/**
 * @license Copyright (c) 2003-2020, CKSource - Frederico Knabben. All rights reserved.
 * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
 */

/**
 * @module core/command
 */

import ObservableMixin from '@ckeditor/ckeditor5-utils/src/observablemixin';
import mix from '@ckeditor/ckeditor5-utils/src/mix';

/**
 * The base class for CKEditor commands.
 *
 * Commands are the main way to manipulate editor contents and state. They are mostly used by UI elements (or by other
 * commands) to make changes in the model. Commands are available in every part of code that has access to
 * the {@link module:core/editor/editor~Editor editor} instance.
 *
 * Instances of registered commands can be retrieved from {@link module:core/editor/editor~Editor#commands `editor.commands`}.
 * The easiest way to execute a command is through {@link module:core/editor/editor~Editor#execute `editor.execute()`}.
 *
 * By default commands are disabled when the editor is in {@link module:core/editor/editor~Editor#isReadOnly read-only} mode.
 *
 * @mixes module:utils/observablemixin~ObservableMixin
 */
export default class Command {
	/**
	 * Creates a new `Command` instance.
	 *
	 * @param {module:core/editor/editor~Editor} editor Editor on which this command will be used.
	 */
	constructor( editor ) {
		/**
		 * The editor on which this command will be used.
		 *
		 * @readonly
		 * @member {module:core/editor/editor~Editor}
		 */
		this.editor = editor;

		/**
		 * The value of the command. A concrete command class should define what it represents for it.
		 *
		 * For example, the `'bold'` command's value indicates whether the selection starts in a bolded text.
		 * And the value of the `'link'` command may be an object with links details.
		 *
		 * It is possible for a command to have no value (e.g. for stateless actions such as `'imageUpload'`).
		 *
		 * A concrete command class should control this value by overriding the {@link #refresh `refresh()`} method.
		 *
		 * @observable
		 * @readonly
		 * @member #value
		 */
		this.set( 'value', undefined );

		/**
		 * Flag indicating whether a command is enabled or disabled.
		 * A disabled command will do nothing when executed.
		 *
		 * A concrete command class should control this value by overriding the {@link #refresh `refresh()`} method.
		 *
		 * It is possible to disable a command from "outside". For instance, in your integration you may want to disable
		 * a certain set of commands for the time being. To do that, you can use the fact that `isEnabled` is observable
		 * and it fires the `set:isEnabled` event every time anyone tries to modify its value:
		 *
		 *		function disableCommand( cmd ) {
		 *			cmd.on( 'set:isEnabled', forceDisable, { priority: 'highest' } );
		 *
		 *			cmd.isEnabled = false;
		 *
		 *			// Make it possible to enable the command again.
		 *			return () => {
		 *				cmd.off( 'set:isEnabled', forceDisable );
		 *				cmd.refresh();
		 *			};
		 *
		 *			function forceDisable( evt ) {
		 *				evt.return = false;
		 *				evt.stop();
		 *			}
		 *		}
		 *
		 *		// Usage:
		 *
		 *		// Disabling the command.
		 *		const enableBold = disableCommand( editor.commands.get( 'bold' ) );
		 *
		 *		// Enabling the command again.
		 *		enableBold();
		 *
		 * @observable
		 * @readonly
		 * @member {Boolean} #isEnabled
		 */
		this.set( 'isEnabled', false );

		/**
		 * Holds identifiers for {@link #forceDisabled} mechanism.
		 *
		 * @type {Set.<String>}
		 * @private
		 */
		this._disableStack = new Set();

		this.decorate( 'execute' );

		// By default every command is refreshed when changes are applied to the model.
		this.listenTo( this.editor.model.document, 'change', () => {
			this.refresh();
		} );

		this.on( 'execute', evt => {
			if ( !this.isEnabled ) {
				evt.stop();
			}
		}, { priority: 'high' } );

		// By default commands are disabled when the editor is in read-only mode.
		this.listenTo( editor, 'change:isReadOnly', ( evt, name, value ) => {
			if ( value ) {
				this.forceDisabled( 'readOnlyMode' );
			} else {
				this.clearForceDisabled( 'readOnlyMode' );
			}
		} );
	}

