Difference between revisions of "WebAssembly/DOM"

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== Accessing the DOM from WebAssembly ==
+
Accessing JS Objects from WebAssembly.
  
=== General architecture ===
+
= JOB =
Create pascal units, containing ‘proxy’ classes: calling a method on a proxy class will call the corresponding class in JS.
 
The proxy classes can be generated by adapting the existing webidl2pas tool.
 
  
==== Problem: data transfer between JS/WebAssembly ====
+
JS Object Bridge - JOB
JS/Webassembly interface only supports passing integers & floats, not objects.
 
  
 +
JOB provides:
 +
* Units to communicate between fpc wasm and pas2js browser to call JS functions, get and set JS properties and set callbacks.
 +
* Units for fpc wasm with common browser classes.
 +
* A tool webidl2pas to help generating new units for JS classes from webidls.
  
'''solution''': Every object is stored in an array with ID
+
== Using JOB ==
ID is used to pass references to object between JS and Webassembly
 
Lifetime is controlled from WebAssembly.
 
By using interfaces, the lifetime of objects can be controlled by compiler.
 
Methods can be called using an invoke mechanism.
 
  
Due to limited type support in Javascript, only a handful of types must be supported by invoke.
+
A JOB program has a webassembly program (fpc wasi) and a browser (pas2js) program.  
  
==== Problem: Callbacks ====
+
The browser side contains the html and registers all global JS variables needed by the webassembly side.
  
An event handler in WebAssembly must be callable from Javascript.  
+
See the pas2js demo/wasienv/button/BrowserButton1.lpi
The '''AddEventListener''' has a single method signature, so a single exported function from webassembly can be used for this:
 
  
all that is needed is to pass the object pointer & method pointer (both integers), plus the ID of the event object.
+
https://gitlab.com/freepascal.org/fpc/pas2js/-/tree/main/demo/wasienv/button
Pointers to methods & instances can be passed between JS and webassembly, this can be used.
 
Alternatively: Using the FPC dispatchstr mechanism, the correct method can be called in Webassembly. To be checked.
 
  
Advantage of this method is that only a couple of webassembly and Javascript exports are needed.
+
=== JS Classes ===
  
=== Implementation details ===
+
Each JS class like '''HTMLButtonElement''' have a FPC class (''TJSHTMLButtonElement'') with a reference counted interface (''IJSHTMLButtonElement'').
 +
 
 +
Normally functions return an interface and expect interfaces as arguments.
 +
 
 +
=== Callbacks ===
 +
 
 +
An event handler in WebAssembly can be called from Javascript.
 +
 
 +
At the moment only methods (of object) are supported.
 +
 
 +
<source lang="pascal">
 +
  TWasmApp = class
 +
    ...
 +
    function OnButtonClick(Event: IJSEvent): boolean;
 +
    ...
 +
  end;
 +
 
 +
function TWasmApp.OnButtonClick(Event: IJSEvent): boolean;
 +
begin
 +
  JSWindow.Alert('You triggered TWasmApp.OnButtonClick');
 +
  Result:=true;
 +
end;
 +
...
 +
  JSButton.addEventListener('click',@OnButtonClick);
 +
...
 +
</source>
 +
 
 +
=== Type casts ===
 +
 
 +
Often a low level interface needs to be type casted to a descendant (or another type). For example ''JSDocument.getElementById'' returns a IJSElement, which is type casted to IJSHTMLElement:
 +
 
 +
<source lang="pascal">
 +
var
 +
  Elem: IJSElement;
 +
  HTMLElem: IJSHTMLElement;
 +
begin
 +
  Elem := JSDocument.getElementById('button');
 +
  // type casting to IJSHTMLElement requires creating a new bridge object using TJSHTMLElement.Cast:
 +
  HTMLElem := TJSHTMLElement.Cast(Elem);
 +
  // Note: since Elem and HTMLElem are reference counted interfaces, the compiler automatically frees temporary objects.
 +
end;
 +
</source>
 +
 
 +
This '''HTMLElement''' has the same '''ObjectId''' as '''Elem''', but it does not own it. It merely keeps a reference to the '''Elem'''. When all references are released, the JS object is released,
 +
 
