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packages/polymer_expressions/README.md

+polymer_expressions
+===================
+
+
+Polymer expressions are an expressive syntax that can be used in HTML templates
+with Dart.
+
+Templates are one feature of Polymer.dart, which is a set of comprehensive UI
+and utility components for building web applications.
+This package is automatically included with the
+[Polymer](https://pub.dartlang.org/packages/polymer) package
+because Polymer expressions are the default expression syntax
+in Polymer Dart apps.
+The [Polymer.dart homepage][home_page]
+contains a list of features, project status,
+installation instructions, tips for upgrading from Web UI,
+and links to other documentation.
+
+
+## Overview
+
+Polymer expressions allow you to write complex binding expressions, with
+property access, function invocation, list/map indexing, and two-way filtering
+like:
+
+```html
+    {{ person.title + " " + person.getFullName() | upppercase }}
+```
+
+### Model-Driven Views (MDV)
+[MDV][mdv] allows you to define templates directly in HTML that are rendered by
+the browser into the DOM. Templates are bound to a data model, and changes to
+the data are automatically reflected in the DOM, and changes in HTML inputs are
+assigned back into the model. The template and model are bound together via
+binding expressions that are evaluated against the model. These binding
+expressions are placed in double-curly-braces, or "mustaches".
+
+Example:
+
+```html
+    <template>
+      <p>Hello {{ person.name }}</p>
+    </template>
+```
+
+MDV includes a very basic binding syntax which only allows a series of
+dot-separate property names.
+
+[mdv]: http://www.polymer-project.org/platform/mdv.html
+
+### Custom binding syntaxes with binding delegate
+
+While MDV's built-in syntax is very basic, it does allow custom syntaxes called
+"binding delegates" to be installed and used. A binding delegate can interpret
+the contents of mustaches however it likes. PolymerExpressions is such a
+binding delegate.
+
+Example:
+
+```html
+    <template bind>
+      <p>Hello {{ person.title + " " + person.getFullName() | uppercase }}</p>
+    </template>
+```
+
+## Usage
+
+### Installing from Pub
+
+Add the following to your pubspec.yaml file:
+
+```yaml
+    dependencies:
+      polymer_expressions: any
+```
+
+Hint: check https://pub.dartlang.org/packages/polymer_expressions for the latest
+version number.
+
+Then import polymer_expressions.dart:
+
+    import 'package:polymer_expressions/polymer_expressions.dart';
+
+### Registering a binding delegate
+
+**Polymer Expressions are now the default syntax for `<polymer-element>` custom
+elements.**
+
+You do not need to manually register the bindingDelegate if your bindings are
+inside a custom element. However, if you want to use polymer_expressions outside
+a custom element, read on:
+
+Binding delegates must be installed on a template before they can be used.
+For example, set the bindingDelegate property of your template
+elements to an instance of PolymerExpressions. The templates will then use the
+PolymerExpressions instance to interpret
+binding expressions.
+
+```dart
+    import 'dart:html';
+    import 'package:polymer_expressions/polymer_expressions.dart';
+
+    main() {
+      var template = query('#my_template');
+      template.bindingDelegate = new PolymerExpressions();
+    }
+```
+
+### Registering top-level variables
+
+Before a top-level variable can be used, it must be registered. The
+PolymerExpressions constructor takes a map of named values to use as variables.
+
+```dart
+    main() {
+      var globals = {
+        'uppercase': (String v) => v.toUpperCase(),
+        'app_id': 'my_app_123',
+      };
+      var template = query('#my_template');
+      template.bindingDelegate = new PolymerExpressions(globals: globals);
+    }
+```
+
+## Features
+
+### The model and scope
+
+Polymer Expressions allow binding to more than just the model assigned to a
+template instance. Top-level variables can be defined so that you can use
+filters, global variables and constants, functions, etc. These variables and the
+model are held together in a container called a Scope. Scopes can be nested,
+which happens when template tags are nested.
+
+### Two-way bindings
+
+Bindings can be used to modify the data model based on events in the DOM. The
+most common case is to bind an &lt;input&gt; element's value field to a model
+property and have the property update when the input changes. For this to work,
+the binding expression must be "assignable". Only a subset of expressions are
+assignable. Assignable expressions cannot contain function calls, operators, and
+any index operator must have a literal argument. Assignable expressions can
+contain filter operators as long as all the filters are two-way transformers.
+
+Some restrictions may be relaxed further as allowed.
+
+Assignable Expressions:
+
+ * `foo`
+ * `foo.bar`
+ * `items[0].description`
+ * `people['john'].name`
+ * `product.cost | convertCurrency('ZWD')` where `convertCurrency` evaluates to
+   a Tranformer object.
