Layering canvas objects in Fabric.js

问题

Is there a way to layer objects on a Fabric.js canvas via the official API? Right now the only way I have found to do it is to manually iterate through the canvas._objects and reorder them so that they get drawn in a specific order. Is there a better way to do this that doesn't (potentially) break the object?

回答1:

[Edit] I've corrected my info below (my bad, I was originally thinking of the KineticJs api).

FabricJS has these API methods that change the z-index of objects:

canvas.sendBackwards(myObject)
canvas.sendToBack(myObject)
canvas.bringForward(myObject)
canvas.bringToFront(myObject)

Under the covers, FabricJs changes the z-index by removing the object from the getObjects() array and splicing it back in the desired position. It has a nice optimization that checks for intersecting objects.

bringForward: function (object) {
       var objects = this.getObjects(),
           idx = objects.indexOf(object),
           nextIntersectingIdx = idx;


       // if object is not on top of stack (last item in an array)
       if (idx !== objects.length-1) {

         // traverse up the stack looking for the nearest intersecting object
         for (var i = idx + 1, l = this._objects.length; i < l; ++i) {

           var isIntersecting = object.intersectsWithObject(objects[i]) ||
                                object.isContainedWithinObject(this._objects[i]) ||
                                this._objects[i].isContainedWithinObject(object);

           if (isIntersecting) {
             nextIntersectingIdx = i;
             break;
           }
         }
         removeFromArray(objects, object);
         objects.splice(nextIntersectingIdx, 0, object);
       }
       this.renderAll();
     },

回答2:

step 1 : You can Use preserveObjectStacking: true

step 2 : then use sendtoback,sendtofront....fabricjs options like below

Answered over here

回答3:

If you want to set a specific ordering for all objects, instead of moving one object forward/backward, iterative calls to bring/send to front/back are slow.

You can use the code of bringToFront as inspiration to speed this use case up: https://github.com/kangax/fabric.js/blob/586e61dd282b22c1d50e15c0361875707e526fd8/src/static_canvas.class.js#L1468

And do this:

fabric.Canvas.prototype.orderObjects = function(compare) {
    this._objects.sort(compare);
    this.renderAll();
}

Where compare defines the sorting, like:

function compare(x,y) {
    return x.getWidth() * x.getHeight() < y.getWidth() * y.getHeight();
}

some_canvas.orderObjects(compare)

If you wish to bring smaller objects to the front.

来源：https://stackoverflow.com/questions/15032497/layering-canvas-objects-in-fabric-js

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