Fabric.js 2.7.0中fabric.curvedText兼容异常求助

Let's break down why you're seeing that selection offset issue and how to fix it:

Background #

When upgrading from Fabric.js 1.7.9 to 2.7.0, the old fabric.curvedText.js library misses critical changes to how Fabric handles object bounding boxes, coordinate calculations, and selection state. Your fixes for delegatedProperties and createAccessors resolved the initial creation errors, but they don't address the core alignment issues for selection handles.

Root Cause #

Fabric 2.x completely revamped how objects calculate their bounding boxes and position selection controls. The original curvedText implementation was built for 1.x's coordinate logic, so when you select the text, Fabric uses outdated bounding box data to place the handles—hence the offset.

Step-by-Step Fixes #

1. Add Curved Text-Specific Dimension Calculation

Fabric 2.x uses _getNonTransformedDimensions to calculate an object's base size (before scaling/rotation). Add this method to the fabric.CurvedText prototype to ensure accurate bounding box calculations:

fabric.CurvedText.prototype._getNonTransformedDimensions = function() {
    // Calculate total angle covered by the text
    const anglePerChar = this.spacing * Math.PI / 180;
    const totalAngle = anglePerChar * this.text.length;
    // Calculate width based on the arc's chord length
    const width = 2 * this.radius * Math.sin(totalAngle / 2);
    // Height accounts for radius + font size (adjust if needed for descenders/ascenders)
    const height = this.radius + this.fontSize * 1.2;
    
    return { x: width, y: height };
};

2. Fix Selection Handle Alignment with setCoords

The setCoords method updates the position of selection handles. Override it to align handles with the curved text's actual bounds:

fabric.CurvedText.prototype.setCoords = function() {
    // Call the base Text class's setCoords first
    fabric.Text.prototype.setCoords.call(this);
    
    const center = this.getCenterPoint();
    const { width, height } = this._getNonTransformedDimensions();
    
    // Update corner handles to match the calculated bounds
    this.oCoords.tl = { x: center.x - width/2, y: center.y - height/2 };
    this.oCoords.tr = { x: center.x + width/2, y: center.y - height/2 };
    this.oCoords.bl = { x: center.x - width/2, y: center.y + height/2 };
    this.oCoords.br = { x: center.x + width/2, y: center.y + height/2 };
    
    // Update midpoint handles
    this.oCoords.mt = { x: center.x, y: center.y - height/2 };
    this.oCoords.mb = { x: center.x, y: center.y + height/2 };
    this.oCoords.ml = { x: center.x - width/2, y: center.y };
    this.oCoords.mr = { x: center.x + width/2, y: center.y };
    
    return this;
};

3. Sync Custom Properties with State

Make sure curved text-specific properties (radius, spacing) are included in the object's state properties so Fabric tracks them correctly. Add this to your fabric.CurvedText class definition:

stateProperties: fabric.Text.prototype.stateProperties.concat('radius', 'spacing'),

4. Verify Render Alignment

Double-check the _render method in fabric.curvedText.js to ensure it uses Fabric 2.x's coordinate system. Ensure the text is rendered relative to the object's left/top or center point consistently. For example, if the original code uses hardcoded offsets, adjust them to use this.left and this.top properly.

Testing the Fix #

After applying these changes, create your curved text again:

let curvedTextObj = new fabric.CurvedText("Hello World", { 
    left: 100, 
    top: 100, 
    textAlign: 'center', 
    radius: 150, 
    fontSize: 20, 
    spacing: 20 
});
this.canvas.add(curvedTextObj);
this.canvas.renderAll();

The selection handles should now align perfectly with the curved text, and dragging/resizing should behave as expected.

内容的提问来源于stack exchange，提问作者Dipak Sumesara