+
+Non-Assignable Expressions:
+
+ * `a + 1`
+ * `!c`
+ * `foo()`
+ * `person.lastName | uppercase` where `uppercase` is a filter function.
+
+### Null-Safety
+
+Expressions are generally null-safe. If an intermediate expression yields `null`
+the entire expression will return null, rather than throwing an exception.
+Property access, method invocation and operators are null-safe. Passing null to
+a function that doesn't handle null will not be null safe.
+
+### Streams
+
+Polymer Expressions have experimental support for binding to streams, and when
+new values are passed to the stream, the template updates. The feature is not
+fully implemented yet.
+
+See the examples in /example/streams for more details.
+
+## Syntax
+
+### Property access
+
+Properties on the model and in the scope are looked up via simple property
+names, like `foo`. Property names are looked up first in the top-level
+variables, next in the model, then recursively in parent scopes. Properties on
+objects can be access with dot notation like `foo.bar`.
+
+The keyword `this` always refers to the model if there is one, otherwise `this`
+is `null`. If you have model properties and top-level variables with the same
+name, you can use `this` to refer to the model property.
+
+### Literals
+
+Polymer Expressions support number, boolean, string, and map literals. Strings
+can use either single or double quotes.
+
+ * Numbers: `1`, `1.0`
+ * Booleans: `true`, `false`
+ * Strings: `'abc'`, `"xyz"`
+ * Maps: `{ 'a': 1, 'b': 2 }`
+
+List literals are planned, see [issue 9](https://github.com/dart-lang/polymer_expressions/issues/9)
+
+### Functions and methods
+
+If a property is a function in the scope, a method on the model, or a method on
+an object, it can be invoked with standard function syntax. Functions and
+Methods can take arguments. Named arguments are not supported. Arguments can be
+literals or variables.
+
+Examples:
+
+ * Top-level function: `myFunction()`
+ * Top-level function with arguments: `myFunction(a, b, 42)`
+ * Model method: `aMethod()`
+ * Method on nested-property: `a.b.anotherMethod()`
+
+### Operators
+
+Polymer Expressions supports the following binary and unary operators:
+
+ * Arithmetic operators: +, -, *, /, %, unary + and -
+ * Comparison operators: ==, !=, <=, <, >, >=
+ * Boolean operators: &&, ||, unary !
+
+Expressions do not support bitwise operators such as &, |, << and >>, or increment/decrement operators (++ and --)
+
+### List and Map indexing
+
+List and Map like objects can be accessed via the index operator: []
+
+Examples:
+
+ * `items[2]`
+ * `people['john']`
+
+Unlike JavaScript, list and map contents are not generally available via
+property access. That is, the previous examples are not equivalent to `items.2`
+and `people.john`. This ensures that access to properties and methods on Lists
+and Maps is preserved.
+
+### Filters and transformers
+
+A filter is a function that transforms a value into another, used via the pipe
+syntax: `value | filter` Any function that takes exactly one argument can be
+used as a filter.
+
+Example:
+
+If `person.name` is "John", and a top-level function named `uppercase` has been
+registered, then `person.name | uppercase` will have the value "JOHN".
+
+The pipe syntax is used rather than a regular function call so that we can
+support two-way bindings through transformers. A transformer is a filter that
+has an inverse function. Transformers must extend or implement the `Transformer`
+class, which has `forward()` and `reverse()` methods.
+
+### Repeating templates
+
+A template can be repeated by using the "repeat" attribute with a binding. The
+binding can either evaluate to an Iterable, in which case the template is
+instantiated for each item in the iterable and the model of the instance is
+set to the item, or the binding can be a "in" iterator expression, in which
+case a new variable is added to each scope.
+
+The following examples produce the same output.
+
+Evaluate to an iterable:
+
+```html
+    <template repeat="{{ items }}">
+      <div>{{ }}</div>
+    </template>
+```
+
+"in" expression:
+
+```html
+    <template repeat="{{ item in items }}">
+      <div>{{ item }}</div>
+    </template>
+```
+
+## Status
+
+The syntax implemented is experimental and subject to change, in fact, it
+**will** change soon. The goal is to be compatible with Polymer's binding
+syntax. We will announce breaking changes on the
+[web-ui@dartlang.org mailing list][web-ui-list].
+
+Please [file issues on Dart project page](http://dartbug.com/new)
+for any bugs you find or for feature requests. Make a note that it applies to
+"package:polymer_expressions"
+
+You can discuss Polymer Expressions on the
+[web-ui@dartlang.org mailing list][web-ui-list].
+
+[web-ui-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/web-ui