 +
=== Typeof ===
 +
 
 +
'''InvokeJSTypeOf'''
 +
 
 +
See the JOBResult_* constants in unit JOB_Shared.
 +
 
 +
 
 +
== Create JOB units using webidl2pas ==
 +
 
 +
Compile the latest and greatest webidl2pas utility from fpc main:
 +
 
 +
utils/pas2js/webidl2pas.lpi
 +
 
 +
Download some webidls for example from
 +
https://hg.mozilla.org/mozilla-central/raw-file/tip/dom/webidl/<JSClassName>
 +
 
 +
Concatenate them into one file ''Foo.webidl''.
 +
 
 +
Create a text file ''FooAlias.txt'' containing the used classes from other units:
 +
<pre>
 +
Object=IJSObject
 +
Set=IJSSet
 +
Map=IJSMap
 +
Function=IJSFunction
 +
Date=IJSDate
 +
RegExp=IJSRegExp
 +
String=IJSString
 +
Array=IJSArray
 +
ArrayBuffer=IJSArrayBuffer
 +
TypedArray=IJSTypedArray
 +
BufferSource=IJSBufferSource
 +
DataView=IJSDataView
 +
JSON=IJSJSON
 +
Error=IJSError
 +
Promise=IJSPromise
 +
TextEncoder=IJSTextEncode
 +
TextDecoder=IJSTextDecoder
 +
</pre>
 +
 
 +
Create a text file ''FooGlobals.txt'' containing all the global JS variables in form Pascal variable name=JS class name,registered name:
 +
<pre>
 +
JSFoo=Foo,foo
 +
</pre>
 +
 
 +
Run the tool:
 +
webidl2pas -f wasmjob -i Foo.webidl --typealiases=@FooAlias.txt --globals=@FooGlobals.txt
 +
 
 +
If the tool stops, because it can not find an identifier or something is not yet supported, you can add another webidl or comment the problematic definition. Then run the tool again.
 +
 
 +
In the browser side you must register the global JS variables:
 +
<source lang="pascal">
 +
  FWADomBridge.RegisterGlobalObject(foo,'foo');
 +
</source>
 +
 
 +
=== Not yet supported webidl elements ===
 +
 
 +
==== Elements from other Pascal units ====
 +
 
 +
At the moment you can only provide a list of type aliases for classes. This does not work for callbacks, arrays, sequences, etc. It would be better to give used units and parse them.
 +
 
 +
==== callback interfaces ====
 +
 
 +
"callback interfaces" are a legacy definition.
 +
Remedy: Replace them manually with a callback
 +
 
 +
==== function returning a Dictionary ====
 +
 
 +
ToDo: Define dictionary as class and interface and return an interface reference.
 +
 
 +
==== function returning a Promise ====
 +
 
 +
ToDo: maybe define generic TJSPromise<T> and IJSPromise<T>
 +
 
 +
==== function returning union ====
 +
 
 +
ToDo: use TVarRec or TJOB_JSValue
 +
 
 +
==== function returning a typed sequence ====
 +
 
 +
At the moment it returns an untyped IJSArray.
 +
 
 +
ToDo: returned a typed array.
 +
 
 +
==== function returning a callback ====
 +
 
 +
==== passing an array as argument ====
 +
 
 +
==== varargs ====
 +
 
 +
Functions allowing to pass an arbitrary number of arguments.
 +
 
 +
Workaround: User can call the InvokeX function directly.
 +
 
 +
==== constructor ====
 +
 
 +
==== getter ====
 +
 
 +
==== callback property ====
 +
 
 +
A property with a callback, e.g. '''onAbort'''
 +
 
 +
At the moment it is only added as a comment. Reading callbacks is not yet supported. Theoretically it could be added as a write only property.
 +
 
 +
==== callback with a TJOB_JSValue argument ====
 +
 
 +
E.g. JOBCallTOnErrorEventHandlerNonNull
 +
 
 +
At the moment argument is allocated and must be freed must be the user. Either free it in the auto generated code or use a reference counted interface.
 +
 
 +
== JOB architecture ==
 +
Pascal units containing ‘proxy’ classes: calling a method on a proxy class will call the corresponding class in JS.
 +
The proxy classes can be generated by the existing '''webidl2pas''' tool with '''-f wasmjob''' flag.
 +
 
 +
=== Data transfer between JS/WebAssembly ===
 +
JS/Webassembly interface only supports passing atomic types like boolean, integers and floats, not objects or strings.
 +
 
