Flutter：如何将 Future<String> 转换为字符串？答案

【问题标题】：Flutter : How To Convert Future<String> to String?Flutter：如何将 Future<String> 转换为字符串？
【发布时间】：2020-05-20 23:43:10
【问题描述】：

我使用 API 从数据库中获取图像。我有这样的回应：

{
    "status": "ok",
    "message": "Logo Client Is Found",
    "data": [
        {
            "fileInfo": "img-1.png"
        }
    ]
}

我想用这段代码从 API 中获取那个值：

 Future<String> getLogoClient() async {
    final response = await _client.get("$_baseUrl/getLogoClient");
    final Map<String, dynamic> responseJson = json.decode(response.body);
    if (responseJson["status"] == "ok") {
      List image = responseJson["data"];
      final imageList =
          image.map((json) => AppInfoModel.logoFromJson(json)).toList();
      final singleImage = imageList.single.fileInfo;
      final String urlImage = "$baseImageUrl/$singleImage";
      print(urlImage);
      return urlImage;
    } else {
      throw CustomError(responseJson["message"]);
    }
  }

打印响应：

http://---/images/info/img-1.png

问题是，我想将值从 Future 转换为 String 我该怎么做？

我已经尝试使用此代码并成功转换了该值：

String urlImageApi = "";
  _getImage() async {
    final result = await appInfoApi.getLogoClient();
    setState(() {
      urlImageApi = result;
    });
  }

  @override
  void initState() {
    super.initState();
    _getImage();
  }

但我得到错误：

I/flutter ( 8408): #644    ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4243
I/flutter ( 8408): #645    Element.rebuild 
package:flutter/…/widgets/framework.dart:3947
I/flutter ( 8408): #646    ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:4206
I/flutter ( 8408): #647    StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:4381
I/flutter ( 8408): #648    ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4201
I/flutter ( 8408): #649    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3194
I/flutter ( 8408): #650    Element.updateChild 
package:flutter/…/widgets/framework.dart:2988
I/flutter ( 8408): #651    ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4243
I/flutter ( 8408): #652    Element.rebuild 
package:flutter/…/widgets/framework.dart:3947
I/flutter ( 8408): #653    ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:4206
I/flutter ( 8408): #654    ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4201
I/flutter ( 8408): #655    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3194
I/flutter ( 8408): #656    Element.updateChild 
package:flutter/…/widgets/framework.dart:2988
I/flutter ( 8408): #657    ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4243
I/flutter ( 8408): #658    Element.rebuild 
package:flutter/…/widgets/framework.dart:3947
I/flutter ( 8408): #659    ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:4206
I/flutter ( 8408): #660    StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:4381
I/flutter ( 8408): #661    ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4201
I/flutter ( 8408): #662    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3194
I/flutter ( 8408): #663    Element.updateChild 
package:flutter/…/widgets/framework.dart:2988
I/flutter ( 8408): #664    ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4243
I/flutter ( 8408): #665    Element.rebuild 
package:flutter/…/widgets/framework.dart:3947
I/flutter ( 8408): #666    ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:4206
I/flutter ( 8408): #667    ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4201
I/flutter ( 8408): #668    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3194
I/flutter ( 8408): #669    Element.updateChild 
package:flutter/…/widgets/framework.dart:2988
I/flutter ( 8408): #670    RenderObjectToWidgetElement._rebuild 
package:flutter/…/widgets/binding.dart:1028
I/flutter ( 8408): #671    RenderObjectToWidgetElement.mount 
package:flutter/…/widgets/binding.dart:999
I/flutter ( 8408): #672    RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> 
package:flutter/…/widgets/binding.dart:942
I/flutter ( 8408): #673    BuildOwner.buildScope 
package:flutter/…/widgets/framework.dart:2412
I/flutter ( 8408): #674    RenderObjectToWidgetAdapter.attachToRenderTree 
package:flutter/…/widgets/binding.dart:941
I/flutter ( 8408): #675    WidgetsBinding.attachRootWidget 
package:flutter/…/widgets/binding.dart:819
I/flutter ( 8408): #676    WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> 
package:flutter/…/widgets/binding.dart:804
I/flutter ( 8408): #685    _Timer._runTimers  (dart:isolate-patch/timer_impl.dart:384:19)
I/flutter ( 8408): #686    _Timer._handleMessage  (dart:isolate-patch/timer_impl.dart:418:5)
I/flutter ( 8408): #687    _RawReceivePortImpl._handleMessage  (dart:isolate-patch/isolate_patch.dart:174:12)
I/flutter ( 8408): (elided 11 frames from package dart:async and package dart:async-patch)
I/flutter ( 8408):
I/flutter ( 8408): Image provider: NetworkImage("", scale: 1.0)
I/flutter ( 8408): Image key: NetworkImage("", scale: 1.0)
I/flutter ( 8408): ════════════════════════════════════════════════════════════════════════════════════════════════════

我想念什么？

【问题讨论】：

将 Future 转换为 String 是什么意思？你不能。
你需要使用 FutureBuilder 来访问未来的字符串值。
可以不用 FutureBuilder 吗？因为我的小部件中的属性只接受字符串。我使用上面的代码成功从 Future 转换为字符串，应用程序仍在运行并成功获取图像，但我得到了上面的错误
你可以。你为什么使用等待？如果您仅在响应后才需要 ui，那么您需要使用 futurebuilder 和 future 否则不需要。
你需要检查空字符串。

标签： api flutter dart

【解决方案1】：

你可以使用 then 方法，并且可以将 Future 转换为 String。

appInfoApi.getLogoClient().then((String result){
setState(() {
      urlImageApi = result;
    });
});

【讨论】：

【解决方案2】：

您收到错误的原因是因为您在initState 上使用await-ing。 Flutter 期望 initState 中的代码是非阻塞的，因为它不能等待 initState 完成执行，Flutter 需要立即渲染 UI 元素。

您可以通过在Future 上使用then 方法来做到这一点，例如here。另一种方法是使用Timer#run 方法，如下所示：

Timer.run(() async {
    String urlImageApi = await appInfoApi.getLogoClient();
    setState(() {
      urlImageApi = result;
    });
})

上面的代码工作的原因是here

【讨论】：

【解决方案3】：

在我的情况下，我的 Future 是 Future，我这样解决：

appInfoApi.getLogoClient().then((result) {
    setState(() {
       if (result is String)
            urlImageApi = result.toString(); //use toString to convert as String
    });
});

【讨论】：