xwidget 0.0.18

A package for building dynamic UIs in Flutter using an expressive, XML based markup language.

Note: This document is very much still a work in progress.

What is XWidget?

XWidget is a not-so-opinionated library for building dynamic UIs in Flutter using an expressive, XML based markup language.

That was a mouth full, so let's break it down. "Not-so-opinionated" means that you're not forced to use XWidget in any particular way. You can use as much or as little of the framework as you want - whatever makes sense for your project. There are, however, a few Best Practices that you should follow to help keep your code organized and your final build size down to a minimum.

An XWidget UI is defined in XML and parsed at runtime. You have access to all the Flutter widgets and classes you are used to working with, including widgets from 3rd party libraries and even your own custom widgets. This is achieved through code generation. You specify which widgets you want to use and XWidget will generate the appropriate classes and functions and make them available as XML elements. You'll have access to all of the Widgets' constructor arguments as element properties just as if you were writing Dart code. You'll even have code completion and access to Widgets' documentation in tooltips, if provided by the author, when you register the generated XML schema with your IDE.

For example:

<Column crossAxisAlignment="start">
    <Text data="Hello World">
        <TextStyle for="style" fontWeight="bold" color="#262626"/>
    </Text>
    <Text>Welcome to XWidget!</Text>
</Column>

Important: Only specify widgets that you actually use in your UI. Specifying unused widgets and helper classes in your configuration will bloat your app size. This is because code is generated for every component you specify and thus neutralizes Flutter's tree-shaking.

Quick Start

This Quick Start guide will help you get up and running with XWidget in just a few minutes. For a more comprehensive description of the various components and features, please see the sections below.

1. Install XWidget using the following command:

   $ flutter pub add xwidget 
   

2. Create an inflater specification file. This is a Dart file that tells XWidget which widgets and helper classes you're planning on using in your fragments. See Inflaters for more.

   // lib/xwidget/inflater_spec.dart
   import 'package:flutter/material.dart';
      
   const inflaters = [
     Column,   
     Container,
     Text,
     TextStyle, 
   ];
   

3. Create a custom configuration file. This is an XML document that configures the inputs and outputs of XWidget's code generator. By default, XWidget looks for a file named xwidget_config.yaml in the project's root folder. Make sure that sources contains the location of the inflater spec you created in step #2. See Configuration for more.

   # xwidget_config.yaml
   inflaters:
      sources: [ "lib/xwidget/inflater_spec.dart" ]
   

4. Generate inflaters and fragment schema. By default, all generated Dart files are written to lib/xwidget/generated. The schema file is written to the project root as xwidget_schema.g.xsd. See Code Generation for more.

   $ dart run xwidget:generate 
   

5. Register the generated schema file with your IDE under the namespace http://www.appfluent.us/xwidget. This will provide validation, code completion, and tooltip documentation while editing your fragments.
   
   

6. Register the generated components in your application's main method. You'll need to import XWidget and the generated code.

   import 'package:xwidget/xwidget.dart';
   import 'xwidget/generated/inflaters.g.dart';
      
   main() async {
      WidgetsFlutterBinding.ensureInitialized();
       
      // load resources i.e. fragments, values, etc.
      await Resources.instance.loadResources("resources");
       
      // register XWidget components
      registerXWidgetInflaters();
      ...
   }
   

7. Modify your project's pubspec.yaml and add resources/fragments/ to assets. There's no need to add each individual fragment; however, if you use fragment folders, you'll need to add each folder here. See Resources for more.

   flutter:
     assets:
       - resources/fragments/
   

8. Create your UI fragment. By default, XWidget looks for fragments under resources/fragments. Fragments are XML documents that are "inflated" at runtime. See Fragments for more.

   <?xml version="1.0"?>
     
   <!-- resources/fragments/hello_world.xml -->
   <Column xmlns="http://www.appfluent.us/xwidget">
       <Text data="Hello World">
           <TextStyle for="style" fontWeight="bold" color="#262626"/>
       </Text>
       <Text>Welcome to XWidget!</Text>
   </Column>
   

9. Inflate your fragment. Where ever you want to render your fragment, simply call XWidget.inflateFragment(...) with the name of your fragment and Dependencies. See Dependencies for more.

   return Container(
     child: XWidget.inflateFragment("hello_world", Dependencies())
   )
   

Example

Please see the example folder of this package. It contains an XWidget version of Flutter's classic starter app. It only scratches the surface of XWidget's capabilities.

