canvas操作插件fabric.js使用方法详解

//1: 获得画布上的所有对象：
var items = canvas.getObjects();

//2: 设置画布上的某个对象为活动对象。
canvas.setActiveObject(items[i]);

//3：获得画布上的活动对象
canvas.getActiveObject()

//4：取消画布中的所有对象的选中状态。
canvas.discardActiveObject();

//5: 设置画布中的对象的某个属性值，比如第 0 个对象的 id
var items = canvas.getObjects();
tems[0].id ="items_id0" 或 items[0].set("id","items_id0")

//6：获得画布中对象的某个属性，比如 第0 个对象的 id
var items = canvas.getObjects();
items[0].id;
//或
items[0].get("id");

//7: 重新渲染一遍画布，当画布中的对象有变更，在最后显示的时候，需要执行一次该操作
canvas.renderAll()

//8: 清除画布中所有对象：
canvas.clear();

//9:清除画布中的活动对象：
 var t = canvas.getActiveObject();
 t && canvas.remove(t);

//10: 设置活动对象在画布中的层级
var t = canvas.getActiveObject();
canvas.sendBackwards(t) //向下跳一层
canvas.sendToBack(t)  //向下跳底层：
canvas.bringForward(t) //向上跳一层：
canvas.bringToFront(t) //向上跳顶层：
//或者：
t.sendBackwards();
t.sendToBack();
t.bringForward();
t.bringToFront();

//10：加载图片时图片缩放到指定的大小。
fabric.Image.fromURL(image_src, function(oImg) {
  oImg.set({
    left:tmp_left,
    top:tmp_top,
    centeredScaling:true,
    cornerSize: 7,
    cornerColor: "#9cb8ee",
    transparentCorners: false,
  });
  oImg.scaleToWidth(image_width);
  oImg.scaleToHeight(image_height);
  canvas.add(oImg).setActiveObject(oImg);
 });

//11：当选择画布中的对象时，该对象不出现在顶层。
canvas.preserveObjectStacking = true;

// 12：为画布通过URL方式添加背景图片
var bg_url = "http://www.xxxx.com/...../bg.png"      
fabric.Image.fromURL( bg_url , function(oImg) {
  oImg.set({
  width: canvas_obj.width, 
  height: canvas_obj.height,
  originX: "left", 
  originY: "top"
  });
  canvas_obj.setBackgroundImage(oImg, canvas_obj.renderAll.bind(canvas_obj));
});

//13: originx: originy:代表坐标系。

//14: 画布对象居中设置：
var t = canvas.getActiveObject();
t.center();  //全部居中
t.centerH();  //水平居中
t.centerV();  //垂直居中
t.setCoords(); //注：必须设coords以上设置才会有效。

//15: 当对象移动时 限制对象的 不超出画布
// canvas moving limit 
function objectMoving(e){
  var obj = e.target;
  if(obj.currentHeight > obj.canvas.height || obj.currentWidth > obj.canvas.width){
      return;
   }    
   obj.setCoords();    
    // top-left corner
    if(obj.getBoundingRect().top <0 || obj.getBoundingRect().left <0){
      obj.top = Math.max(obj.top, obj.top-obj.getBoundingRect().top);
      obj.left = Math.max(obj.left, obj.left-obj.getBoundingRect().left);
     }
     // bot-right corner
     if(obj.getBoundingRect().top+obj.getBoundingRect().height > obj.canvas.height 
     || obj.getBoundingRect().left+obj.getBoundingRect().width > obj.canvas.width){
      obj.top = Math.min(obj.top, obj.canvas.height-obj.getBoundingRect().height+obj.top-obj.getBoundingRect().top);
         obj.left = Math.min(obj.left, obj.canvas.width-obj.getBoundingRect().width+obj.left-obj.getBoundingRect().left);
      }
}

//16：当画布对象放大时限制其操出边界：
//注意当创建对象到画布上时必须先加上：
 obj.saveState();
 //在对象修改时方法里就可以调用了。
 function objectModified (e) {
  let obj = e.target;
  let boundingRect = obj.getBoundingRect(true);
  if (boundingRect.left <0
    || boundingRect.top <0
    || boundingRect.left + boundingRect.width > obj.canvas.getWidth()
    || boundingRect.top + boundingRect.height > obj.canvas.getHeight()) {
    obj.top = obj._stateProperties.top;
    obj.left = obj._stateProperties.left;
    obj.angle = obj._stateProperties.angle;
    obj.scaleX = obj._stateProperties.scaleX;
    obj.scaleY = obj._stateProperties.scaleY;
    obj.setCoords();
    obj.saveState();
  }else{
  obj.saveState();
}
  }
//17:当生成json对象时，背景图片显示出来。
fabric.Image.fromURL( bgImg, function(oImg) {
   oImg.set({
     width: 400,
     height:400,
     left:0,
     top:0,
     originX: "left",
     originY: "top",
     opacity:0
   });
   //当toObject方法时显示背景图片。
   oImg.toObject = (function(toObject) {
     return function() {
      return fabric.util.object.extend(toObject.call(this), {
        opacity:1
      });
     };
  })(oImg.toObject); 

