GC in Sciter’s Script

Excellent article by Ken Fox outlining architecture of different Garbage Collection strategies.

Sciter’s script uses “Copying Collector”. Its work is visualized as:

Copying GC at work

And DOM (tree of objects without cycles) is using Reference Counting that reflects in external native DOM API by pair of SciterUseElement and SciterUnUseElement (a.k.a. AddRef/Release).

Sciter for AngularJS practitioners. Directives.

Directives in AngularJS

From AngularJS documentation :

At a high level, directives are markers on a DOM element (such as an attribute, element name, comment or CSS class) that tell AngularJS’s HTML compiler ($compile) to attach a specified behavior to that DOM element (e.g. via event listeners), or even to transform the DOM element and its children.

And here is a typical AngularJS directive implementation:

app.directive('myCustomer', function() {
  return {
    template: 'Name: {{name}} Address: {{address}}', // content
    scope: { name:"", address:"" },  // internal data model
    link: function(scope, elem, attrs) { // initialization 
      elem.bind('click', function() { ... });
      elem.bind('dblclick', function() { ... });
    }
  };
});

It says that each <my-customer> element will have

  • Name: ... Address: ... content;
  • click and dblclick event handlers.

In Sciter

Behaviors

Declarative code-to-element binding in Sciter is made by CSS. Sciter’s prototype property is used for that purpose:

my-customer {
  prototype: MyCustomer url(my-customer.tis);
  display: block;
  ...
}

In plain text: all <my-customer> elements are rendered as block element and will have class MyCustomer assigned to them.  The class will be loaded from my-customer.tis file (if it was not loaded before):

class MyCustomer : Element 
{
  function attached() // called when element gets this class - "link" in terms of Angular 
  {
     // content initialization: 
     this.$content(Name: <output.name/> Address: <output.address/>);
  }
  // event handlers:
  event click { ... /* 'this' here is the element*/ }
  event dblclick { ... }
}

As the code is assigned by CSS then you can use full power of CSS selectors to assign code classes to elements.

Aspects

Another option to bind code with elements is to use so called aspects. Aspect here is just a script function that gets executed when its selector matched the element first time:

[collapsible] { aspect: Collapsible url(my-aspects.tis); }
[collapsible]:collapsed > :last-child { display:none; } // last child is invisible when collapsed

And Collapsible here is a simple function that handles click event and triggers :collapsed state flag switch:

function Collapsible() { 
  this << event click {
     if( this.state.collapsed ) this.state.expanded = true;
     else this.state.collapsed = true;    
  }
}

Having such aspect defined we can define collapsible element:

<div collapsible>
  click here to see content
  <p>Content</p>
</div>

So if you have multiple collapsible sections in your design then just add “collapsible” attribute to them.

Sciter Script, hidden treasures

There is otherwise keyword in script that is used in loop statements so this:

var emptyArr = [];

for(var el in emptyArr)
  stdout.println(el);
otherwise
  stdout.println("nothing seen, array is empty!");

will print nothing seen, array is empty!.

Statement or block after otherwise will be executed if body of the loop was not executed.

+plus and +formation, what’s the difference?

+plus

+plus is the way to define and support mapping between DOM tree and data tree rooted at some object or namespace. So if you have these two declarations, markup:

<style>@import url(plus.css)</style>
<section model="person">
    <label>First</label> <input(name.first)>
    <label>Second</label> <input(name.last)>
    <label>Age</label> <input|integer(age)>
</section>

and script

namespace person {
  var name = { first: "Albert", last:"Einshtein" };
  var age = 53;
}

then the +plus will establish live two-way mapping (binding)  between DOM elements above and the data structure.

When the user changes value of input(name.first) field  (short form of <input name="name.first"> in Sciter) the following happens:

  1. Sciter generates “change” DOM event;
  2. +plus handles that event and
  3. updates value of person.name.first in data namespace.

And when data gets changed by some other code like

person.name.first = "Some other name";

then pretty much similar flow occurs:

  1. Sciter determines that filed first  has changed on observable object.
  2. Sciter calls attached function-observer – each object/array inside bound namespace has an observer function attached to it.
  3. And that function-observer updates bound DOM element.

As you see, in order binding to work both-ways, each data node (object, array) in bound namespace has to have corresponding observer attached to it.

+formation

The main difference from the above is that the +formation does not require separate data structure.  The formation is a data structure by itself naturally mapped to markup structure.

