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DrawerJS architecture overview

src/Drawer.js

Main file which contains DrawerJS constructor function.

This file contains all core functionality of Drawer: code for instantiating a new instance of Drawer and setting up events and plugins, options management code, state management code.

Most important topics:

DrawerJS states

DrawerJS instance always has one of two states: preview mode or edit mode.

Preview mode

In normal mode DrawerJS is a simple <img> tag in the DOM. It shows an image that was drawn by encoding canvas data to base64 string and setting it as a src attribute on underlying <img>.

That image has a click handler attached to it. When it is triggered DrawerJS goes into edit mode.

Edit mode

When in edit mode, DrawerJS becomes a <canvas> element. Underlying <img> becomes hidden and user sees actual drawings on canvas. Drawing is performed using fabric.js library.

A set of toolbar is also displayed around the canvas. Toolbars contain tools and options for user to draw.

When DrawerJS goes into edit mode, it attaches a click handler to document.body. When click occurs outside of DrawerJS it means that is loosed focus and preview mode is activated.

State management functions

There are two functions that manage state: [_startEditing] and [_stopEditing]

Here we will cover only [_startEditing] because [_stopEditing] does all the same but in reverse order.

[_startEditing] function is called from DrawerJS code when user clicks on the image (drawerjs instance).

The purpose of the function is:

  • Create <canvas> element, append it to DOM and position it correctly above the preview image.

  • Construct new fabric.js canvas wrapper around raw html5 <canvas> element.

  • Pass the data about objects on canvas to fabric.js wrapper object. [Drawer.prototype.loadCanvas] function which is located at Drawer.Storage.js used for that purpose.

    [Drawer.prototype.loadCanvas] deserializes canvas data (objects/sizes/colors/dimensions) using fabric.js loadFromJSON method, fixes objects properties, triggers EVENT_LOADED_FROM_JSON event and calls [Drawer.prototype.onCanvasLoaded] function.

    When all post-load job is done, EVENT_CANVAS_READY gets triggered on DrawerJS object. This is the moment where all registered plugins get notified about canvas readyness and prepare themselves for edit mode.

Canvas data

To persist canvas data between switching states DrawerJS serializes all fabric.js objects from canvas to JSON and stores it in <img> attribute data-canvas-serialized.

This allows saving image as-is on the backend or on any other storage and then restore DrawerJS from that image.

For example: when image is loaded from some storage and DrawerJS is attached to it, it looks at data-canvas-serialized attribute. When user clicks on image, DrawerJS goes into edit mode and restores all canvas objects and user is able to manipulate them.

Plugin system

All drawing tools as well as most of other features (toolbars, shape options) are not tied to DrawerJS and are separate modules.

Drawer uses [Drawer.prototype.loadPlugins] function to load plugins and store their instances in _pluginsInstances private variable.

When plugin is instantiated DrawerJS passes a hashmap of options to it from own options.pluginsConfig[pluginName] property.

DrawerJS uses [Events] system to communicate with plugins.

All plugins receive DrawerJS instance in their constructor and should subscribe to any required events with `Drawer.on(EVENT_NAME) method.

When event occurs (for example, EVENT_EDIT_START) DrawerJS will trigger all event listeners in all plugins subscribed to a particular event. It's a plugin's responsibility to react to an event, for example, to show some UI or change state.

Documentation for all plugins can be found in a plugin director (see Directories).

Here are quick links for most complex plugins: