Streaming JSON

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JSON (JavaScript Object Notation) is a text-based, standardized data format. As the name implies, JSON documents are valid JavaScript code and can be directly converted into JavaScript objects. However, JSON can be used for data exchange regardless of the programming language used.

This tutorial explains how to load JSON data into a Free Pascal program and process it there. It also explains how to convert data from the program into JSON (eg to send it to a web browser).

General requirements

Loading and storing (streaming) objects is done with the fpjsonrtti unit. It also makes sense to use the Classes unit (see below for details).

The uses statement should therefore contain at least these two units:

uses Classes, fpjsonrtti;

Currently (May 2014) there are some differences between Free Pascal's streaming system and JSON:

• JSON is a case-sensitive data format. It follows that the properties of the Free Pascal objects must be written in the same case as the JSON properties.
• No properties can be defined with DefineProperties. No references to methods (event handlers) can be saved.²
• TCollection and TStrings can used as a replacements for arrays.

Demo programs are provided with Free Pascal Compiler source code in the packages/fcl-json/examples directory.

This JSON structure is used in the examples which follow:

{
  "id"     : 123,                                                // an integer
  "obj"    : { "name": "Hello world!" },                         // an object
  "coll"   : [ { "name": "Object 1" }, { "name": "Object 2" } ], // two objects in a TCollection
  "strings": [ "Hello 1", "Hello 2" ]                            // a string list
}

It can be defined in your Free Pascal program using a constant assignment as follows:

const JSON_TESTDATA =
'{'+LineEnding+
'  "id": 123,'+LineEnding+
'  "obj": { "name": "Hello world!" },'+LineEnding+
'  "coll": [ { "name": "Object 1" }, { "name": "Object 2" } ],'+LineEnding+
'  "strings": [ "Hello 1", "Hello 2" ]'+LineEnding+
'}';

Data Structure

The base class for the data is TPersistent from the Classes unit, runtime type information (RTTI) is created for it and all subclasses. These are essential for streaming. Since fpjsonrtti does not integrate into the streaming system, any other class translated with the compiler switch {$M+} can also be used.

All properties to be read must be declared as a property in the published section of the class. As a rule, you can use read and write to refer directly to a data field (the variable). If you want, you can of course use getter and setter methods.

The following class definition results from the JSON structure:

type
  TNameObject = class(TCollectionItem) // class for the 'obj' property and TCollection 
  private
    fName: String;
  published
    property name: String read fName write fName;
  end;  

  TBaseObject = class(TPersistent)  // class for the entire JSON structure
  private
    fid: Integer;
    fObj: TNameObject;
    fColl: TCollection;
    fStrings: TStrings;
  public
    constructor Create;
    destructor Destroy; override;
  published                         // all properties must be published 
    property id: Integer read fid write fid;
    property obj: TNameObject read fObj write fObj;
    property coll: TCollection read fColl;
    property strings: TStrings read fStrings;
  end;

The TNameObject class was derived from TCollectionItem. This means that it can be used both for the obj property and in the collection. If this is not desired, then two different classes must be defined here.

The TCollection and the string list must be created in the constructor of the TBaseObject class and released in the destructor.

constructor TBaseObject.Create;
begin
  // Create Collection and StringList
  fColl    := TCollection.Create(TNameObject);
  fStrings := TStringList.Create;
  fObj     := TNameObject.Create(nil);
end;

destructor TBaseObject.Destroy;
begin
  // Release Collection and StringList
  fColl.Free;
  fStrings.Free;
  fObj.Free;
  inherited Destroy;
end;

If you do not want any more functionality in the data classes, their definition is now complete.

Load JSON

With the method Procedure JSONToObject(Const JSON : TJSONStringType; AObject : TObject); in the TJSONDeStreamer class you can assign JSON data directly to an existing object. Before you call the method, you must create TJSONDeStreamer and the target object.

The following method loads the data from the JSON structure JSON_TESTDATA in the object o and then outputs the current values ​​of the properties to the console.

procedure DeStreamTest;
var
  DeStreamer: TJSONDeStreamer;
  o: TBaseObject;
  no: TNameObject;
  s: String;
begin
  WriteLn('DeStream test');
  WriteLn('======================================');

  // Create the DeStreamer object and target object
  DeStreamer := TJSONDeStreamer.Create(nil);
  o := TBaseObject.Create;
  try
    // Load JSON data into object o
    DeStreamer.JSONToObject(JSON_TESTDATA, o);
    // output ID
    WriteLn(o.id);
    // output object name
    WriteLn(o.obj.name); 
    // output the names of all objects
    for TCollectionItem(no) in o.coll do
      Writeln(no.name);
    // output all strings
    for s in o.strings do
      WriteLn(s);

  // Cleanup
  finally
    o.Destroy;
    DeStreamer.Destroy;
  end;
end;

Saving JSON

The class TJSONStreamer is used to convert an object into JSON text. Here the method Function ObjectToJSONString(AObject : TObject) : TJSONStringType; is used.

