am正在尝试获取数据，然后将其显示在列表视图中。。首先，我会获取一个哈希值，然后将其与我保存的哈希进行比较，如果相同，那么我会获取保存的json数据，如果不相同，我会调用api。。。像这样：
Future<String> get _localPath async {
final directory = await getApplicationDocumentsDirectory();
return directory.path;
}
Future<File> get _localFile async {
final path = await _localPath;
File f = File('$path/mypollshash.txt');
if (f.existsSync()) {
print('exists');
String contents = await f.readAsString();
content = contents;
fetchHash();
} else {
print('not exists');
fetch();
}
return f;
}
Future checkfileexist() async {
try {
final file = await _localFile;
String contents = await file.readAsString();
content = contents;
} catch (e) {
//return 'nothing';
}
}
Future<File> writehash(String hash) async {
final file = await _localFile;
return file.writeAsString('$hash', mode: FileMode.write);
}
Future<File> get _localjson async {
final path = await _localPath;
return File('$path/mypolls.json');
}
Future<File> writejson(String json) async {
final file = await _localjson;
return file.writeAsString('$json', mode: FileMode.write);
}
Future readjson() async {
try {
final file = await _localjson;
String contents = await file.readAsString();
content = contents;
setState(() {
polls = pollsFromJson(content);
});
writejson(pollsToJson(polls));
writehash(polls.hash);
print('here');
// return contents;
} catch (e) {
print('there');
print(e);
// If we encounter an error, return 0
//return 'nothing';
}
}
fetch() async {
String data =
await DefaultAssetBundle.of(context).loadString("assets/mypolls.json");
setState(() {
polls = pollsFromJson(data);
});
writejson(pollsToJson(polls));
writehash(polls.hash);
}
fetchHash() async {
String data = await DefaultAssetBundle.of(context)
.loadString("assets/pollshash.json");
print(content);
final pollshash = pollshashFromJson(data);
if (content == pollshash.hash) {
print('take from the saved json');
readjson();
} else {
print('call api');
fetch();
}
}
我在这里叫它：
@override
void initState() {
super.initState();
checkfileexist();
}
问题是，一旦页面启动，我将得到长度为空的红色错误flutter屏幕。。因为数据尚未提取。。然后将显示列表。。
我如何获取数据，然后建立列表，这样我就不会得到错误？
而且，自从我收到这些消息以来，这些功能不断重复：
I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里I/flutter(3556(：存在I/flutter(3556(：I/flutter(3556(：调用apiI/flutter(3556(：存在I/flutter(3556(：d1f4bd60f52991d100adafa416f48b52I/flutter(3556(：从保存的json中获取I/flutter(3556(：在这里….
我怎么能只调用一次？页面首次启动的时间？
您需要使用FutureBuilder
https://docs.flutter.io/flutter/widgets/FutureBuilder-class.html请参阅示例代码，其中"_calculation"是数据源。
它需要被包裹在一个未来中。
试试这个
Widget _myBuild() {
return FutureBuilder(
future: _fetchData(),
builder: (context, snapshot) {
if (snapshot.hasData) {
// Update UI with data
} else if (snapshot.hasError) {
// show error
}
// show progress indicator
CircularProgressIndicator();
});
}