template_engine 1.0.0

A Dart package to parse and render templates, using (nested) variables and custom tags.

template_engine

A flexible Dart library to parse templates and render output such as:

• Html
• Programming code
• Markdown
• Xml, Json, Yaml
• Etc...

Features

• Template expressions that can contain (combinations of):
  • Data types
  • Constants
  • Variables
  • Operators
  • Functions
• All of the above can be customized or you could add your own.

Getting started

See: Installing

Examples

import 'package:template_engine/template_engine.dart';

void main() {
  var template = TextTemplate('Hello {{ERROR}}.');
  // See also FileTemplate and WebTemplate
  var engine = TemplateEngine();
  var parseResult = engine.parse(template, {'name': 'world'});
  // Here you could additionally mutate or validate the parseResult if needed.
  print(engine.render(parseResult)); // should print 'Hello world.';
}

Note the documentation of the template_engine package was generated by itself. See: tool/generate_documentation.dart

For more see: Examples

Tags

Tags are specific texts in templates that are replaced by the TemplateEngine with other information.

A tag:

• Starts with some bracket and/or character combination, e.g.: {{
• Followed by some contents
• Ends with some closing bracket and/or character combination, e.g.: }}

A tag example: {{customer.name}}

By default the TemplateEngine tags start with {{ and end with }} brackets, just like the popular template engines Mustache and Handlebars.

You can also define alternative tag brackets in the TemplateEngine constructor parameters. See TemplateEngine.tagStart and TemplateEngine.tagEnd.

It is recommended to use a start and end combination that is not used elsewhere in your templates, e.g.: Do not use < > as tag start and end if your template contains HTML or XML.

The TemplateEngine comes with default tags. You can replace or add your own tags by manipulating the the TemplateEngine.tags field.

ExpressionTag
description:	Evaluates an expression that can contain: * Data Types (e.g. boolean, number or String) * Constants (e.g. pi) * Variables (e.g. person.name ) * Operators (e.g. + - * /) * Functions (e.g. cos(7) ) * or any combination of the above
expression example:	The volume of a sphere = {{ round( (3/4) * pi * (radius ^ 3) )}}.
code example:	tag_expression_parser_test.dart

Data types in tag expressions

A data type defines what the possible values an expression, such as a variable, operator or a function call, might take.

The TemplateEngine supports several default DataTypes.

Custom DataTypes

You can adopt existing DataTypes or add your own custom DataTypes by manipulating the TemplateEngine.dataTypes field. See Example.

Available DataTypes

String
description:	A form of data containing a sequence of characters
syntax:	A string is declared with a chain of characters, surrounded by two single (') or double (") quotes to indicate the start and end of a string. In example: 'Hello' or "Hello"
example:	string_test.dart

Number
description:	A form of data to express the size of something.
syntax:	A number is declared with: * optional: positive (e.g. +12) or negative (e.g. -12) prefix or no prefix (12=positive) * one or more digits (e.g. 12) * optional fragments (e.g. 0.12) * optional: scientific notation: the letter E is used to mean"10 to the power of." (e.g. 1.314E+1 means 1.314 * 10^1which is 13.14).
example:	num_test.dart

Boolean
description:	A form of data with only two possible values: true or false
syntax:	A boolean is declared with the word true or false. The letters are case insensitive.
example:	bool_test.dart

Constants in tag expressions

A Constant is a value that does not change value over time.

The TemplateEngine comes with several mathematical constants.

Custom Constants

You can create and add your own Constants by manipulating the TemplateEngine.constants field. See Example.

Available Constants

e
description:	Base of the natural logarithms.
return type:	double
code example:	e_test.dart

ln10
description:	Natural logarithm of 10.
return type:	double
code example:	ln10_test.dart

ln2
description:	Natural logarithm of 2.
return type:	double
code example:	ln2_test.dart

log10e
description:	Base-10 logarithm of e.
return type:	double
code example:	log10e_test.dart

log2e
description:	Base-2 logarithm of e.
return type:	double
code example:	log2e_test.dart

pi
description:	The ratio of a circle's circumference to its diameter
return type:	double
code example:	pi_test.dart

Variables in tag expressions

A Variable is a named container for some type of information (like number, boolean, String etc...)

• Variables are stored as key, value pairs in a dart Map<String, Object> where:
  • String=Variable name
  • Object=Variable value
• Variables can be used in a tag expression
• Initial variable values are passed to the TemplateEngine.render method
• Variables can be modified during rendering

The variable name:

• must be unique and does not match a other [Tag] syntax
• must start with a letter, optionally followed by letters and or digits
• is case sensitive (convention: use camelCase)

Variables can be nested. Concatenate variable names separated with dot's to get the variable value of a nested variable.

E.g.:
Variable map: {'person': {'name': 'John Doe', 'age',30}}
Variable Name person.name: refers to the variable value of 'John Doe'

Examples:

• Variable Example
• Nested Variable Example

Functions in tag expressions

A function is a piece of dart code that performs a specific task. So a function can basically do anything that dart code can do.

A function can be used anywhere in an tag expression wherever that particular task should be performed.

The TemplateEngine supports several default functions.

Custom Functions

You can adopt existing functions or add your own custom functions by manipulating the TemplateEngine.functionGroups field. See Example.