 +
'''Solution''':
 +
* Global objects like '''document''' and '''window''' are registered and queried by name.
 +
* Every object is stored in an array with ID: TJOBObjectID
 +
* ID is used to pass references to object between JS and Webassembly
 +
* Lifetime is controlled from WebAssembly.
 +
* By using interfaces, the lifetime of objects are controlled by the compiler.
 +
* Methods can be called using an invoke mechanism.
 +
* Due to limited type support in Javascript, only a handful of types must be supported by invoke, e.g. undefined, null, boolean, number, unicodestring, Object, callbacks and the union JSValue.
 +
 
 +
=== Function Arguments ===
 +
 
 +
When calling a JS function from wasm, you can pass the following types/constants:
 +
 
 +
* boolean
 +
* integers (limited to double, because all numbers in JS are double, so up to 54 bits)
 +
* double
 +
* nil
 +
* string (utf8 converted to utf16)
 +
* unicodestring
 +
* widestring
 +
* PChar - using strlen to get the size and utf8 converted to utf16
 +
* PWideChar - using strlen to get the size
 +
* TJSObject and IJSObject - its ObjectID is passed to the JS side, where the corresponding JS object is used
 +
* JSUndefined
 +
* TJOB_JSValue
 +
 
 +
=== Function Result ===
 +
 
 +
Calling a JS function is done via the ''InvokeJS*Result'' functions, e.g. ''aJSDate.InvokeJSUnicodeStringResult('toLocaleDateString',[])'' which returns a ''UnicodeString''.
 +
 
 +
If the function does not exist, an ''EJSInvoke'' exception is raised.
 +
 
 +
If the function returns the JS undefined value, JOB returns the default value, e.g. InvokeJSUnicodeStringResult returns the empty string, InvokeJSDoubleResult returns NaN, InvokeJSObjectResult returns nil, InvokeJSBooleanResult returns false.
 +
 
 +
If the function returns an incompatible type, e.g. InvokeJSUnicodeStringResult returns a number, an ''EJSInvoke'' exception is raised.
 +
 
 +
To retrieve any kind of JS value, use InvokeJSValueResult.
 +
 
 +
=== Callbacks ===
 +
 
 +
An event handler in WebAssembly can be called from Javascript.
 +
 
 +
At the moment only methods (of object) are supported.
 +
 
 +
Every function type needs a callback, which decodes the arguments and encode the result.
 +
 
 +
For example TJSEventHandler:
 +
 
 +
<source lang="pascal">
 +
type
 +
  TJSEventHandler = function(Event: IJSEventListenerEvent): boolean of object;
 +
...
 +
function JOBCallTJSEventHandler(const aMethod: TMethod; var H: TJOBCallbackHelper): PByte;
 +
var
 +
  Event: IJSEventListenerEvent;
 +
begin
 +
  // get arguments. First as IJSEventListenerEvent
 +
  Event:=H.GetObject(TJSEventListenerEvent) as IJSEventListenerEvent;
 +
  // call the method and encode the result
 +
  Result:=H.AllocBool(TJSEventHandler(aMethod)(Event));
 +
end;
 +
</source>
 +
 
 +
TJOBCallbackHelper provides functions to decode arguments and encode the result.
 +
 
 +
Passing a method as argument works like this:
 +
 
 +
<source lang="pascal">
 +
type
 +
  IJSEventTarget = interface
 +
    ['{1883145B-C826-47D1-9C63-47546BA536BD}']
 +
    procedure addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
 +
  end;
 +
 
 +
  TJSEventTarget = class(TJSObject,IJSEventTarget)
 +
    procedure addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
 +
  end;
 +
...
 +
procedure TJSEventTarget.addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
 +
var
 +
  m: TJOB_JSValueMethod;
 +
begin
 +
  // combine the users method and the callback into one argument m
 +
  m:=TJOB_JSValueMethod.Create(TMethod(aListener),@JOBCallTJSEventHandler);
 +
  try
 +
    // call the JS function addEventListener(aName,m)
 +
    InvokeJSNoResult('addEventListener',[aName,m]);
 +
  finally
 +
    m.Free;
 +
  end;
 +
end;
 +
</source>
 +
 
 +
== Implementation details ==
  
 
Here are some technical notes describing the various architectural decisions.
 
Here are some technical notes describing the various architectural decisions.
  
