Streaming JSON/zh CN
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JSON (JavaScript Object Notation）是一种基于文本的标准化数据格式。顾名思义，JSON 文档是有效的 JavaScript 代码，可以直接转换为 JavaScript 对象。但是，无论使用哪种编程语言，JSON 都可用于数据交换。
本教程解释了如何将 JSON 数据加载到免费的 Pascal 程序中并在那里进行处理。它还解释了如何将数据从程序转换为JSON（例如将其发送到Web浏览器）。
General requirements
加载和存储（流式处理）对象是通过 fpjsonrtti 单元完成的。使用类单元也很有意义（有关详细信息，请参见下文）。
因此，uses 语句应至少包含以下两个单元：
uses Classes, fpjsonrtti;
目前（2014 年 <> 月）Free Pascal 的流媒体系统和 JSON 之间存在一些差异：
- JSON 是区分大小写的数据格式。因此，Free Pascal对象的属性必须以与JSON属性相同的方式编写。
- 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.
