Processing tree

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Dart library for building and executing static trees of execution created in runtime.

When to use it

The main idea behind usage of this library is when there is a need to build builder in runtime. So for example there is a bunch of input files which will build some objects, but parse operation is complex and heavy. So intention is to do read and parse only once, and then use created builder any time needed.

Getting started

Installation Follow the installation instructions on dart packages.

Import the library into your Dart code using:

import 'package:processing_tree/processing_tree.dart';

Use one of provided builders to prepare structure of tree (please refer to example) and binding with your code which will be executed while using tree. Finally call build function on builder to obtain instance of TreeProcessor. From this moment tree is ready to processing data call process function on obtained TreeProcessor instance to perform execution.


Currently only possibility to build tree is to use one of provided builders, or write your own one using existing builders as base for it.


The most easy and flexible builder is a TreeBuilder, it allows direct adding nodes to it's parents. The first call od addNode with parent equals null is used to define root. Every next call to addNode requires passing parent node for newly added. In this way whole structure of tree can be done through builder API. In the end single call to build returns instance of TreeProcessor which can be used to execute tree.


More advanced version of TreeBuilder is a StackedTreeBuilder it allows to work with building process in more organized way. This reduce need to store intermediate parent references to build tree. In the beginning you need to pass data needed to create root of tree into StackedTreeBuilder constructor. After this moment you can add or navigate on constructed tree structure. By calling method push new node is attached to tree, and became parent for further operations. If added node should not be parent for next operations call addChild, this creates new leaf in destination tree. To navigate through children in current level please call to prevSibling and nextSibling. When all children on given level are add call to levelUp will select parent of current parent as a node for next operation. In the end single call to build returns instance of TreeProcessor which can be used to execute tree.


This is the most advanced builder which mix xml deserialization and tree building. It allows to parse any custom XML into processing tree. To achieve this there is need of data provider which can interpret meaning of nodes and select proper delegates and data for it. To achieve this two approaches are implemented. For simple trees use DelegateProvider for greater level of control use BuildCoordinator.

Using XmlTreeBuilder with DelegateProvider

Parsing of data using DelegateProvider implementation if designed for xml's for which selecting delegate can be easy determined by xml element name. Override method delegate which returns delegate to be used for found xml element. Second method which need to be overridden is delegateData. It accepts two arguments a delegateName same as previous method, and Map<String,dynamic>. This method allow to process data read from xml element attributes into form which is expected by delegate. As a result of this method any object can be used, it will be later passed to delegate while executing processing tree. Whooaaa... maybe some example :)

    <int value="12"/>
    <double value="42"/>

While parsing this xml 3x call to delegate will be done, passed delegateName will be add, int and double. Responsibility of delegate delegate method is to return proper delegates which know what to do with data. But what data? Again for this xml there will be 3x call to delegateData with following values:

  • delegateName: "add", data: {}
  • delegateName: "int", data: {"value": "12"}
  • delegateName: "double", data: {"value": "42"} Now we need to understands what will be resulting processing tree? It will look like:
             PNDelegate for add + data
             /                        \
  PNDelegate for int + data     PNDelegate for double + data

What will be in data? In short: it will be result od method delegateData. So we can:

dynamic delegateData(String delegateName, Map<String, dynamic> rawData) {
   if (delegateName == "int") {
      return rawData;

Sure, this will work if your delegate will look like:

Action _intDelegate(dynamic context, dynamic data) {
  int value = int.tryParse(data["value"]);
  //.... do something with it
  return Action.proceed;

In this example data associated with int node is Map<String,dynamic> so every time we execute processing tree we will do String to int conversion. Can we do it better? Yes, consider:

dynamic delegateData(String delegateName, Map<String, dynamic> rawData) {
   if (delegateName == "int") {
      return int.tryParse(data["value"]);

Now our delegate can look like:

Action _intDelegate(dynamic context, dynamic data) {
  int value = data;
  //.... do something with it
  return Action.proceed;

There is one more thing... Each call to delegateData will have it own map, so it's perfectly fine to return it if more data are collected from node. However you might want to preprocess this data in map. Consider:

  <SomeXmlNode int="22" string="Some text" bool="true"/>

And some code to initial preprocess:

dynamic delegateData(String delegateName, Map<String, dynamic> rawData) {
   if (delegateName == "int") {
      rawData["int"] = int.tryParse(data["int"]);
      rawData["bool"] = "true" == data["bool"];
      return rawData

And this is just a beginning... For more heavy magic read next section :)

Using XmlTreeBuilder with BuildCoordinator

There are two reasons why you are reading this. Curiosity or DelegateProvider is not enough... Well let's meet his stronger brother BuildCoordinator. All what is described for provider is true, but a little different and with more details. Instead of two methods delegate and delegateData now there is one called requestData. It has following responsibilities:

  • return delegate to be used (same as DelegateProvider)
  • return data pack for delegate (same as DelegateProvider)
  • return type of node (covered later)
  • optionally return any object you want. (covered later) All those information should be returned in object ParsedItem. The decision what to return can be based on more more detailed info stored in input parameter of type State. What is inside state:
  • parentNodeName - name of xml element which owns currently processing xml node,
  • delegateName - name of currently processing xml node
  • constValDepth - how deep in branch of const values currently processing xml node is (covered later),
  • isLeaf - if true then this xml node has no children
  • data - data collected from xml node attributes (same as DelegateProvider) The idea behind returning delegate and data for it, is exactly this same as for DelegateProvider.