Setup

Specification

Inflater specification

Icon Specification

Includes

Configuration

By default, XWidget's code generator looks for a custom configuration file named xwidget_config.yaml in the project root. This custom configuration is layered on top of XWidget's own default configuration which handles most of the configuration burden. See package:xwidget/res/default_config.yaml for details.

Since the default config does most of the heavy lifting, the typical config can be relatively simple like this example:

# custom config - xwidget_config.yaml
inflaters:
   imports: [
      "dart:ui",
      "package:flutter/foundation.dart",
      "package:flutter/gestures.dart",
   ]
   sources: [ "lib/xwidget/inflater_spec.dart" ]
   includes: [ "lib/xwidget/inflater_spec_includes.dart" ]

icons:
   sources: [ "lib/xwidget/icon_spec.dart" ]

There are four top-level mappings that configure each of the four generated outputs: inflaters, schema, controllers, and icons.

Inflaters Configuration

# Responsible for configuring inputs and outputs to generate inflaters.
inflaters:
   
  # The file path to save the generated code. The output contains all inflater classes and
  # a library function to to register them. The default value can be overwritten.
  # 
  # DEFAULT: "lib/xwidget/generated/inflaters.g.dart"
  target:

  # A list of additional imports to include in the generated output. Sometimes the code
  # generator can't determine all the required imports. This happens because of a current
  # limitation in dealing with default values for constructor arguments. This option
  # allows manual configuration when needed. Custom imports are appended to XWidget's
  # default list.
  #
  # DEFAULT: [ "package:xwidget/xwidget.dart" ]
  imports: [ ]

  # List of list inflater specification source files. Specifications tell XWidget which objects
  # to create inflaters for.
  #
  # DEFAULT: none
  sources: [ ]

  # DEFAULT none
  includes: [ ]

  # DEFAULT: See 'package:xwidget/res/default_config.yaml'
  constructor_exclusions: [ "<class_name | * for any>:<constructor_argument_name>", ]

  # DEFAULT: See 'package:xwidget/res/default_config.yaml'
  constructor_arg_defaults:
    "<class_name | * for any>:<constructor_argument_name>": "<value>"

  # DEFAULT: See 'package:xwidget/res/default_config.yaml'
  constructor_arg_parsers:
  
     # EXAMPLES:
     # - "double": "double.parse(value)"
     # - "Alignment": "parseAlignment(value)"
     # - "*:width": "parseWidth(value)"
    "<constructor_argument_type | * for any>(: <constructor_argument_name>)": "<parser_function_call>"

// example inflater specification.
import 'package:flutter/material.dart';

// Best Practice: Keep declarations in alphabetical order. It makes it much easier to
// quickly determine what has been added and what is missing.

const inflaters = [
  AppBar,
  Center,
  Column,
  FloatingActionButton,
  Icon,
  MaterialApp,
  Scaffold,
  Text,
  TextStyle,
  ThemeData,
];

Schema Configuration

# Responsible for configuring inputs and outputs to generate the inflater schema. Register the
# generated schema with your IDE to get code completion and documentation while editing UI markup. 
schema:
   
  # DEFAULT: "xwidget_schema.g.xsd"
  target:
     
  # DEFAULT: "xwidget|res/schema_template.xsd"   
  template: 

  # DEFAULT: See 'package:xwidget/res/default_config.yaml'
  types:
     # EXAMPLES
     # - "bool": "boolAttributeType"
     # - "BoxFit": "BoxFitAttributeType"
    "<constructor_argument_type>": "<schema_defined_type>"

  # DEFAULT: See 'package:xwidget/res/default_config.yaml'
  attribute_exclusions: [
                           
    # EXAMPLES:
    # - "*:child"                       
    "<class_name | * for any>:<constructor_argument_name | * for any>",
  ]

Controllers Configuration

controllers:
   
  # DEFAULT: "lib/xwidget/generated/controllers.g.dart" 
  target:

  # DEFAULT: [ "package:xwidget/xwidget.dart" ]
  imports: [ ]

  # DEFAULT: [ "lib/xwidget/controllers/**.dart" ]
  sources: [ ]

Icons Configuration

icons:

  # DEFAULT: "lib/xwidget/generated/icons.g.dart"
  target:

  # DEFAULT: [ "package:xwidget/xwidget.dart" ]
  imports: [ ]

  # List of list icon specification source files. Specifications tell XWidget which icons
  # to register.
  #
  # DEFAULT: none
  sources: [ ]

// example icon specification
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

// List icons individually to reduce app size.
const icons = [
  Icons.add,
  Icons.delete,
];

// You can also include the entire icon set by simply referencing the
// enclosing class. This assumes that each icon is a declared as a static
// field of type IconData.
const iconSets = [
  CupertinoIcons,
];

Code Generation

Add documentation here.