A tool is created to generate an interface from the .webidl files. These files for example exist in
+
The webidl2pas tool was extended to generate an interface from the .webidl files (-f wasmjob). These files for example exist in
 
the mozilla firefox repo on github: [https://github.com/mozilla/gecko-dev/tree/master/dom/webidl WebIDL]
 
the mozilla firefox repo on github: [https://github.com/mozilla/gecko-dev/tree/master/dom/webidl WebIDL]
  
 
<source lang="pascal">
 
<source lang="pascal">
IElement = interface ['someawfulGUID'] (IJSObject);
+
IJSElement = interface(IJSObject)
 +
  ['someawfulGUID']
 
   function childElementCount : Integer;
 
   function childElementCount : Integer;
   function firstElementChild : IElement;
+
   function firstElementChild : IJSElement;
 
   // all other
 
   // all other
 
end;
 
end;
 
</source>
 
</source>
 
Only the interfaces are exposed in the API to access the DOM.
 
  
 
In implementation, the following kind of code can be found:
 
In implementation, the following kind of code can be found:
Line 49: Line 296:
 
<source lang="pascal">
 
<source lang="pascal">
 
// Hand crafted in e.g. JSObject unit
 
// Hand crafted in e.g. JSObject unit
TJSObject = class(TInterfacedObject)
+
  TJSObject = class(TInterfacedObject,IJSObject)
begin
+
  public
  constructor CreateFromID(aID: NativeInt);
+
    constructor CreateFromID(aID: TJOBObjectID); virtual;
  destructor destroy; override;
+
    destructor Destroy; override;
  Property ObjectID : NativeInt;
+
    property ObjectID: TJOBObjectID read FObjectID;
  function InvokeJSNativeIntResult(aName : string; Const args : Array of const) : NativeInt;
+
    // call a function
  function InvokeJSStringResult(aName : string; Const args : Array of const) : String;
+
    procedure InvokeJSNoResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeSetType = jisCall); virtual;
  function InvokeJSObjResult(aName : string; aResultClass: TDOMOBjectClass; Const args : Array of const) : TDDOMOBject;
+
    function InvokeJSBooleanResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): Boolean; virtual;
end;
+
    function InvokeJSDoubleResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): Double; virtual;
 +
    function InvokeJSUnicodeStringResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): UnicodeString; virtual;
 +
    function InvokeJSObjectResult(const aName: string; Const Args: Array of const; aResultClass: TJSObjectClass; Invoke: TJOBInvokeGetType = jigCall): TJSObject; virtual;
 +
    function InvokeJSValueResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): TJOB_JSValue; virtual;
 +
    function InvokeJSUtf8StringResult(const aName: string; Const args: Array of const; Invoke: TJOBInvokeGetType = jigCall): String; virtual;
 +
    function InvokeJSLongIntResult(const aName: string; Const args: Array of const; Invoke: TJOBInvokeGetType = jigCall): LongInt; virtual;
 +
    // read a property
 +
    function ReadJSPropertyBoolean(const aName: string): boolean; virtual;
 +
    function ReadJSPropertyDouble(const aName: string): double; virtual;
 +
    function ReadJSPropertyUnicodeString(const aName: string): UnicodeString; virtual;
 +
    function ReadJSPropertyObject(const aName: string; aResultClass: TJSObjectClass): TJSObject; virtual;
 +
    function ReadJSPropertyUtf8String(const aName: string): string; virtual;
 +
    function ReadJSPropertyLongInt(const aName: string): LongInt; virtual;
 +
    function ReadJSPropertyValue(const aName: string): TJOB_JSValue; virtual;
 +
    // write a property
 +
    procedure WriteJSPropertyBoolean(const aName: string; Value: Boolean); virtual;
 +
    procedure WriteJSPropertyDouble(const aName: string; Value: Double); virtual;
 +
    procedure WriteJSPropertyUnicodeString(const aName: string; const Value: UnicodeString); virtual;
 +
    procedure WriteJSPropertyUtf8String(const aName: string; const Value: String); virtual;
 +
    procedure WriteJSPropertyObject(const aName: string; Value: TJSObject); virtual;
 +
    procedure WriteJSPropertyLongInt(const aName: string; Value: LongInt); virtual;
 +
    // create a new object using the new-operator
 +
    function NewJSObject(Const Args: Array of const; aResultClass: TJSObjectClass): TJSObject; virtual;
 +
  end;
 
</source>
 
</source>
  
The various '''Invoke*''' functions encode the arguments in a memory block so they can be
+
The various '''Invoke*''' functions encode the arguments in a memory block so they can be read on the JS side, then calls a '''Invoke_*Result''' function which lives in
read on the JS side, then calls a JSInvokeNNN function which lives in
 
 
Javascript, and which is imported from the browser.  
 
