How can I rotate a Container widget in 2D around a

问题:

I'd like to perform a very simple 2D rotation of a Container widget (that contains a handful of other widgets.) This widget would be rotated around a single fixed point in the center, with no deformations.

I tried using the transform property with Matrix4.rotationZ, which somewhat works – but the anchor point is in the top-left corner, not in the center. Is there an easy way to specify that anchor point?

Furthermore, is there an easier way to 2D-rotate this widget that doesn't require Matrix4?

var container = new Container (
  child: new Stack (
    children: [
      new Image.asset ( // background photo
        "assets/texture.jpg",
        fit: ImageFit.cover,
      ),
      new Center (
        child: new Container (
          child: new Text (
            "Lorem ipsum",
            style: new TextStyle(
              color: Colors.white,
              fontSize: 42.0,
              fontWeight: FontWeight.w900
            )
          ),
          decoration: new BoxDecoration (
            backgroundColor: Colors.black,
          ),
          padding: new EdgeInsets.fromLTRB(16.0, 16.0, 16.0, 16.0),
          transform: new Matrix4.rotationZ(0.174533), // rotate -10 deg
        ),
      ),
    ],
  ),
  width: 400.0,
  height: 200.0,
);

回答1:

Apply a translation (to and from the fulcrum) before and after the rotation.

You can create yourself a little widget that does this, and thus solve your other problem at the same time (hiding the Matrix4).

回答2:

Per Ian's advice, I gave the following a try. It appears to work, at least in my limited testing.

The container is nested within a Transform widget. Using alignment allows one to adjust the transform-origin in relative terms, i.e., in the center, the top left, etc.

var container = new Container (
  child: new Stack (
    children: [
      new Image.asset ( // background photo
        "assets/texture.jpg",
        fit: ImageFit.cover,
      ),
      new Center (
        child: new Transform (
          child: new Container (
            child: new Text (
              "Lorem ipsum",
              style: new TextStyle(
                color: Colors.white,
                fontSize: 42.0,
                fontWeight: FontWeight.w900,
              )
            ),
            decoration: new BoxDecoration (
              backgroundColor: Colors.black,
            ),
            padding: new EdgeInsets.fromLTRB(16.0, 16.0, 16.0, 16.0),              
          ),
          alignment: FractionalOffset.center, // set transform origin
          transform: new Matrix4.rotationZ(0.174533), // rotate -10 deg
        ),
      ),
    ],
  ),
  width: 400.0,
  height: 200.0,
);