    }
}

• 网络请求回调执行时要先判断Fragment是否已挂载，避免视图销毁后更新控件触发异常，可以在回调开头添加if (!isAdded) return判断。
• ProgressBar可以默认设置为android:visibility="gone"，避免初始化阶段的视觉闪烁。

内容的提问来源于stack exchange，提问作者Mayur Panchal