Manipulating JSON

JavaScript Object Notation (JSON) is a lightweight data-interchange format. As the reference site says, It is easy for humans to read and write. It is easy for machines to parse and generate. It is based on a subset of the JavaScript programming language, but it is not limited to JavaScript in any way. Indeed, JSON is a text format that is completely language agnostic. These properties make JSON an ideal data-interchange language for many uses. In recent years, JSON has become on a par with XML in many applications, especially when data size matters, because of its intrinsic conciseness and simplicity.

Getting ready

JSON provides the following five datatypes—String, Number, Object, Array, Boolean, and Null.

This simplicity is an advantage when you have to read a JSON string into some kind of language-specific structure, because every modern language supports the JSON datatypes as simple types, or as a HashMap (in the case of JSON objects) or List (in the case of JSON arrays). So, it makes sense that a data format that is interchangeable with programming languages is also based on these types and structures.

Since version 2009, Delphi has provided built-in support for JSON. The System.JSON.pas unit contains all the JSON types with a nice object-oriented interface. In this recipe, you'll see how to generate, modify, and parse a JSON string.

How to do it...

Let's look at the following steps:

1. Create a new VCL application and drop in three TButton and a TMemo. Align all the buttons as a toolbar at the top of the form and the memo to all the remaining from client areas.

2. From left to right, name the buttons btnGenerateJSON, btnModifyJSON, and btnParseJSON.
3. We'll use static data as our data source. A simple matrix is enough for this recipe. Just after the start of the implementation section of the unit, write the following code:

type 
  TCarInfo = ( 
    Manufacturer = 1, 
    Name = 2, 
    Currency = 3, 
    Price = 4); 
 
var 
  Cars: array [1 .. 4] of 
     array [Manufacturer .. Price] of string = ( 
    ('Ferrari','360 Modena','EUR', '250000'), 
    ('Ford', 'Mustang', 'USD', '80000'), 
    ('Lamborghini', 'Countach', 'EUR','300000'), 
    ('Chevrolet', 'Corvette', 'USD', '100000') 
  ); 

4. TMemo is used to show our JSON files and our data. To keep things clear, create a public property called JSON on the form and map its setter and getter to the Memo1.Lines.Text property. Use the following code:

//...other form methods declaration  
private 
  procedure SetJSON(const Value: String); 
  function GetJSON: String; 
public 
  property JSON: String read GetJSON write SetJSON; 
end; 
 
//...then in the implementation section 
function TMainForm.GetJSON: String; 
begin 
  Result := Memo1.Lines.Text; 
end; 
 
procedure TMainForm.SetJSON(const Value: String); 
begin 
  Memo1.Lines.Text := Value; 
end; 

5. Now, create event handlers for each button and write the code that follows. Pay attention to the event names:

procedure TMainForm.btnGenerateJSONClick(Sender: TObject); 
var 
  i: Integer; 
  JSONCars: TJSONArray; 
  Car, Price: TJSONObject; 
begin 
  JSONCars := TJSONArray.Create; 
  try 
    for i := Low(Cars) to High(Cars) do 
    begin 
      Car := TJSONObject.Create; 
      JSONCars.AddElement(Car); 
      Car.AddPair('manufacturer', 
         Cars[i][TCarInfo.Manufacturer]); 
      Car.AddPair('name', Cars[i][TCarInfo.Name]); 
      Price := TJSONObject.Create; 
      Car.AddPair('price', Price); 
      Price.AddPair('value', 
         TJSONNumber.Create(
           Cars[i][TCarInfo.Price].ToInteger)); 
      Price.AddPair('currency',
         Cars[i][TCarInfo.Currency]); 
    end; 
    JSON := JSONCars.ToJSON; 
  finally 
    JSONCars.Free; 
  end; 
end; 
 