$ dart run xwidget:generate 

$ dart run xwidget:generate --help 

$ dart run xwidget:generate --config "my_config.yaml" 

$ dart run xwidget:generate --only inflaters,controllers,icons 

$ dart run xwidget:generate --allow-deprecated 

Inflaters

Inflaters are the heart of XWidget. They are responsible for building the UI at runtime by parsing attribute values and constructing the components defined in fragments. In other words, they are the primary mechanism by which your XML markup gets transformed from this:

<Container height="50" width="50">
   <Text data="Hello world!"/>
</Container>

into the widget tree represented by this:

Container({
   height: 50,
   width: 50,
   child: Text("Hello world!")
});

A good analogy is the relationship between a recipe, a chef and a meal. The recipe describes how to create the meal. It lists the ingredients, preparation instructions, etc. The chef does all the work described in the recipe. The meal is the finished product. Your XML markup is the recipe, the inflaters are the chefs in the kitchen, and the instantiated widget tree is the meal, which is then served to the end user.

Inflaters are generated from a user (a developer using XWidget) defined specification written in Dart. The specification is very simple and its sole purpose is to tell the code generator which widgets to generate code for. Once the code has been generated, the widgets can be referenced in your markup.

You can create inflaters for basically anything that is a class and has a public constructor. For example, BoxDecoration and TextStyle are not widgets, they're helper classes that style widgets.

// a very simple inflater specification
import 'package:flutter/material.dart';

const inflaters = [
  Container,
  Text,
  TextStyle, 
];

You can add as many components as required by your application; however, you should only specify components that you actually need. Specifying unused components in your configuration will unnecessarily increase your app size. This is because code is generated for every component you specify and thus neutralizes Flutter's tree-shaking.

There are four built-in inflaters: <Controller>, <DynamicBuilder>, <ListOf>, and ValueListener. You can read more about them in the [Custom Inflaters] section.

Parsers

Add documentation here.

Custom Inflaters

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XML Schema

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Code Completion & Tooltip Documentation

Add documentation here.

Dependencies

In the context of XWidget, dependencies are data, objects, and functions needed to render a fragment. The Dependencies object, at its core, is just a map of dependencies as defined above. Every inflate method call requires a Dependencies object. It can be a new instance or one that was received from a previous inflate method invocation.

Dependencies objects have a few characteristics that make them a little more interesting than plain old maps.

1. Values can be referenced using dot/bracket notation for easy access to nested collections. Nulls are handled automatically. If the underlying collection does not exist, reads will resolve to null and writes will create the appropriate collections and store the data.
   
   

   Dart example:

   // example using setValue
   final dependencies = Dependencies();
   dependencies.setValue("users[0].name", "John Flutter");
   dependencies.setValue("users[0].email", "[e-mail]");
      
   print(dependencies.getValue("users[0].name"));
   print(dependencies.getValue("users[0].email"));
   

   Or you could use the constructor:

   // example setting values via Dependencies constructor
   final dependencies = Dependencies({
     "users[0].name": "John Flutter",
     "users[0].email": "[e-mail]"
   });
      
   print(dependencies.getValue("users[0].name"));
   print(dependencies.getValue("users[0].email"));
   

   Markup usage example:

   <!-- example iterating over a collection -->
   <forEach var="user" items="${users}">
     <Row>
       <Text data="${user.name}"/>
       <Text data="${user.email}"/>
     </Row>
   </forEach>
   

2. Supports global data. Sometimes you just need to access data from multiple parts of an application without a lot of fuss. Global data are accessible across all Dependencies instances by adding a global prefix to the key notation.
   
   

   Dart example:

   // example setting global values
   final dependencies = Dependencies({
     "global.users[0].name": "John Flutter",
     "global.users[0].email": "[e-mail]"
   });
     
   print(dependencies.getValue("global.users[0].name"));
   print(dependencies.getValue("global.users[0].email"));
   

   Markup usage example:

   <!-- example iterating over a global collection -->
   <forEach var="user" items="${global.users}">
     <Row>
       <Text data="${user.name}"/>
       <Text data="${user.email}"/>
     </Row>
   </forEach>
   

3. When combined with the ValueListener custom widget, the UI can listen for data changes and update itself. In the following example, if the user's email address changes, then the Text widget is rebuilt.

   <!-- example listening to value changes -->
   <ValueListener varName="user.email">
       <Text data="${user.email}"/>
   </ValueListener>
   

Note: Dependencies also supports the bracket operator []; however, it behaves like an ordinary map.

