How to write a map to a YAML file in Dart - dart

Question

I have a map of key value pairs in Dart. I want to convert it to YAML and write into a file.
I tried using YAML package from dart library but it only provides methods to load YAML data from a file. Nothing is mentioned on how to write it back to the YAML file.
Here is an example:
void main() {
var map = {
"name": "abc",
"type": "unknown",
"internal":{
"name": "xyz"
}
};
print(map);
}
Expected output:
example.yaml
name: abc
type: unknown
internal:
name: xyz
How to convert the dart map to YAML and write it to a file?

Answer 1

It's a bit late of a response but for anyone else looking at this question I have written this class. It may not be perfect but it works for what I'm doing and I haven't found anything wrong with it yet. Might make it a package eventually after writing tests.
class YamlWriter {
/// The amount of spaces for each level.
final int spaces;
/// Initialize the writer with the amount of [spaces] per level.
YamlWriter({
this.spaces = 2,
});
/// Write a dart structure to a YAML string. [yaml] should be a [Map] or [List].
String write(dynamic yaml) {
return _writeInternal(yaml).trim();
}
/// Write a dart structure to a YAML string. [yaml] should be a [Map] or [List].
String _writeInternal(dynamic yaml, { int indent = 0 }) {
String str = '';
if (yaml is List) {
str += _writeList(yaml, indent: indent);
} else if (yaml is Map) {
str += _writeMap(yaml, indent: indent);
} else if (yaml is String) {
str += "\"${yaml.replaceAll("\"", "\\\"")}\"";
} else {
str += yaml.toString();
}
return str;
}
/// Write a list to a YAML string.
/// Pass the list in as [yaml] and indent it to the [indent] level.
String _writeList(List yaml, { int indent = 0 }) {
String str = '\n';
for (var item in yaml) {
str += "${_indent(indent)}- ${_writeInternal(item, indent: indent + 1)}\n";
}
return str;
}
/// Write a map to a YAML string.
/// Pass the map in as [yaml] and indent it to the [indent] level.
String _writeMap(Map yaml, { int indent = 0 }) {
String str = '\n';
for (var key in yaml.keys) {
var value = yaml[key];
str += "${_indent(indent)}${key.toString()}: ${_writeInternal(value, indent: indent + 1)}\n";
}
return str;
}
/// Create an indented string for the level with the spaces config.
/// [indent] is the level of indent whereas [spaces] is the
/// amount of spaces that the string should be indented by.
String _indent(int indent) {
return ''.padLeft(indent * spaces, ' ');
}
}
Usage:
final writer = YamlWriter();
String yaml = writer.write({
'string': 'Foo',
'int': 1,
'double': 3.14,
'boolean': true,
'list': [
'Item One',
'Item Two',
true,
'Item Four',
],
'map': {
'foo': 'bar',
'list': ['Foo', 'Bar'],
},
});
File file = File('/path/to/file.yaml');
file.createSync();
file.writeAsStringSync(yaml);
Output:
string: "Foo"
int: 1
double: 3.14
boolean: true
list:
- "Item One"
- "Item Two"
- true
- "Item Four"
map:
foo: "bar"
list:
- "Foo"
- "Bar"

Answer 2

package:yaml does not have YAML writing features. You may have to look for another package that does that – or write your own.
As as stopgap, remember JSON is valid YAML, so you can always write out JSON to a .yaml file and it should work with any YAML parser.

Answer 3

I ran into the same issue and ended up hacking together a simple writer:
// Save the updated configuration settings to the config file
void saveConfig() {
var file = _configFile;
// truncate existing configuration
file.writeAsStringSync('');
// Write out new YAML document from JSON map
final config = configToJson();
config.forEach((key, value) {
if (value is Map) {
file.writeAsStringSync('\n$key:\n', mode: FileMode.writeOnlyAppend);
value.forEach((subkey, subvalue) {
file.writeAsStringSync(' $subkey: $subvalue\n',
mode: FileMode.writeOnlyAppend);
});
} else {
file.writeAsStringSync('$key: $value\n',
mode: FileMode.writeOnlyAppend);
}
});
}