procedure TMainForm.btnModifyJSONClick(Sender: TObject); 
var 
  JSONCars: TJSONArray; 
  Car, Price: TJSONObject; 
begin 
  JSONCars := TJSONObject.ParseJSONValue(JSON)  
as TJSONArray; 
  try 
    Car := TJSONObject.Create; 
    JSONCars.AddElement(Car); 
    Car.AddPair('manufacturer', 'Hennessey'); 
    Car.AddPair('name', 'Venom GT'); 
    Price := TJSONObject.Create; 
    Car.AddPair('price', Price); 
    Price.AddPair('value', TJSONNumber.Create(600000)); 
    Price.AddPair('currency', 'USD'); 
    JSON := JSONCars.ToJSON; 
  finally 
    JSONCars.Free; 
  end; 
end; 
 
procedure TMainForm.btnParseJSONClick(Sender: TObject); 
var 
  JSONCars: TJSONArray; 
  i: Integer; 
  Car, JSONPrice: TJSONObject; 
  CarPrice: Double; 
  s, CarName, CarManufacturer, CarCurrencyType: string; 
begin 
  s := ''; 
  JSONCars := TJSONObject.ParseJSONValue(JSON)  
                                         as TJSONArray; 
  if not Assigned(JSONCars) then 
    raise Exception.Create('Not a valid JSON'); 
  try 
    for i := 0 to JSONCars.Count - 1 do 
    begin 
      Car := JSONCars.Items[i] as TJSONObject; 
      CarName := Car.GetValue('name').Value; 
      CarManufacturer := 
         Car.GetValue('manufacturer').Value; 
      JSONPrice := Car.GetValue('price') as TJSONObject; 
      CarPrice := (JSONPrice.GetValue('value') as 
         TJSONNumber).AsDouble; 
      CarCurrencyType := JSONPrice.GetValue('currency') 
      .Value 
      s := s + Format( 
        'Name = %s' + sLineBreak + 
        'Manufacturer = %s' + sLineBreak + 
        'Price = %.0n%s' + sLineBreak + 
        '-----' + sLineBreak, 
        [CarName, CarManufacturer, 
        CarPrice, CarCurrencyType]); 
    end; 
    JSON := s; 
  finally 
    JSONCars.Free; 
  end; 
end; 

6. Run the application by hitting F9 (or by going to Run | Run).
7. Click on the btnGenerateJSON button, and you should see a JSON array and some JSON objects in the memo.
8. Click on the btnModifyJSON button, and you should see one more JSON object inside the outer JSON array in the memo.
9. Click on the last button, and you should see the same data as before, but with a normal text representation.
10. After the third click, you should see something similar to the following screenshot:

Figure 1.13: Text representation of the JSON data generated and modified

There's more...

In JSON objects, the Owned property determines whether the parent is responsible for the destruction of the object. This property by default is True, meaning all contained instances are owned by their parent. This is why, usually, if you have a combination of various JSON objects, you free only the last parent.

Although not the fastest or the most standards-compliant on the market, JSON usability is important because other Delphi technologies, such as DataSnap, use it. Luckily, there are a lot of alternative JSON parsers for Delphi, if you find you have trouble with the standard one.

Other notable JSON parsers are the following:

• SuperObject (https://github.com/hgourvest/superobject)
• The one included in Delphi Web Script library can be found at https://bitbucket.org/egrange/dwscript/
• A fast JSON parser from Andreas Hausladen can be found at https://github.com/ahausladen/JsonDataObjects

If your main concern is speed, then check out these alternative JSON parsers.

There are also a lot of serialization libraries that use JSON as a serialization format. In general, every parser has its own way to serialize an object to JSON. Find your favorite. Just as an example, in Chapter 5, The Thousand Faces of Multithreading, in the Using tasks to make your customer happy recipe, you will see an open source library containing a set of serialization helpers using the default Delphi JSON parser.

However, JSON is not the right tool for every interchange or data-representation job. XML has been creating other technologies that can help if you need to search, transform, and validate your data in a declarative way. In JSON land, there is no such level of standardization, apart from the format itself. However, over the years, there has been an effort to include at least the XML schema counterpart in JSON, and you can find more details at http://json-schema.org/.

One of the reasons JSON was chosen over XML (in transfer protocol scenarios) is that JSON results in less data for the same amount of information.