Fragments

Add documentation here.

Controllers

Add documentation here.

Expression Language (EL)

Add documentation here.

Operators

Below is the operator precedence and associativity table. Operators are executed according to their precedence level. If two operators share an operand, the operator with higher precedence will be executed first. If the operators have the same precedence level, it depends on the associativity. Both the precedence level and associativity can be seen in the table below.

Level	Operator	Category	Associativity
10	() [] .	Function call, scope, array/member access
9	-expr !expr	Unary Prefix
8	* / ~/ %	Multiplicative	Left-to-right
7	+ -	Additive	Left-to-right
6	< > <= >=	Relational
5	== !=	Equality
4	&&	Logical AND	Left-to-right
3	||	Logical OR	Left-to-right
2	expr1 ?? expr2	If null	Left-to-right
1	expr ? expr1 : expr2	Conditional (ternary)	Right-to-left

Built-In Functions

Name	Arguments	Returns	Description	Examples
contains	dynamic value dynamic searchValue	bool		${contains('I love XWidget', 'love'} ${contains(dependencyValue, 'hello'}
containsKey	Map? map dynamic searchKey	bool
containsValue	Map? map dynamic searchValue	bool
diffDateTime	DateTime left DateTime right	Duration
durationInDays	Duration value	int
durationInHours	Duration value	int
durationInMinutes	Duration value	int
durationInSeconds	Duration value	int
durationInMills	Duration value	int
endsWith	String value String searchValue	bool
eval	String? value	dynamic
formatDateTime	String format DateTime dateTime	String
isEmpty	dynamic value	bool
isNotEmpty	dynamic value	bool
isNotNull	dynamic value	bool
isNull	dynamic value	bool
length	dynamic value	length
matches	String value String regExp	bool
now	none	DateTime
nowInUtc	none	DateTime
startsWith	String value String searchValue	bool
substring	String value int start [int end = -1]	String
toDateTime	dynamic value	DateTime
toDuration	String value	Duration
toString	dynamic value	String

Custom Functions

Custom functions are functions that you define and add to your Dependencies. They behave like built-in functions except that they are bound to a single Dependencies instance. Custom functions can have up to 10 required and/or optional arguments.

For example:

dependencies["addNumbers"] = addNumbers

int addNumbers(int n1, [int n2 = 0, int n3 = 0, int n4 = 0, int n5 = 0]) {
   return n1 + n2 + n3 + n4 + n5;
}

Example usage:

<Text data="${addNumbers(2,8,4}"/>

Resources

Add documentation here.

Strings

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Ints

Add documentation here.

Doubles

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Bools

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Colors

Add documentation here.

Fragments

Add documentation here.

Tags

Tags are XML elements that do not, themselves, add components to the widget tree. They provide common structure and control elements for constructing the UI such as conditionals, iteration, fragment inclusion, etc. They are always represented in lowercase to distinguish them from inflaters.

<builder>

A tag that wraps its children in a builder function.

This tag is extremely useful when the parent requires a builder function, such as PageView.builder. Use vars, multiChild, and nullable attributes to define the builder function signature. When the builder function executes, the values of named arguments defined in vars are stored as dependencies in the current Dependencies instance. The values of placeholder arguments (_) are simply ignored. The BuildContext is never stored as a dependency, even if explicitly named, because it would cause a memory leak.

Attribute	Description	Required	Default
dependenciesScope	Defines the method for passing Dependencies to immediate children. Valid values are new, copy, and inherit.	no	inherit
for	The name of the parent's attribute that will be assigned the builder function.	yes	null
multiChild	Whether the builder function should return an array of widgets or a single widget.	no	false
nullable	Whether the builder function can return null.	no	false
vars	A comma separated list of builder function arguments. Values of named arguments are stored as dependencies. Supports up to five arguments.	no	null

Example usage:

<PageView.builder>
    <builder for="itemBuilder" vars="_,index" nullable="true">
        <Container>
          <Text data="${index}"/>
        </Container>
    </builder>
</PageView.builder>

<callback>

This tag allows you to bind an event handler with custom arguments. If you don't need to pass any arguments, then just bind the handler using EL, like so: <TextButton onPressed="${onPressed}"/>. This is sufficient in most cases.

The callback tag creates an event handler function for you and executes the action when the event is triggered. action is an EL expression that is evaluated at the time of the event. Do not enclose the expression in curly braces ${...}, otherwise it will be evaluated immediately upon creation instead of when the event is fired.