In the following procedure, an object is created, filled with the test data, and then converted to JSON. The JSON text is output on the console. The order in which the properties are output cannot be specified.

procedure StreamTest;
var
  Streamer: TJSONStreamer;
  o: TBaseObject;
  JSONString: String;
begin
  WriteLn('Stream test');
  WriteLn('======================================');

  Streamer := TJSONStreamer.Create(nil);
  o := TBaseObject.Create;
  try
    // Setup data
    o.id := 123;
    o.obj.name := 'Hello world!';
    TNameObject(o.coll.Add).name := 'Object 1';
    TNameObject(o.coll.Add).name := 'Object 2';
    o.strings.Add('Hello 1');
    o.strings.Add('Hello 2');

    Streamer.Options := Streamer.Options + [jsoTStringsAsArray]; // Save strings as JSON array
    // convert to JSON and output to console
    JSONString := Streamer.ObjectToJSONString(o);
    WriteLn(JSONString);

  // Cleanup
  finally
    o.Destroy;
    Streamer.Destroy;
  end;
end;

Custom properties

Now that we can save and load to JSON, we can as well customize how the property is saved. By default a TColor (TGraphicsColor) is saved as a number, but for example we want to make it a string, so is more human readable.

This example uses BGRABitmap library, that has a function to convert from string to color easily. The streamed control in this case is a button control from BGRAControls package.

This customizes how TColor is saved and loaded.

Saving code:

procedure TForm1.Button1Click(Sender: TObject);
var
  Streamer: TJSONStreamer;
  JSONString: String;
begin
  Streamer := TJSONStreamer.Create(nil);
  try
    Streamer.OnStreamProperty:=@OnStreamProperty;
    JSONString := Streamer.ObjectToJSONString(BCButton1.StateNormal);
    Memo1.Lines.Text := JSONString;
  finally
    Streamer.Destroy;
  end;
end;

procedure TForm1.OnStreamProperty(Sender: TObject; AObject: TObject;
  Info: PPropInfo; var Res: TJSONData);
var
  bgracolor: TBGRAPixel;
begin
  if (Info^.PropType^.Name = 'TGraphicsColor') then
  begin
    bgracolor := ColorToBGRA(TColor(GetPropValue(AObject, Info, False)));
    Res.Free;
    Res := TJSONString.Create('rgb('+IntToStr(bgracolor.red)+','+IntToStr(bgracolor.green)+','+IntToStr(bgracolor.blue)+')');
  end;
end;

Loading code:

procedure TForm1.Button2Click(Sender: TObject);
var
  DeStreamer: TJSONDeStreamer;
  s: String;
begin
  DeStreamer := TJSONDeStreamer.Create(nil);
  try
    DeStreamer.OnRestoreProperty:=@OnRestoreProperty;
    DeStreamer.JSONToObject(Memo1.Lines.Text, BCButton1.StateNormal);
  finally
    DeStreamer.Destroy;
  end;
end;

procedure TForm1.OnRestoreProperty(Sender: TObject; AObject: TObject;
  Info: PPropInfo; AValue: TJSONData; var Handled: Boolean);
var
  bgracolor: TBGRAPixel;
begin
  Handled := False;
  if (Info^.PropType^.Name = 'TGraphicsColor') then
  begin
    Handled := True;
    bgracolor := StrToBGRA(AValue.AsString);
    SetPropValue(AObject, Info, BGRAToColor(bgracolor));
  end;
end;

Conclusion

With the knowledge presented, simple and complex JSON data structures can be loaded into Free Pascal programs. Should any pre- or post-processing of the JSON data be necessary, the text data can be first loaded from the jsonparser unit into a JSON data structure using the TJSONParser class and then manipulated as desired with the fpJSON unit.

The Options property of the TJSONStreamer class can be used to control how the output maps its own data structures in JSON.

See Also

• JSON
• fcl-json
• Streaming components

Notes & References

1. http://lists.freepascal.org/fpc-pascal/2013-January/036254.html
2. http://lists.lazarus.freepascal.org/pipermail/lazarus/2011-January/058878.html