	/**
	 * Refreshes the command. The command should update its {@link #isEnabled} and {@link #value} properties
	 * in this method.
	 *
	 * This method is automatically called when
	 * {@link module:engine/model/document~Document#event:change any changes are applied to the document}.
	 */
	refresh() {
		this.isEnabled = true;
	}

	/**
	 * Disables the command.
	 *
	 * Command may be disabled by multiple features or algorithms (at once). When disabling a command, unique id should be passed
	 * (e.g. feature name). The same identifier should be used when {@link #clearForceDisabled enabling back} the command.
	 * The command becomes enabled only after all features {@link #clearForceDisabled enabled it back}.
	 *
	 * Disabling and enabling a command:
	 *
	 *		command.isEnabled; // -> true
	 *		command.forceDisabled( 'MyFeature' );
	 *		command.isEnabled; // -> false
	 *		command.clearForceDisabled( 'MyFeature' );
	 *		command.isEnabled; // -> true
	 *
	 * Command disabled by multiple features:
	 *
	 *		command.forceDisabled( 'MyFeature' );
	 *		command.forceDisabled( 'OtherFeature' );
	 *		command.clearForceDisabled( 'MyFeature' );
	 *		command.isEnabled; // -> false
	 *		command.clearForceDisabled( 'OtherFeature' );
	 *		command.isEnabled; // -> true
	 *
	 * Multiple disabling with the same identifier is redundant:
	 *
	 *		command.forceDisabled( 'MyFeature' );
	 *		command.forceDisabled( 'MyFeature' );
	 *		command.clearForceDisabled( 'MyFeature' );
	 *		command.isEnabled; // -> true
	 *
	 * **Note:** some commands or algorithms may have more complex logic when it comes to enabling or disabling certain commands,
	 * so the command might be still disabled after {@link #clearForceDisabled} was used.
	 *
	 * @param {String} id Unique identifier for disabling. Use the same id when {@link #clearForceDisabled enabling back} the command.
	 */
	forceDisabled( id ) {
		this._disableStack.add( id );

		if ( this._disableStack.size == 1 ) {
			this.on( 'set:isEnabled', forceDisable, { priority: 'highest' } );
			this.isEnabled = false;
		}
	}

	/**
	 * Clears forced disable previously set through {@link #forceDisabled}. See {@link #forceDisabled}.
	 *
	 * @param {String} id Unique identifier, equal to the one passed in {@link #forceDisabled} call.
	 */
	clearForceDisabled( id ) {
		this._disableStack.delete( id );

		if ( this._disableStack.size == 0 ) {
			this.off( 'set:isEnabled', forceDisable );
			this.refresh();
		}
	}

	/**
	 * Executes the command.
	 *
	 * A command may accept parameters. They will be passed from {@link module:core/editor/editor~Editor#execute `editor.execute()`}
	 * to the command.
	 *
	 * The `execute()` method will automatically abort when the command is disabled ({@link #isEnabled} is `false`).
	 * This behavior is implemented by a high priority listener to the {@link #event:execute} event.
	 *
	 * In order to see how to disable a command from "outside" see the {@link #isEnabled} documentation.
	 *
	 * This method may return a value, which would be forwarded all the way down to the
	 * {@link module:core/editor/editor~Editor#execute `editor.execute()`}.
	 *
	 * @fires execute
	 */
	execute() {}

	/**
	 * Destroys the command.
	 */
	destroy() {
		this.stopListening();
	}

	/**
	 * Event fired by the {@link #execute} method. The command action is a listener to this event so it's
	 * possible to change/cancel the behavior of the command by listening to this event.
	 *
	 * See {@link module:utils/observablemixin~ObservableMixin#decorate} for more information and samples.
	 *
	 * **Note:** This event is fired even if command is disabled. However, it is automatically blocked
	 * by a high priority listener in order to prevent command execution.
	 *
	 * @event execute
	 */
}

mix( Command, ObservableMixin );

// Helper function that forces command to be disabled.
function forceDisable( evt ) {
	evt.return = false;
	evt.stop();
}