With the `Canvas` class fabric.js can handle a layer of selection / interaction.
Fabric can handle also multiple selection using a special class called `ActiveSelection`.
When a mouse interaction is performed by a user that utilize an app based on fabricJS, there are
some built in selection functionalities.
What follow is a default overview of the selection process:
- Left click on an object selects it.
- Click and drag on the canvas creates a rectangular selection. All the bounding boxes intersecting with
this rectangle will be selected on mouse up creating a multi selection.
- Clicking on objects with shift selects it or add it to the current selection, changing from a selected
object to a multi selection of 2 objects.
An object is selected and behaves as selected when is on a canvas and is referenced on that canvas _activeObject
property.
A multi selection is represented by an ActiveSelection object, that is a special class derived from the Group
class, referenced in the _activeObject property of the canvas.
There is one active object at time and it is either an object or a multi selection.
Developers can create/destroy or change selection, outside of user mouse interactions in the following ways:
canvas.setActiveObject(object) sets the object passed as argument as the active one. The current selected
object gets discarded.
canvas.discardActiveObject() Remove the current selection.
canvas.getActiveObject() Returns a reference to the current active object.
canvas.getActiveObjects() Returns an array containing a reference to the current selected objects, one or many.
None of this methods render the canvas again, so you have to call canvas.requestRenderAll after them to see the changes.
canvas._setActiveObject() and canvas._discardActiveObject() are two private method used from the non private ones
to make the selection job. They do not contain the event firing code, and they are not chainable. if you need to handle a selection process
but you do not want to fire the side effects you inserted in the selection events, you may try to use those.
// rect1 and rect2 are 2 object on a canvas, canvas is the canvas instance
var selection = new fabric.ActiveSelection([rect1, rect2], {
canvas: canvas
});
canvas.setActiveObject(selection);
// rect1 and rect2 are 2 object on a canvas, grouped in a multi selection
// rect3 is another object on the canvas
var selection = canvas.getActiveObject();
if (selection.type === 'activeSelection') {
selection.addWithUpdate(rect3)
}
So an object that is on a canvas can be an activeObject. Many object in a canvas can be grouped in an
ActiveSelection object and behave like a multi seleciton. The objects inside the multi selection are still
direct children of the canvas, and the ActiveSelection object, even if referenced in the `_activeObject` property
of the canvas, it is not included in the canvas objects. canvas.getObjects() will not contain
the ActiveSelection object. The active selection has a mandatory canvas property that has to be the actual canvas
or it will not work.
Future non breaking updates to fabric may simplify this aspect, changing the canvas property
of the multi selection implictly in some method.
A take away point is that the ActiveSelection is a service class that behaves in some predetermined way and that has little
space for customization.
discardActiveObject on a multi selection triggers lot of side effects.
If you are planning to do something with objects inside an active selection and reuse them, and then trash the activeSelection,
first discardActiveObject, then do what you want to do with the objects, then in case select a new object.
The following methods are no more available since the introduction of ActiveSelection:
setActiveGroup(group) getActiveGroup(); deactivateAll(); discardActiveGroup(); deactivateAllWithDispatch();
Regarding selections fabric provides callbacks and events.
Events fired by objects are:
selected deselectedEvents fired by canvas are:
selection:created selection:updated before:selection:cleared selection:cleared // deprecated object:selectedFor each object are available 2 callbacks
onDeselect onSelectThose callbacks are empty and meant to be overridden, and should not used to execute much logic, but just to return a boolean to cancel the current process depending on dynamic conditions for which mutating the properties of objects like selectable, evented or others, is not comfortable or anyway brings to weird code paths.
Those events are meant to intercept user interaction with the selection of fabric.
You should expect the canvas to fire selection:created every time the user pass from a situation
of no selection to something selected. Each object involved in the seleciton process, one or more, by click or
selected event.selection:created fires, it receives as first argument of the callback an object that contains
property 'selected', with an array of all the selected fabric instances.selection:updated fires. The event receives as first argument of the
callback an object that contains both 'selected' and 'deselected'
property contaning an array of the objects that entered or exited the selection status. Each object involved in the process fires its
own single event.selection:cleared fires, getting in
arguments an object with the 'deselected' property. object:selected is considered deprecated and will be removed in the next major version of fabricjs.