Javascript, and which is imported from the browser.  
  
Line 67: Line 336:
  
 
* Negative IDs are special: window, document.
 
* Negative IDs are special: window, document.
* positive IDs use a '''wasmObjects['id']''' to look for the object.
+
* positive IDs are temporary objects created via the '''InvokeJSObjectResult''', see below.
  
The '''Invoke*''' function decodes the arguments and uses '''TJSFunction.apply''' to execute the requested function.
+
The '''Invoke_*Result''' pas2js function decodes the arguments and uses '''TJSFunction.apply''' to execute the requested function.
The result is put in a memory block, encoded in the same way as incoming arguments.
+
The result is checked for the requested type and then returned to the wasm.
  
If the result is an object, an ID is generated (simple counter), the result
+
If the result is an object, an ID is generated (simple counter), the result value is stored in an array '''FLocalObjects'''.  
value is stored in the array '''wasmObjects['id']''' .  
 
  
The ID is returned to the webassembly, which will use the ID to create a TJSObject descendent.
+
The ID is returned to the webassembly, which will use the ID to create a ''TJSObject'' descendent.
  
The destructor of '''TJSObject''' calls a '''releaseObject''' function in javascript
+
The destructor of '''TJSObject''' calls a '''__job_release_object''' function in javascript if the '''ObjectID''' is positive. The '''ReleaseObject''' function simply sets '''FLocalObjects[id]''' to
if the '''ObjectID''' is positive. The '''releaseObject''' function simply sets '''wasmObjects['id']''' to
+
null, so the browser also releases it.
null. (so the browser also releases it)
 
  
 
The above is a basic invoke mechanism for Javascript code.
 
The above is a basic invoke mechanism for Javascript code.
  
This basic mechanism is then used by a modified version of the webidl
+
This basic mechanism is then used by a modified version of the webidl program to generate proxy definitions. For each object in Javascript, 2 definitions are generated:  
program to generate proxy definitions. For each object in Javascript, 2
 
definitions are generated:  
 
  
 
* The interface (see above for an example)
 
* The interface (see above for an example)
* An implementation object as below, descendent of '''TJSObject'''
+
* An implementation object as below, descendant of '''TJSObject'''
  
 
<source lang="pascal">
 
<source lang="pascal">
 
// Generated from webIDL in jsweb/jsdom unit.
 
// Generated from webIDL in jsweb/jsdom unit.
 
   
 
   
TElementImpl = class(TJSObject,IElement)
+
IJSElement = interface(IJSNode)
 +
  function childElementCount : Integer;
 +
  function firstElementChild : IJSElement;
 +
end;
 +
 
 +
TJSElementImpl = class(TJSObject,IJSElement)
 
   function childElementCount : Integer;
 
   function childElementCount : Integer;
   function firstElementChild : IElement;
+
   function firstElementChild : IJSElement;
 
   // all other
 
   // all other
 
end;
 
end;
 
   
 
   
+
function TJSElementImpl.childElementCount : Integer;
function TElementImpl.childElementCount : Integer;
 
 
begin
 
begin
   Result:=InvokeJSStringResult('childElementCount',[]).AsInteger;
+
   Result:=ReadJSPropertyLongInt('childElementCount');
 
end;
 
end;
 
   
 
   
function TElementImpl.firstElementChild : IElement;
+
function TJSElementImpl.firstElementChild : IJSElement;
 
begin
 
begin
   Result:=InvokeJSObjResult('firstElementChild',TElementImpl,[]) as IElement;
+
   Result:=ReadJSPropertyObject('firstElementChild',TJSElementImpl) as IJSElement;
 
end;
 
end;
 
</source>
 
</source>
 +
 +
=ToDos=
 +
 +
* read/write array elements
 +
* store/cache callbacks to support removeEventListener
 +
* extend webidl2pas to produce code for wasm-job
 +
 +
[[Category:WebAssembly]]

Latest revision as of 09:33, 29 July 2022

Accessing JS Objects from WebAssembly.

JOB

JS Object Bridge - JOB

JOB provides:

  • Units to communicate between fpc wasm and pas2js browser to call JS functions, get and set JS properties and set callbacks.
  • Units for fpc wasm with common browser classes.
  • A tool webidl2pas to help generating new units for JS classes from webidls.

