The Question Mark - blog by Mark Volkmann

The JSON package defines the classes Json, JsonObject, and JsonError. It also adds the method jsonWriteOn: to many classes including Array2D, Association, CharacterSequence, Collection, Dictionary, False, Integer, Number, Text, True, and UndefinedObject.

To install the JSON package, enter Feature require: #JSON in a Workspace and “Do it”.

Custom classes whose instances need to be serialized to JSON should implement the instance method jsonWriteOn:. This method describes the instance variables that should be included. Here’s how it could be implemented for a Dog class with instance variables id, name, and breed.

jsonWriteOn: aWriteStream
    {
        #id->id.
        #name->name.
        #breed->breed
    } asDictionary jsonWriteOn: aWriteStream

Here’s how we can get a JSON string for a Dog object.

json := Json render: dog

Custom classes that need to deserialize JSON to a new instance should implement the class method fromJson: (suggested name) that takes a stream of JSON data and returns a new object created from it. Here’s an example that uses the Dog class method id:name:breed: to create a new instance.

fromJson: aStream
    | jsonObject |
    jsonObject := Json readFrom: aStream.
    ^Dog
        id: (jsonObject at: #id)
        name: (jsonObject at: #name)
        breed: (jsonObject at: #breed)

Here’s how we can parse a JSON string to get a Dog object.

newDog := Dog fromJson: json readStream