Available Functions

Math Functions

exp
description:	Returns the natural exponent e, to the power of the value
return type:	number
expression example:	{{exp(7)}} should render: 1096.6331584284585
code example:	exp_test.dart
parameter:	value	number	mandatory

log
description:	Returns the natural logarithm of the value
return type:	number
expression example:	{{log(7)}} should render: 1.9459101490553132
code example:	log_test.dart
parameter:	value	number	mandatory

sin
description:	Returns the sine of the radians
return type:	number
expression example:	{{sin(7)}} should render: 0.6569865987187891
code example:	sin_test.dart
parameter:	radians	number	mandatory

asin
description:	Returns the values arc sine in radians
return type:	number
expression example:	{{asin(0.5)}} should render: 0.5235987755982989
code example:	asin_test.dart
parameter:	value	number	mandatory

cos
description:	Returns the cosine of the radians
return type:	number
expression example:	{{cos(7)}} should render: 0.7539022543433046
code example:	cos_test.dart
parameter:	radians	number	mandatory

acos
description:	Returns the values arc cosine in radians
return type:	number
expression example:	{{acos(0.5)}} should render: 1.0471975511965979
code example:	acos_test.dart
parameter:	value	number	mandatory

tan
description:	Returns the the tangent of the radians
return type:	number
expression example:	{{tan(7)}} should render: 0.8714479827243188
code example:	tan_test.dart
parameter:	radians	number	mandatory

atan
description:	Returns the values arc tangent in radians
return type:	number
expression example:	{{atan(0.5)}} should render: 0.4636476090008061
code example:	atan_test.dart
parameter:	value	number	mandatory

sqrt
description:	Returns the positive square root of the value.
return type:	number
expression example:	{{sqrt(9)}} should render: 3.0
code example:	sqrt_test.dart
parameter:	value	number	mandatory

round
description:	Returns the a rounded number.
return type:	number
expression example:	{{round(4.445)}} should render: 4
code example:	round_test.dart
parameter:	value	number	mandatory

String Functions

length
description:	Returns the length of a string
return type:	number
expression example:	{{length("Hello")}} should render: 5
code example:	length_test.dart
parameter:	string	String	mandatory

Documentation Functions

engine.tag.documentation
description:	Generates markdown documentation of all the tags within a TemplateEngine
return type:	String
expression example:	{{ engine.tag.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

engine.dataType.documentation
description:	Generates markdown documentation of all the data types that can be used within a ExpressionTag of a TemplateEngine
return type:	String
expression example:	{{ engine.dataType.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

engine.constant.documentation
description:	Generates markdown documentation of all the constants that can be used within a ExpressionTag of a TemplateEngine
return type:	String
expression example:	{{ engine.constant.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

engine.function.documentation
description:	Generates markdown documentation of all the functions that can be used within a ExpressionTag of a TemplateEngine
return type:	String
expression example:	{{ engine.function.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

engine.operator.documentation
description:	Generates markdown documentation of all the operators that can be used within a ExpressionTag of a TemplateEngine
return type:	String
expression example:	{{ engine.operator.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

engine.example.documentation
description:	Generates markdown documentation of all the examples. This could be used to generate example.md file.
return type:	String
expression example:	{{ engine.example.documentation() }}
parameter:	titleLevel	number	optional (default=1)	The level of the tag title

Operator

An operator behaves generally like functions, but differs syntactically or semantically.

Common simple examples include arithmetic (e.g. addition with +) and logical operations (e.g. &).

An operator can be used anywhere in an tag expression wherever that particular Operator should be performed.

The TemplateEngine supports several standard operators.

Custom Operators

You can adopt existing operators or add your own custom operators by manipulating the TemplateEngine.operatorGroups field. See custom_operator_test.dart.

Available Operators

Parentheses

operator: ( ... )
description:	Groups expressions together so that the are calculated first
expression example:	{{(2+1)*3}} should render: 9
code example:	parentheses_test.dart

Prefixes

operator: +
parameter type: number
description:	Optional prefix for positive numbers
expression example:	{{+3}} should render: 3
code example:	positive_test.dart

operator: -
parameter type: number
description:	Prefix for a negative number
expression example:	{{-3}} should render: -3
code example:	negative_test.dart

operator: !
parameter type: boolean
description:	Prefix to invert a boolean, e.g.: !true =false
expression example:	{{!true}} should render: false
code example:	not_test.dart

Multiplication

operator: ^
parameter type: number
description:	Calculates a number to the power of the exponent number
expression example:	{{2^3}} should render: 8
code example:	num_power_test.dart
parameter type: boolean
description:	Logical XOR with two booleans
expression example:	{{true^false}} should render: true
code example:	bool_xor_test.dart

operator: *
parameter type: number
description:	Multiplies 2 numbers
expression example:	{{2*3}} should render: 6
code example:	num_multiply_test.dart

operator: /
parameter type: number
description:	Divides 2 numbers
expression example:	{{6/4}} should render: 1.5
code example:	num_divide_test.dart

operator: %
parameter type: number
description:	Calculates the modulo (rest value of a division)
expression example:	{{8%3}} should render: 2
code example:	num_modulo_test.dart

operator: &
parameter type: boolean
description:	Logical AND operation on two booleans
expression example:	{{true&true}} should render: true
code example:	bool_and_test.dart
parameter type: String
description:	Concatenates two strings
expression example:	{{"Hel"&"lo"}} should render: Hello
code example:	string_concatenate_test.dart

Additions

operator: +
parameter type: number
description:	Adds two numbers
expression example:	{{2+3}} should render: 5
code example:	num_addition_test.dart
parameter type: String
description:	Concatenates two strings
expression example:	{{"Hel"+"lo"}} should render: Hello
code example:	string_concatenate_test.dart

operator: -
parameter type: number
description:	Subtracts two numbers
expression example:	{{5-3}} should render: 2
code example:	num_subtract_test.dart

operator: |
parameter type: boolean
description:	Logical OR operation on two booleans
expression example:	{{false|true}} should render: true
code example:	bool_or_test.dart