Using JOB

A JOB program has a webassembly program (fpc wasi) and a browser (pas2js) program.

The browser side contains the html and registers all global JS variables needed by the webassembly side.

See the pas2js demo/wasienv/button/BrowserButton1.lpi

https://gitlab.com/freepascal.org/fpc/pas2js/-/tree/main/demo/wasienv/button

JS Classes

Each JS class like HTMLButtonElement have a FPC class (TJSHTMLButtonElement) with a reference counted interface (IJSHTMLButtonElement).

Normally functions return an interface and expect interfaces as arguments.

Callbacks

An event handler in WebAssembly can be called from Javascript.

At the moment only methods (of object) are supported.

  TWasmApp = class
    ...
    function OnButtonClick(Event: IJSEvent): boolean;
    ...
  end;

function TWasmApp.OnButtonClick(Event: IJSEvent): boolean;
begin
  JSWindow.Alert('You triggered TWasmApp.OnButtonClick');
  Result:=true;
end;
...
  JSButton.addEventListener('click',@OnButtonClick);
...

Type casts

Often a low level interface needs to be type casted to a descendant (or another type). For example JSDocument.getElementById returns a IJSElement, which is type casted to IJSHTMLElement:

var 
  Elem: IJSElement;
  HTMLElem: IJSHTMLElement;
begin
  Elem := JSDocument.getElementById('button');
  // type casting to IJSHTMLElement requires creating a new bridge object using TJSHTMLElement.Cast:
  HTMLElem := TJSHTMLElement.Cast(Elem);
  // Note: since Elem and HTMLElem are reference counted interfaces, the compiler automatically frees temporary objects.
end;

This HTMLElement has the same ObjectId as Elem, but it does not own it. It merely keeps a reference to the Elem. When all references are released, the JS object is released,

Typeof

InvokeJSTypeOf

See the JOBResult_* constants in unit JOB_Shared.


Create JOB units using webidl2pas

Compile the latest and greatest webidl2pas utility from fpc main:

utils/pas2js/webidl2pas.lpi

Download some webidls for example from https://hg.mozilla.org/mozilla-central/raw-file/tip/dom/webidl/<JSClassName>

Concatenate them into one file Foo.webidl.

Create a text file FooAlias.txt containing the used classes from other units:

Object=IJSObject
Set=IJSSet
Map=IJSMap
Function=IJSFunction
Date=IJSDate
RegExp=IJSRegExp
String=IJSString
Array=IJSArray
ArrayBuffer=IJSArrayBuffer
TypedArray=IJSTypedArray
BufferSource=IJSBufferSource
DataView=IJSDataView
JSON=IJSJSON
Error=IJSError
Promise=IJSPromise
TextEncoder=IJSTextEncode
TextDecoder=IJSTextDecoder

Create a text file FooGlobals.txt containing all the global JS variables in form Pascal variable name=JS class name,registered name:

JSFoo=Foo,foo

Run the tool:

webidl2pas -f wasmjob -i Foo.webidl --typealiases=@FooAlias.txt --globals=@FooGlobals.txt

If the tool stops, because it can not find an identifier or something is not yet supported, you can add another webidl or comment the problematic definition. Then run the tool again.

In the browser side you must register the global JS variables:

  FWADomBridge.RegisterGlobalObject(foo,'foo');

Not yet supported webidl elements

Elements from other Pascal units

At the moment you can only provide a list of type aliases for classes. This does not work for callbacks, arrays, sequences, etc. It would be better to give used units and parse them.

callback interfaces

"callback interfaces" are a legacy definition. Remedy: Replace them manually with a callback

function returning a Dictionary

ToDo: Define dictionary as class and interface and return an interface reference.

function returning a Promise

ToDo: maybe define generic TJSPromise<T> and IJSPromise<T>

function returning union

ToDo: use TVarRec or TJOB_JSValue

function returning a typed sequence

At the moment it returns an untyped IJSArray.

ToDo: returned a typed array.

function returning a callback

passing an array as argument

varargs

Functions allowing to pass an arbitrary number of arguments.

Workaround: User can call the InvokeX function directly.

constructor

getter

callback property

A property with a callback, e.g. onAbort

At the moment it is only added as a comment. Reading callbacks is not yet supported. Theoretically it could be added as a write only property.

callback with a TJOB_JSValue argument

E.g. JOBCallTOnErrorEventHandlerNonNull

At the moment argument is allocated and must be freed must be the user. Either free it in the auto generated code or use a reference counted interface.