This markup:

<section(person)>
    <label>First</label> <input(name.first)>
    <label>Second</label> <input(name.last)>
    <label>Age</label> <input|integer(age)>
</section

gets mapped by formation() function to the [formation] tree structure:

And code can access that tree directly as:

var root = formation( self );
// setting whole formation:
root.person.value = {
   name : {
     first: "Albert",
     last: "Einshtein"
   },
   age:69
 };

// or setting particular field in the formation:
root.person.name.first.value = "Some other name";

Note: root.person.name.first is a direct reference to <input> DOM element thus .value is required.

The formation is a data structure derived from actual DOM tree (its projection)  and so you can use normal DOM events to be notified when some data changes by the user:

root.person.name.first.on("change", function() {...}) // or
self.on("change", "section[name=person]", function() {...}) // any change in person fields.

Resume

Essentially +plus and +formation are aimed to the same task – update UI from code and update/notify code about some changes in UI.

  • +plus
    • pros: Allows to bind arbitrary data structure to arbitrary DOM tree.
    • cons: Can be memory and CPU consuming on large data and DOM trees.
  • +formation
    • pros: Fast and not CPU consuming.  Yet allows to access “fields of interest” by using dot notation (“paths in formation”):  root.person.name.first.state.disabled = true;
    • cons: data structure you get (the formation tree) is bound with DOM structure. So when you change DOM structure then  formation paths in code may change.

Optimising ‘+=’ and ‘+’ operators for strings.

This simple code:

var s = "";
for(var i = 0; i < 50000; ++i) {
  s += "*";
}

code used to run in Sciter in 874ms.

After implementing simple and obvious optimization it runs now in just 4ms.

String concatenation was implemented as simple string allocation of size(r) = size(s1) + size(s2) and memcpy of s1 and s2 into that new string r.

Memory allocation in script is cheap - just increment memory pointer by size of requested memory chunk and you are done. So in theory that naïve strcat shall be lightning fast. But it is definitely not. There is a hidden cost of GC cycles - at the moment of concatenation r,s1 and s2 shall be present in memory.

So "0123456789" + "A" needs memory for 10 + 1 + 11 = 22 characters. And on each iteration that number increases - the VM is busy cleaning garbage most of the time.

So when someone from C# and Java crowds is saying you that memory allocation is cheap take it with a grain of salt.

Sciter assistant for Visual Studio Code

Early BETA of Sciter Assistant for Microsoft Visual Code by ConrLab.

MS-visual-code-sciter

Microsoft Visual Code, Sciter assistant in actions 

Supports syntax and editing of Sciter HTML and CSS flavours and TIScript.

Code completion in CSS including @mixin/@const constructs and script for this and variables starting from el....

If to update user settings (File->Preferences->User Settings) like this:
code-preferences

then by using Ctrl-K, Ctrl-S sequence you can run it in sciter.exe.

scapp.exe – standalone Sciter executable.

Adding scapp.exe to the SDK.

The ScApp is an executable that contains Sciter engine linked statically – has no external dependencies.

scapp.exe can be started without parameters or with a file name that has either .htm, .tis or .scapp extension:

> scapp.exe

or

> scapp.exe tetris.scapp

If it starts without parameters then it will try to look for either main.htm, main.tis or main.scapp in current folder.

If .htm[l] file is given Sciter window will be created and the document will be loaded to it.

The .scapp file is a zip file containing /main.tis at root. The main.tis is expected to contain code creating windows and calling Sciter.run(); – GUI message pump loop:

  // create first window, it may call Sciter.requestQuit(0) to exit from the loop below
  var window = View.window { ... };
  // run message pump loop of the application
  return Sciter.run(); 

Essentially it will just execute the script in the same way as tiscript.exe does.

At the mean time, event handler assignment…

Sciter supports event handler assignment in jQuery style:

element.on("event", "selector", function(evt) {...});

That looks mostly OK but for some reason I feel it as “aesthetically non pleasant” so thinking about alternatives.

One feasible option/idea may look like this:

// subscribe function for "click" event on element
element << function "click" (evt) {...};

That above is a direct equivalent of

element.on("click", function (evt) {...});

And in full form, with selector

// subscribe function for "click" event on <button id=first> element
self << function ["click","button#first"] (evt) {...};

The main challenge here is to stay in constraints of JS syntax/grammar …