Flutter: create an Object from Type or String

Question

I want to create an Object from an object type defined in Type or from an object name defined in String. The following example uses a String to hold the object type. But I think this is not that elegant - even with Type, this if block would increase dramatically for a lot of object types...

I didn't find a better solution for this yet. How can I create that object dynamically from the specified object type?

if (model == 'Event') {
  data = Event.fromMap(result);
}
else if (model == 'Content') {
  data = Content.fromMap(result);
}
else if (...) {
  // ...
}

You can't, but you could compress your code with a switch statement in a function. Each case contains just a return. — Richard Heap
– Richard Heap, Commented Feb 16, 2020 at 14:07
Ok, thank you for the answer. It's not ideal, but at least a small optimization to my code above... — mixable
– mixable, Commented Feb 18, 2020 at 13:22

Ademir Villena Zevallos · Accepted Answer · 2020-02-16 14:07:32Z

This is other approach.

class Event{
  Event.fromMap(_map){
    print('This is an event');
    print(_map);
  }
}

class Content{
  Content.fromMap(_map){
    print('This is a content');
    print(_map);
  }
}

Map<String, Function> types = {
  'Event' : (_map)=>Event.fromMap(_map),
  'Content' : (_map)=>Content.fromMap(_map),
};

void main() {
  var a = types['Event']({'test':'success_event'});
  print(a.runtimeType);
  var b = types['Content']({'test':'success_content'});
  print(b.runtimeType);
}

Its a bit more scalable (since only depends on add the class constructor into the map).

The explanation:

class Event{
  Event.fromMap(_map){
    print('This is an event');
    print(_map);
  }
}

class Content{
  Content.fromMap(_map){
    print('This is a content');
    print(_map);
  }
}

Here we are creating the test classes. nothing important.

Map<String, Function> types = {
  'Event' : (_map)=>Event.fromMap(_map),
  'Content' : (_map)=>Content.fromMap(_map),
};

Here we are defining a Map. Why? Because it allows us to access some value through some key in constant time. In this case, the keys are the Strings 'Event', 'Content', but also can be types as you wanted. For simplicity, let them be Strings. The values are Function's, in this example only getting as parameter a _map (because the Class constructors in the example require one parameter _map). So, if you need more types, only add the type and the function encapsulating the constructor for that type.

void main() {
  var a = types['Event']({'test':'success_event'});
  print(a.runtimeType);
  var b = types['Content']({'test':'success_content'});
  print(b.runtimeType);
}

Finally you can instantiate the classes easily. Only with the type string and passing to the function the values you want (In this example a map with a key 'test').

In your example would be something like:

data = types[model](result);

Thank you for the answer. This might be a solution I can life with.

Eric Aya · Accepted Answer · 2023-03-28 08:18:28Z

0

I also got stuck with a similar problem. Here's how I fixed it:

Create Objects of a class :

class a{ 
a.fromMap(_map){}}

class b{
b.fromMap(_map){}}

Map<String,Dynamic> obj (){
return { 
"a":a,
"b":b}

Now we can utilize the obj object to perform the desired operations

Hope this helps

edited Mar 28, 2023 at 8:18

Eric Aya

70.2k36 gold badges190 silver badges266 bronze badges

answered Mar 27, 2023 at 20:30

Sarthak Sarkar

351 bronze badge

Collectives™ on Stack Overflow

Flutter: create an Object from Type or String

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related