JOB architecture

Pascal units containing ‘proxy’ classes: calling a method on a proxy class will call the corresponding class in JS. The proxy classes can be generated by the existing webidl2pas tool with -f wasmjob flag.

Data transfer between JS/WebAssembly

JS/Webassembly interface only supports passing atomic types like boolean, integers and floats, not objects or strings.

Solution:

  • Global objects like document and window are registered and queried by name.
  • Every object is stored in an array with ID: TJOBObjectID
  • ID is used to pass references to object between JS and Webassembly
  • Lifetime is controlled from WebAssembly.
  • By using interfaces, the lifetime of objects are controlled by the compiler.
  • Methods can be called using an invoke mechanism.
  • Due to limited type support in Javascript, only a handful of types must be supported by invoke, e.g. undefined, null, boolean, number, unicodestring, Object, callbacks and the union JSValue.

Function Arguments

When calling a JS function from wasm, you can pass the following types/constants:

  • boolean
  • integers (limited to double, because all numbers in JS are double, so up to 54 bits)
  • double
  • nil
  • string (utf8 converted to utf16)
  • unicodestring
  • widestring
  • PChar - using strlen to get the size and utf8 converted to utf16
  • PWideChar - using strlen to get the size
  • TJSObject and IJSObject - its ObjectID is passed to the JS side, where the corresponding JS object is used
  • JSUndefined
  • TJOB_JSValue

Function Result

Calling a JS function is done via the InvokeJS*Result functions, e.g. aJSDate.InvokeJSUnicodeStringResult('toLocaleDateString',[]) which returns a UnicodeString.

If the function does not exist, an EJSInvoke exception is raised.

If the function returns the JS undefined value, JOB returns the default value, e.g. InvokeJSUnicodeStringResult returns the empty string, InvokeJSDoubleResult returns NaN, InvokeJSObjectResult returns nil, InvokeJSBooleanResult returns false.

If the function returns an incompatible type, e.g. InvokeJSUnicodeStringResult returns a number, an EJSInvoke exception is raised.

To retrieve any kind of JS value, use InvokeJSValueResult.

Callbacks

An event handler in WebAssembly can be called from Javascript.

At the moment only methods (of object) are supported.

Every function type needs a callback, which decodes the arguments and encode the result.

For example TJSEventHandler:

type
  TJSEventHandler = function(Event: IJSEventListenerEvent): boolean of object;
...
function JOBCallTJSEventHandler(const aMethod: TMethod; var H: TJOBCallbackHelper): PByte;
var
  Event: IJSEventListenerEvent;
begin
  // get arguments. First as IJSEventListenerEvent
  Event:=H.GetObject(TJSEventListenerEvent) as IJSEventListenerEvent;
  // call the method and encode the result
  Result:=H.AllocBool(TJSEventHandler(aMethod)(Event));
end;

TJOBCallbackHelper provides functions to decode arguments and encode the result.

Passing a method as argument works like this:

type
  IJSEventTarget = interface
    ['{1883145B-C826-47D1-9C63-47546BA536BD}']
    procedure addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
  end;

  TJSEventTarget = class(TJSObject,IJSEventTarget)
    procedure addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
  end;
...
procedure TJSEventTarget.addEventListener(const aName: UnicodeString; const aListener: TJSEventHandler);
var
  m: TJOB_JSValueMethod;
begin
  // combine the users method and the callback into one argument m
  m:=TJOB_JSValueMethod.Create(TMethod(aListener),@JOBCallTJSEventHandler);
  try
    // call the JS function addEventListener(aName,m)
    InvokeJSNoResult('addEventListener',[aName,m]);
  finally
    m.Free;
  end;
end;

Implementation details

Here are some technical notes describing the various architectural decisions.

