Prevent to read file many times - dart

Question

I am trying to write an i18n app. The program read a json file, that contains translation from languages and it based on json structure.
{
"EN": {
"TEXT1": "Hello",
"TEXT2": "March"
},
"DE": {
"TEXT1": "Hallo",
"TEXT2": "März"
}
}
My program read the json file in async way with the file class, the whole code
import 'dart:io';
import 'dart:async';
import 'package:json_object/json_object.dart';
abstract class I18n {
static _I18n _i18n;
factory I18n(String file, String lang) {
if(_i18n == null) {
_i18n = new _I18n(file, lang);
return _i18n;
}
return _i18n;
}
Future<String> getTextByMap(String textId);
}
class _I18n implements I18n {
File _file;
String _lang;
JsonObject _jsonContainer;
JsonObject _jsonFiltered;
Future<JsonObject> _imme;
// Parameters:
// file: The whole path and filename
// lang: Expected language
_I18n(String file, this._lang) {
this._file = new File(file);
}
// Read file and return the content of file.
Future<String> _readFileFromStream() {
var com = new Completer();
this._file.exists()
.then((fileExists) {
if(!fileExists) {
throw new StateError('File not found');
}
return this._file.readAsString()
.then((stream) => com.complete(stream));
});
return com.future;
}
void _convertContentToJson(String stream) {
this._jsonContainer = new JsonObject.fromJsonString(stream);
}
Future<JsonObject> _prepareData() {
return this._readFileFromStream().then((stream) {
_convertContentToJson(stream);
this._jsonFiltered = this._jsonContainer[this._lang];
return this._jsonFiltered;
});
}
Future<String> getTextByMap(String textId) {
return this._prepareData().then((filterd) {
return filterd[textId];
});
}
}
and the main code
import 'package:i18n/i18n.dart';
void main() {
var i18n = new I18n('../hello.json', 'EN');
i18n.getTextByMap('TEXT1').then((val) => print(val));
i18n.getTextByMap('TEXT2').then((val) => print(val));
}
Everything here, happen in dart async way, read json file etc. And everytime, when i call the method
i18n.getTextByMap('TEXT1').then((val) => print(val));
it gonna read the json file again and again. I tried to rewrite the method to prevent reading json file many times
Future<String> getTextByMap(String textId) {
if(this._jsonFiltered == null)
{
return this._prepareData().then((filterd) {
return filterd[textId];
});
}
return new Future(() => this._jsonFiltered[textId]);
}
but it doesn't work too, because dart works in async way.
My question is, how can i keep this json file content in an object? Read json file only one time and keep the contents in an object, it is better then read json file everytime, that is my opinion.
It could do everything in sync way, then i wouldn't have such as problem but this is not dart terminology.
In which order do dart execute I/O operations, like this?
Future
I/O Events

Answer 1

My solution would be to create a class with a factory constructor. The factory constructor always returns a object of that file.
Your problem is that futures are parallel. So both calls are executed in parallel. The solution is to let the first future complete and then do other stuff to be able to get cached results.
Then you can have a read() method that reads the value of the file if it is not present in the classes "contents" attribute for example - or if that attribute is not null, it loads the file in background.
In both cases a completer or future is returned you can listen on.
EDIT Example Code:
example_async_file_factory.dart
import 'dart:io';
import 'dart:async';
class FileHolder {
String _contents = null;
String path;
static Map<String, FileHolder> _files;
factory FileHolder(String path) {
if (_files == null) {
_files = {};
}
if (_files.containsKey(path)) {
return _files[path];
} else {
final fh = new FileHolder._internal(path);
_files[path] = fh;
return fh;
}
}
FileHolder._internal(this.path);
Future<String> getContents() {
if(_contents != null) {
print("cached");
return new Future.value(_contents);
} else {
print("read");
File f = new File(this.path);
Future<String> future = f.readAsString();
Completer completer = new Completer();
future.then((String c) {
_contents = c;
completer.complete(_contents);
});
return completer.future;
}
}
}
void main() {
FileHolder f = new FileHolder("example_async_file_factory.dart");
f.getContents().then((String contents) {
print(contents.length);
FileHolder f2 = new FileHolder("example_async_file_factory.dart");
f2.getContents().then((String contents) {
print(contents.length);
});
f2.getContents().then((String contents) {
print(contents.length);
});
f.getContents().then((String contents) {
print(contents.length);
});
});
}
Output:
read
1411
cached
cached
cached
1411
1411
1411
Regards
Robert