Item type, what is it?

In previous section there is something about return type of node, a lot of details can be found in documentation for ParsedItemType, it's important now to understands why it's introduced. It appears that xml representation of algorithms contains some nodes which are just structured data which need to be parsed, but doesn't need any delegate to execute/process it. So in short some xml nodes can be "consumed" while parsing and not be part of processing tree. Quick example:

    <color value="red"/>
    <text value="text to show"/>

Looking at this you got impression that only element which do any action is ShowText, other elements are just constant data which need to be processed once and that's it. This is the real reason why BuildCoordinator was introduced. So if text and color values should be consumed while parsing, return ParsedItemType.constValue as an item type, otherwise if element should be add to tree return ParsedItemType.owner. That's it... almost... wait, should I return delegate for constValue type node? Well... yes, but: This delegate will be called immediately while parsing! Whoooaaa.. why? Let's look at this, consider building tree of Widget:

    <Text data="text to show"/>

Let's assume both ListView and Text are Widget``s. If Text`` will be marked as constValue then we can process data for it, but we don't have Widget instance... Builder doesn't understands what each xml node really means. Here comes delegate, it know what to do with data. In the path of execution will be like:

  1. Parse xml, found ListView, call requestData on coordinator.
  2. Coordinator returns delegate which can build ListView + info that this is ParsedItemType.owner
  3. Parse xml, found Text, call requestData on coordinator.
  4. Coordinator returns delegate which can build Text + info that this is ParsedItemType.constValue
  5. Parser see that this element is a ParsedItemType.constValue, so call immediately delegate returned be coordinator.
  6. Delegate takes data from xml element Text build widget and returns it.
  7. Parser got result from delegate, add this result to ListView data collection.

It look fuzzy, it look overcomplicated, and there lack of many details... You need to experiment to understands this. The good news is: that's it! No. Just kidding. Here is more :D Keep reading.

Mystery solved: what is extObj in ParsedItem?

If you missed this explanation here it goes. Sometimes some extra actions need to be done after parsing of all sub elements of current element. Here comes second override from BuildCoordinator called step. This method is called several times while parsing xml element. When it's discovered when processing of children begin and end, and when parsing is finalized. Each time as a parameter you get ParsedItemType instance, but this can be not enough. Sometimes some extra data should be associated with this parsing node. This is extObj returned from requestData. This is anything you like, it's transparent for builder and will not be cached anywhere. It's only referenced by ParsedItemType while parse process, later all ParsedItemType instances are disposed.

Mystery solved: what is constValDepth in ParsedItem?

This is easy :) Any const value element can have many children (each must be a const value). If there is need to know if currently parsing Const Value typed element, and how deep in this branch parser is read value of constValDepth.

Execution of processing tree (TreeProcessor)

Each builder in the end returns instance of TreeProcessor which can be used to execute tree. There are two types of execution normal and inverted. Since those method are significant different more info about execution models are in next sections.

Normal execution

By default each builder returns instance of TreeProcessor prepared to normal execution. Running of tree is done through method called process, it accepts one argument of any type. This method will pass this argument to each delegate in processing tree as a first parameter. Then after executing of delegate decision what to do next based on returned Action value is taken. Please refer to documentation of type Action to understands what will happen for every value. In general normal execution is considered as going from root down to each leaf visiting each node and it's delegate (Preorder Traversal). This execution allow usage of all Action enum values. So it allows to mark place in tree and return back to it for process again some branch, or it allows to repeat execution of current node.

Inverted execution

Sometimes it's demanded to process tree from bottom to up, collecting values from lower layers and processing it. To do so call inverted method on TreeProcessor, as a result another instance of TreeProcessor is returned. This new instance will perform inverted tree execution after call to process. What's the difference? In inverted execution again each nodes will be visited but starting from the leaves. It's guaranteed that each delegate for node which have more then one child, will be called after all of children was processed. Order of visiting children is same as for normal node, but delegate is called after children are visited.

I'm executing... where is my result?

Well this is complicated. In general there might be no result, all depends what your delegates really do. But there is one special place to search for results. In class TreeProcessor method process takes one argument of any type. This is object which is passed to all delegates while executing tree, it is called context in PNDelegate signature. So

TreeProcessor processor =;
ResultCollector result = ResultCollector();

Action myDelegate(dynamic context, dynamic data) {
  ResultCollector result = context; //here you go!
  //... perform something
  result.addData(...);  //call, store, whatever


Is it really possible to use anywhere? Maybe... Take look at Yet Another Layout Builder maybe it will succeed.


This library came to my mind after few weeks of working with xml_layout package. After understanding what it gives and how it works, I found few things which I didn't like and decided to do it in my way. However after some thinking I decided to split my work for two parts. First prepare some generic library which allows me to build and execute algorithms using it representation stored in independent way. This lib is a result of this work.


Library for creating static trees of execution.