为什么FutureBuilder在Flutter中没有接到电话? [英] Why FutureBuilder doesn't get call in Flutter?

问题描述

我的屏幕上有一个按钮，每当我单击_fetchPost()时，我都会呼叫它.此方法调用fetchPost().在var post = ...行上设置断点时，我能够看到我的解析对象.但是，我的问题是builder不会被调用.

I have a button in my screen and I am calling _fetchPost() whenever I click it. This method calls fetchPost(). I am able to see my parsed object when I set break point on var post = ... line. However, my problem is builder won't be called.

我是Flutter的新手，所以我的代码错了吗?

I am very new in Flutter, so is my code wrong?

void _fetchPost() {
    FutureBuilder<Post>(
      future: fetchPost(),
      builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
        if (snapshot.hasData) {
          String res = snapshot.data.parm1.toString() + snapshot.data.op + snapshot.data.parm2.toString();
          setState(() {
            _result = res;
          });
        } else if (snapshot.hasError) {
          setState(() {
            _result = snapshot.error;
          });
        }

        // By default, show a loading spinner
        return CircularProgressIndicator();
      },
    );
  }

  Future<Post> fetchPost() async {
    final response = await http.get('http://test.ethorstat.com/test.ashx');
    if (response.statusCode == 200) {
      // If server returns an OK response, parse the JSON
      var post = Post.fromJason(json.decode(response.body));
      return post;
    } else {
      // If that response was not OK, throw an error.
      throw Exception('Failed to load post!');
    }
  }

推荐答案

FutureBuilder 是小工具.这意味着要使其正常工作，您需要先将其插入小部件树.

FutureBuilder is a widget. This means that in order for it to work, you need to insert it into the widget tree first.

通过将FutureBuilder作为子级返回到另一个窗口小部件或直接返回到窗口小部件的build函数，将FutureBuilder添加到窗口小部件树:

Either you add your FutureBuilder to the widget tree by returning it as a child to another widget or directly to a build function of a widget:

class FetchWidget extends StatelessWidget {

  @override
  Widget build(BuildContext context) => FutureBuilder<Post>(
    future: fetchPost(),
    builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
      if (snapshot.hasData) {
        String res = snapshot.data.parm1.toString() + snapshot.data.op + snapshot.data.parm2.toString();
        setState(() { // setState will not work here, I only used this StatelessWidget to show what I mean with inserting it into the build tree
          _result = res;
        });
      } else if (snapshot.hasError) {
        setState(() {
          _result = snapshot.error;
        });
      }

      // By default, show a loading spinner
      return CircularProgressIndicator();
    },
  );

  Future<Post> fetchPost() async {
    final response = await http.get('http://test.ethorstat.com/test.ashx');
    if (response.statusCode == 200) {
      // If server returns an OK response, parse the JSON
     var post = Post.fromJason(json.decode(response.body));
     return post;
    } else {
      // If that response was not OK, throw an error.
     throw Exception('Failed to load post!');
    }
  }
}

这不会编译，因为我只将您的代码复制到了没有setState的StatelessWidget中.我只想通过说将[...]添加到小部件树" 来表明我的意思.

This will not compile because I only copied your code into a StatelessWidget, which does not have setState. I only wanted to show what I mean by saying "add [...] to the widget tree".

或者您可以将常规async await用于尝试执行的操作:

Or you can just use regular async await for what you are trying to do:

void _fetchPost() async {
  final Post fetchedPost = await fetchPost();
  final String res = fetchedPost.parm1.toString() + fetchedPost.op + fetchedPost.parm2.toString();
  setState(() {
    _result = res;
  });
}

我不确定您要如何实现fetchPost，但是如果要使用FutureBuilder，则需要将其插入小部件树并将小部件返回到builder.

I am not sure how you want to implement your fetchPost, but if you want to use FutureBuilder, you need to insert it into the widget tree and return a widget to the builder.

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