我如何轉換&#39;未來<String?> &#39;到&#39;未來<String> ？ 在顫振？

Question

所以基本上我正在使用network_info_plus包來讀取設備當前連接的 Wifiname。 我想使用FutureBuilder從await info.getWifiName();獲取未來 . 問題是該方法返回一個Future<String?> 。 將其轉換為Future<String>? ?

例如

    NetworkInfo info = NetworkInfo();
    ...
    FutureBuilder<String>(
    future: info.getWifiName(), // Error: The argument type 'Future<String?>' 
                                // can't be assigned to the parameter type 'Future<String>?'
    builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
        ...

Answer 1

如果你看一下 getWifiName() 函數的內部實現

  /// Obtains the wifi name (SSID) of the connected network
  ///
  /// Please note that it DOESN'T WORK on emulators (returns null).
  ///
  /// From android 8.0 onwards the GPS must be ON (high accuracy)
  /// in order to be able to obtain the SSID.
  Future<String?> getWifiName() {
    return _platform.getWifiName();
  }

它返回一個 Future<String?> 。 對於 FutureBuilder，您有兩種選擇

這是在 mac 應用程序上運行此代碼的默認顫振應用程序的演示

你可以這樣稱呼它

FutureBuilder<String?>(
      future: info.getWifiName(),
      builder: (context, snapshot) {
        if (snapshot.hasData &&
            snapshot.connectionState == ConnectionState.done) {
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                const Text(
                  'You are connected to WIFI ',
                ),
                Text(
                  '${snapshot.data}',
                  style: Theme.of(context).textTheme.headline4,
                ),
              ],
            ),
          );
        }
        return const Center(
          child: CircularProgressIndicator(),
        );
      })

或者您可以選擇在使用 FutureBuilder 時不顯式聲明數據類型

FutureBuilder(
      future: info.getWifiName(),
      builder: (context, snapshot) {
        if (snapshot.hasData &&
            snapshot.connectionState == ConnectionState.done) {
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                const Text(
                  'You are connected to WIFI ',
                ),
                Text(
                  '${snapshot.data}',
                  style: Theme.of(context).textTheme.headline4,
                ),
              ],
            ),
          );
        }
        return const Center(
          child: CircularProgressIndicator(),
        );
      })

請注意，您並沒有真正轉換這里的數據類型，只是說明它並使 FutureBuilder 知道從給定的未來期望什么數據類型。