The webidl2pas tool was extended to generate an interface from the .webidl files (-f wasmjob). These files for example exist in the mozilla firefox repo on github: WebIDL

IJSElement = interface(IJSObject)
  ['someawfulGUID']
  function childElementCount : Integer;
  function firstElementChild : IJSElement;
  // all other
end;

In implementation, the following kind of code can be found:

// Hand crafted in e.g. JSObject unit
  TJSObject = class(TInterfacedObject,IJSObject)
  public
    constructor CreateFromID(aID: TJOBObjectID); virtual;
    destructor Destroy; override;
    property ObjectID: TJOBObjectID read FObjectID;
    // call a function
    procedure InvokeJSNoResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeSetType = jisCall); virtual;
    function InvokeJSBooleanResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): Boolean; virtual;
    function InvokeJSDoubleResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): Double; virtual;
    function InvokeJSUnicodeStringResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): UnicodeString; virtual;
    function InvokeJSObjectResult(const aName: string; Const Args: Array of const; aResultClass: TJSObjectClass; Invoke: TJOBInvokeGetType = jigCall): TJSObject; virtual;
    function InvokeJSValueResult(const aName: string; Const Args: Array of const; Invoke: TJOBInvokeGetType = jigCall): TJOB_JSValue; virtual;
    function InvokeJSUtf8StringResult(const aName: string; Const args: Array of const; Invoke: TJOBInvokeGetType = jigCall): String; virtual;
    function InvokeJSLongIntResult(const aName: string; Const args: Array of const; Invoke: TJOBInvokeGetType = jigCall): LongInt; virtual;
    // read a property
    function ReadJSPropertyBoolean(const aName: string): boolean; virtual;
    function ReadJSPropertyDouble(const aName: string): double; virtual;
    function ReadJSPropertyUnicodeString(const aName: string): UnicodeString; virtual;
    function ReadJSPropertyObject(const aName: string; aResultClass: TJSObjectClass): TJSObject; virtual;
    function ReadJSPropertyUtf8String(const aName: string): string; virtual;
    function ReadJSPropertyLongInt(const aName: string): LongInt; virtual;
    function ReadJSPropertyValue(const aName: string): TJOB_JSValue; virtual;
    // write a property
    procedure WriteJSPropertyBoolean(const aName: string; Value: Boolean); virtual;
    procedure WriteJSPropertyDouble(const aName: string; Value: Double); virtual;
    procedure WriteJSPropertyUnicodeString(const aName: string; const Value: UnicodeString); virtual;
    procedure WriteJSPropertyUtf8String(const aName: string; const Value: String); virtual;
    procedure WriteJSPropertyObject(const aName: string; Value: TJSObject); virtual;
    procedure WriteJSPropertyLongInt(const aName: string; Value: LongInt); virtual;
    // create a new object using the new-operator
    function NewJSObject(Const Args: Array of const; aResultClass: TJSObjectClass): TJSObject; virtual;
  end;

The various Invoke* functions encode the arguments in a memory block so they can be read on the JS side, then calls a Invoke_*Result function which lives in Javascript, and which is imported from the browser.

That function does the actual call: it uses ObjectID to look for the Self object in an array:

  • Negative IDs are special: window, document.
  • positive IDs are temporary objects created via the InvokeJSObjectResult, see below.

The Invoke_*Result pas2js function decodes the arguments and uses TJSFunction.apply to execute the requested function. The result is checked for the requested type and then returned to the wasm.

If the result is an object, an ID is generated (simple counter), the result value is stored in an array FLocalObjects.

The ID is returned to the webassembly, which will use the ID to create a TJSObject descendent.

The destructor of TJSObject calls a __job_release_object function in javascript if the ObjectID is positive. The ReleaseObject function simply sets FLocalObjects[id] to null, so the browser also releases it.

The above is a basic invoke mechanism for Javascript code.

This basic mechanism is then used by a modified version of the webidl program to generate proxy definitions. For each object in Javascript, 2 definitions are generated:

  • The interface (see above for an example)
  • An implementation object as below, descendant of TJSObject
// Generated from webIDL in jsweb/jsdom unit.
 
IJSElement = interface(IJSNode)
  function childElementCount : Integer;
  function firstElementChild : IJSElement;
end;

TJSElementImpl = class(TJSObject,IJSElement)
  function childElementCount : Integer;
  function firstElementChild : IJSElement;
  // all other
end;
 
function TJSElementImpl.childElementCount : Integer;
begin
  Result:=ReadJSPropertyLongInt('childElementCount');
end;
 
function TJSElementImpl.firstElementChild : IJSElement;
begin
  Result:=ReadJSPropertyObject('firstElementChild',TJSElementImpl) as IJSElement;
end;

ToDos

  • read/write array elements
  • store/cache callbacks to support removeEventListener
  • extend webidl2pas to produce code for wasm-job