 canvas_obj.setBackgroundImage(oImg, canvas_obj.renderAll.bind(canvas_obj));  
}, { crossOrigin: "Anonymous" });

//18：创建蒙版层
//获取蒙版位置属性（非必要）：
var maskLayerTop  = parseInt($(this).attr("data-top"));
var maskLayerLeft  = parseInt($(this).attr("data-left"));
var maskLayerWidth = parseInt($(this).attr("data-width"));
var maskLayerHeight = parseInt($(this).attr("data-height"));
//创建蒙版
var clipMask = new fabric.Rect({
  originX: "left",
  originY: "top",
  left: maskLayerLeft,
  top: maskLayerTop,
  width: maskLayerWidth,
  height: maskLayerHeight,
  fill: "rgba(0,0,0,0)", 
  strokeWidth:0,
  selectable: false
});  

clipMask.set({
  clipFor: "pug"
});

canvas_obj.add(clipMask);                
function degToRad(degrees) {
  return degrees * (Math.PI / 180);
}   

function findByClipName(name){
  return _(canvas_obj.getObjects()).where({
    clipFor: name
  }).first()
}  

canvas_obj.clipByName = function(ctx) {
  this.setCoords();
  var clipObj = findByClipName(this.clipName);
  var scaleXTo1 = (1 / this.scaleX);
  var scaleYTo1 = (1 / this.scaleY);
  var skewXReverse = - this.skewX;
  var skewYReverse = - this.skewY;
  ctx.save();                  
  var ctxLeft = -( this.width / 2 ) + clipObj.strokeWidth;
    var ctxTop = -( this.height / 2 ) + clipObj.strokeWidth;
    var ctxWidth = clipObj.width - clipObj.strokeWidth;
    var ctxHeight = clipObj.height - clipObj.strokeWidth;
  ctx.translate( ctxLeft, ctxTop );
  ctx.scale(scaleXTo1, scaleYTo1);
  ctx.transform(1, skewXReverse, skewYReverse, 1, 0, 0);
  ctx.rotate(degToRad(this.angle * -1));                  
  ctx.beginPath();                  
  ctx.rect(
    clipObj.left - this.oCoords.tl.x,
    clipObj.top - this.oCoords.tl.y,
    clipObj.width,
    clipObj.height
  );
  ctx.closePath();                  
  ctx.restore();
} 
//调用的地方：
//文字层设置蒙版
 var t = new fabric.Text("Your Text", {
      id: first_level+"-text-input"+unique_id,
      cornerSize: 7,
      cornerColor: "#9cb8ee",
      transparentCorners: false,
      textAlign:"center",
      clipName: "pug",
      clipTo: function(ctx) { 
      return _.bind(tmp_canvas_obj.clipByName, t)(ctx) 
      }
});
// 图片层设置蒙版：
// add the forntimage to the canvas
fabric.Image.fromURL(image_src, function(oImg) {
  oImg.set({
    id:first_level+"-image-input"+unique_id,
    left:tmp_left,
    top:tmp_top,
    centeredScaling:true,
    cornerSize: 7,
    cornerColor: "#9cb8ee",
    transparentCorners: false,
    clipName: "pug",
    clipTo: function(ctx) { 
      return _.bind(tmp_canvas_obj.clipByName, oImg)(ctx) 
    }

  });

//19：生成的图片缩放到指定的尺寸。
  oImg.scaleToWidth(image_width);
  oImg.scaleToHeight(image_height);

//20：to object 时添加 id属性。
  oImg.toObject = (function(toObject) {
       return function() {
        return fabric.util.object.extend(toObject.call(this), {
         id: this.id,
        });
       };
  })(oImg.toObject);
  oImg.id = first_level+"-image-input"+unique_id;               
  oImg.saveState();
  tmp_canvas_obj.add(oImg).setActiveObject(oImg);
}, { crossOrigin: "Anonymous" });
tmp_canvas_obj.renderAll();