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+# Extending Netlify CMS
+The Netlify CMS exposes an `window.CMS` global object that you can use to register custom widgets, previews and editor plugins. The available methods are:
+
+* **registerPreviewStyle** Register a custom stylesheet to use on the preview pane.
+* **registerPreviewTemplate** Registers a template for a collection.
+* **registerWidget** lets you register a custom widget.
+* **registerEditorComponent** lets you add a block component to the Markdown editor
+
+**Writing React Components inline**
+
+Both registerPreviewTemplate and registerWidget requires you to provide a React component. If you have a build process in place for your project, it is possible to integrate webpack and Babel for a complete React build flow.
+
+Although possible, it may be cumbersome or even impractical to add a React build phase. For this reason, Netlify CMS exposes two React constructs globally to allow you to create components inline: ‘createClass’ and ‘h’ (alias for React.createElement).
+
+
+## `registerPreviewStyle`
+
+Register a custom stylesheet to use on the preview pane.
+
+`CMS.registerPreviewStyle(file);`
+
+**Params:**
+
+* file: css file path.
+
+**Example:**
+
+`CMS.registerPreviewStyle("/example.css");`
+
+
+## `registerPreviewTemplate`
+
+Registers a template for a collection.
+
+`CMS.registerPreviewTemplate(collection, react_component);`
+
+**React Component Props:**
+
+* collection: The name of the collection which this preview component will be used for.
+* react_component: A React component that renders the collection data. Three props will be passed to your component during render:
+  * entry: Immutable collection containing the entry data.
+  * widgetFor: Returns the appropriate widget preview component for a given field.
+  * getAsset: Returns the correct filePath or in-memory preview for uploaded images.
+
+**Example:**
+
+```html
+<script>
+var PostPreview = createClass({
+  render: function() {
+    var entry = this.props.entry;
+    var image = entry.getIn(['data', 'image']);
+    var bg = this.props.getAsset(image);
+    return h('div', {},
+      h('h1', {}, entry.getIn(['data', 'title'])),
+      h('img', {src: bg.toString()}),
+      h('div', {"className": "text"}, this.props.widgetFor('body'))
+    );
+  }
+});
+
+CMS.registerPreviewTemplate("posts", PostPreview);
+</script>
+```
+
+### Accessing Metadata
+Preview Components also receive an additional prop: `fieldsMetaData`. It contains aditional information (besides the plain plain textual value of each field) that can be useful for preview purposes. 
+
+For example, the Relation widget passes the whole selected relation data in fieldsMetaData.
+
+```js
+export default class ArticlePreview extends React.Component {
+  render() {
+    const {entry, fieldsMetaData} = this.props;
+    const author = fieldsMetaData.getIn(['authors', data.author]);
+
+    return <article>
+      <h2>{ entry.getIn(['data', 'title']) }</h2>
+      {author && <AuthorBio author={author.toJS()}/>}
+    </article>
+  }
+}
+```
+
+## `registerWidget`
+
+lets you register a custom widget.
+
+`CMS.registerWidget(field, control, [preview])`
+
+**Params:**
+
+* field: The field type which this widget will be used for.
+* control: A React component that renders the editing interface for this field. Two props will be passed:
+  * value: The current value for this field.
+  * onChange: Callback function to update the field value.
+* preview (optional): A React component that renders the preview of how the content will look. A `value` prop will be passed to this component.
+
+
+**Example:**
+
+```html
+<script>
+var CategoriesControl = createClass({
+  handleChange: function(e) {
+    this.props.onChange(e.target.value.split(',').map((e) => e.trim()));
+  },
+
+  render: function() {
+    var value = this.props.value;
+    return h('input', { type: 'text', value: value ? value.join(', ') : '', onChange: this.handleChange });
+  }
+});
+
+CMS.registerWidget('categories', CategoriesControl);
+</script>
+```
+
+## `registerEditorComponent`
+
+lets your register a block level component for the Markdown editor
+
+`CMS.registerEditorComponent(definition)`
+
+**Params**
+
+* definition: The component definition, must specify: id, label, fields, patterns, fromBlock, toBlock, toPreview
+
+**Example:**
+
+```js
+CMS.registerEditorComponent({
+  // Internal id of the component
+  id: "youtube",
+  // Visible label
+  label: "Youtube",
+  // Fields the user need to fill out when adding an instance of the component
+  fields: [{name: 'id', label: 'Youtube Video ID', widget: 'string'}],
+  // Pattern to identify a block as being an instance of this component
+  pattern: /^{{<\s?youtube (\S+)\s?>}}/,
+  // Function to extract data elements from the regexp match
+  fromBlock: function(match) {
+    return {
+      id: match[1]
+    };
+  },
+  // Function to create a text block from an instance of this component
+  toBlock: function(obj) {
+    return '{{< youtube ' + obj.id + ' >}}';
+  },
+  // Preview output for this component. Can either be a string or a React component
+  // (Component gives better render performance)
+  toPreview: function(obj) {
+    return (
+      '<img src="http://img.youtube.com/vi/' + obj.id + '/maxresdefault.jpg" alt="Youtube Video"/>'
+    );
+  }
+});
+```
diff --git a/docs/validation.md b/docs/validation.md
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+# Collection Field Validation
+
+## Available validations to use on config.yaml:
+
+- Presence: By default all widgets are required, unless specified in the config. Example:
+`- {label: "Subtitle", name: "subtitle", widget: "string", required: false}`
+
+- Pattern: Field configuration can specify a regex pattern with the appropriate error message. Example:
+`- {label: "Title", name: "title", widget: "string", pattern: [".{10,}", "Should have more than 10 characters"] }`
+
+
+## Advanced Guide (For widget authors).
+
+The widget control can optionally implement an `isValid` method to perform custom validations, in addition to presence and pattern. The `isValid` method will be automatically called, and it can return either a boolean value, an object with an error message or a promise. Examples:
+
+**Boolean**
+No errors:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return true;
+  };
+```
+
+Existing error:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return false;
+  };
+```
+
+**Object with `error` (Useful for returning custom error messages)**
+Existing error:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return { error: 'Your error message.' };
+  };
+```
+
+**Promise**
+You can also return a promise from isValid. While the promise is pending, the widget will be marked as in error. When the promise resolves, the error is automatically cleared.
+
+```javascript
+  isValid = () => {
+    return this.existingPromise;
+  };
+```
+
+Note: Do not create a promise inside isValid - isValid is called right before trying to persist. This means that even if a previous promise was already resolved, when the user hits 'save', `isValid` will be called again - if it returns a new Promise it will be immediately marked as in error until the new promise resolves.