If the handler function defines arguments in its signature, you must declare those arguments using the vars attribute. This attribute takes a comma separated list of argument names. When the handler is triggered, argument values are added to Dependencies using the specified name as the key, and can be referenced in the action EL expression, if needed. They're also accessible anywhere else that instance of Dependencies is available. If you don't need the values, then use and underscore (_) in place of the name. Doing so will ignore the values and they won't be added to Dependencies e.g. ...vars="_,index".... BuildContext is never added to Dependencies even when named, because this would cause a memory leak.

Attribute	Description	Required	Default
action	The El expression to evaluate when the event handler is triggered.	yes	null
dependenciesScope	Defines the method for passing Dependencies to immediate children. Valid values are new, copy, and inherit.	no	inherit
for	The name of the parent's attribute that will be assigned the event handler.	yes	null
returnVar	The storage destination within Dependencies for the return value of action.	no	null
vars	A comma separated list of handler function arguments. Values of named arguments are stored as dependencies. Supports up to five arguments.	no	null

<TextButton>
   <callback for="onPressed" action="doSomething('Hello World')"/>
   <Text>Press Me</Text>
</TextButton>

<debug>

A simple tag that logs a debug message

Attribute	Description	Required	Default
message	The message to log.	yes	null

<debug message="Hello world!"/>

<forEach>

Add documentation here.

Attribute	Description	Required	Default
dependenciesScope	Defines the method for passing Dependencies to immediate children. Valid values are new, copy, and inherit.	no	inherit
indexVar		no	null
items		yes	null
multiChild		no	null
nullable		no	null
var		yes	null

<forEach var="user" items="${users}">
   
</forEach>

<forLoop>

Add documentation here.

Attribute	Description	Required	Default
begin		no	0
dependenciesScope	Defines the method for passing Dependencies to immediate children. Valid values are new, copy, and inherit.	no	inherit
end		no	0
step		no	1
var		yes	null

<forLoop var="index" begin="1" end="5">
   
</forLoop>

<fragment>

A tag that renders a UI fragment

Attribute	Description	Required	Default
dependenciesScope	Defines the method for passing Dependencies to immediate children. Valid values are new, copy, and inherit.	no	inherit
for	The name of the parent's attribute that will be assigned the fragment's output.	no	null
name	The Resources name of the fragment.	yes	null

<AppBar>
  <fragment for="leading" name="profile/icon"/>
</AppBar>

<if>/<else>

Add documentation here.

Attribute	Description	Required	Default
test	EL expression tht must evaluate to a bool value.	yes	null

<if test="${}">
    <fragnent name=""/>    
    <else>
       <fragnent name=""/>
    </else>
</if>

<variable>

Add documentation here.

Attribute	Description	Required	Default
name		yes	null
value		yes	null

<variable name="" value=""/>

Logging

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Best Practices

Do use fragment folders

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Don't specify unused widgets

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Do check-in generated files into source control

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Recommended folder structure

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project
├── lib
│   └── xwidget          # holds all specification files used in code generation
│       ├── controllers  # holds all custom controllers
│       └── generated    # holds all generated .g.dart files
└── resources
    ├── fragments  # holds all fragments
    └── values     # holds all resource values i.e strings.xml, bools.xml, colors.xml, etc.

Tips and Tricks

Regenerate inflaters after upgrading Flutter

Add documentation here.

Use controllers to create reusable components

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FAQ

1. What problems does XWidget solve?

The first and most obvious answer is that it gives applications the flexibility to create and modify its UI at runtime. An app might want to give its users the ability to download a different look-and-feel or create dynamic forms all without a redeployment. You're only limited by the existing functionality of your custom controllers, since they're static Dart code.

It provides better separation between business and presentation layers out of the box. Sometimes developers struggle with the best way to separate these concerns. XWidget inherently addresses these problems in an uncomplicated way with fragments and controllers.

This may just be our opinion, but building views in code just feels clunky. We find it more enjoyable to write our UIs using markup - it feels more natural and it's certainly a lot easier to read. The experience should only get better as we improve IDE integration.

Roadmap

The primary focus right now is documentation, critical bug fixes, more documentation, minor improvements, and oh, even more documentation. The implementation is already fairly stable, but lacks test coverage. Once the documentation is complete, we'll bump the minor version and concentrate on testing.

0.0.x Releases (2023)

• Write README and API documentation
• Critical bug fixes
• Minor improvements

0.x Releases (2024)

• Write unit, widget, UI, and performance tests
• Critical and major bug fixes
• Refine and add documentation as needed

1.0.0 Release (mid 2024)

• Stable release

Known Issues

None at the moment :)