问如何用Fabric.js实现画布平移

EN

function startPan(event) {
  if (event.button != 2) {
    return;
  }
  var x0 = event.screenX,
      y0 = event.screenY;
  function continuePan(event) {
    var x = event.screenX,
        y = event.screenY;
    fc.relativePan({ x: x - x0, y: y - y0 });
    x0 = x;
    y0 = y;
  }
  function stopPan(event) {
    $(window).off('mousemove', continuePan);
    $(window).off('mouseup', stopPan);
  };
  $(window).mousemove(continuePan);
  $(window).mouseup(stopPan);
  $(window).contextmenu(cancelMenu);
};
function cancelMenu() {
  $(window).off('contextmenu', cancelMenu);
  return false;
}
$(canvasWrapper).mousedown(startPan);

canvas.toggleDragMode(true); // Start panning
canvas.toggleDragMode(false); // Stop panning

const STATE_IDLE = 'idle';
const STATE_PANNING = 'panning';
fabric.Canvas.prototype.toggleDragMode = function(dragMode) {
  // Remember the previous X and Y coordinates for delta calculations
  let lastClientX;
  let lastClientY;
  // Keep track of the state
  let state = STATE_IDLE;
  // We're entering dragmode
  if (dragMode) {
    // Discard any active object
    this.discardActiveObject();
    // Set the cursor to 'move'
    this.defaultCursor = 'move';
    // Loop over all objects and disable events / selectable. We remember its value in a temp variable stored on each object
    this.forEachObject(function(object) {
      object.prevEvented = object.evented;
      object.prevSelectable = object.selectable;
      object.evented = false;
      object.selectable = false;
    });
    // Remove selection ability on the canvas
    this.selection = false;
    // When MouseUp fires, we set the state to idle
    this.on('mouse:up', function(e) {
      state = STATE_IDLE;
    });
    // When MouseDown fires, we set the state to panning
    this.on('mouse:down', (e) => {
      state = STATE_PANNING;
      lastClientX = e.e.clientX;
      lastClientY = e.e.clientY;
    });
    // When the mouse moves, and we're panning (mouse down), we continue
    this.on('mouse:move', (e) => {
      if (state === STATE_PANNING && e && e.e) {
        // let delta = new fabric.Point(e.e.movementX, e.e.movementY); // No Safari support for movementX and movementY
        // For cross-browser compatibility, I had to manually keep track of the delta

        // Calculate deltas
        let deltaX = 0;
        let deltaY = 0;
        if (lastClientX) {
          deltaX = e.e.clientX - lastClientX;
        }
        if (lastClientY) {
          deltaY = e.e.clientY - lastClientY;
        }
        // Update the last X and Y values
        lastClientX = e.e.clientX;
        lastClientY = e.e.clientY;

        let delta = new fabric.Point(deltaX, deltaY);
        this.relativePan(delta);
        this.trigger('moved');
      }
    });
  } else {
    // When we exit dragmode, we restore the previous values on all objects
    this.forEachObject(function(object) {
      object.evented = (object.prevEvented !== undefined) ? object.prevEvented : object.evented;
      object.selectable = (object.prevSelectable !== undefined) ? object.prevSelectable : object.selectable;
    });
    // Reset the cursor
    this.defaultCursor = 'default';
    // Remove the event listeners
    this.off('mouse:up');
    this.off('mouse:down');
    this.off('mouse:move');
    // Restore selection ability on the canvas
    this.selection = true;
  }
};

// Create the canvas

let canvas = new fabric.Canvas('fabric')
canvas.backgroundColor = '#f1f1f1';

// Add a couple of rects

let rect = new fabric.Rect({
  width: 100,
  height: 100,
  fill: '#f00'
});
canvas.add(rect)

rect = new fabric.Rect({
  width: 200,
  height: 200,
  top: 200,
  left: 200,
  fill: '#f00'
});
canvas.add(rect)

// Handle dragmode change

let dragMode = false;
$('#dragmode').change(_ => {
  dragMode = !dragMode;
  canvas.toggleDragMode(dragMode);
});

<div>
  <label for="dragmode">
    Enable panning
    <input type="checkbox" id="dragmode" name="dragmode" />
  </label>
</div>
<canvas width="300" height="300" id="fabric"></canvas>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.7.15/fabric.min.js"></script>