Flutter Alignment FractionalOffset AlignmentDirectional

Alignment
 

const Alignment(this.x, this.y)
    : assert(x != null),
      assert(y != null);

参数x：-1：最左边 1:最右边 0：中间
参数y：-1：最上边 1:最下边 0：中间

比如：Alignment(0,0)代表控件的中心
坐标系如下图：

 

Container(
          width: 300,
          height: 100,
          color: Color(0xFFFF0000),
          alignment: Alignment(1,0),
          child: Text('Alignment(1,0)',
          textDirection: TextDirection.rtl,),
        )

效果：

 Alignment定义了我们常用的位置：

/// The top left corner.
  static const Alignment topLeft = Alignment(-1.0, -1.0);

  /// The center point along the top edge.
  static const Alignment topCenter = Alignment(0.0, -1.0);

  /// The top right corner.
  static const Alignment topRight = Alignment(1.0, -1.0);

  /// The center point along the left edge.
  static const Alignment centerLeft = Alignment(-1.0, 0.0);

  /// The center point, both horizontally and vertically.
  static const Alignment center = Alignment(0.0, 0.0);

  /// The center point along the right edge.
  static const Alignment centerRight = Alignment(1.0, 0.0);

  /// The bottom left corner.
  static const Alignment bottomLeft = Alignment(-1.0, 1.0);

  /// The center point along the bottom edge.
  static const Alignment bottomCenter = Alignment(0.0, 1.0);

  /// The bottom right corner.
  static const Alignment bottomRight = Alignment(1.0, 1.0);

FractionalOffset

FractionalOffset继承Alignment，他们2个区别就是坐标系不一样，Alignment的原点是中心，而FractionalOffset原点是左上角。

Container(
          width: 300,
          height: 100,
          color: Color(0xFFFF0000),
          alignment: FractionalOffset(0,0),
          child: Text('FractionalOffset(0,0)',
          textDirection: TextDirection.rtl,),
        ),

效果：

FractionalOffset定义了我们常用的位置：

/// The top left corner.
  static const FractionalOffset topLeft = FractionalOffset(0.0, 0.0);

  /// The center point along the top edge.
  static const FractionalOffset topCenter = FractionalOffset(0.5, 0.0);

  /// The top right corner.
  static const FractionalOffset topRight = FractionalOffset(1.0, 0.0);

  /// The center point along the left edge.
  static const FractionalOffset centerLeft = FractionalOffset(0.0, 0.5);

  /// The center point, both horizontally and vertically.
  static const FractionalOffset center = FractionalOffset(0.5, 0.5);

  /// The center point along the right edge.
  static const FractionalOffset centerRight = FractionalOffset(1.0, 0.5);

  /// The bottom left corner.
  static const FractionalOffset bottomLeft = FractionalOffset(0.0, 1.0);

  /// The center point along the bottom edge.
  static const FractionalOffset bottomCenter = FractionalOffset(0.5, 1.0);

  /// The bottom right corner.
  static const FractionalOffset bottomRight = FractionalOffset(1.0, 1.0);

AlignmentDirectional

AlignmentDirectional 的坐标系和Alignment比较像，原点在中心，不过AlignmentDirectional的起始位置和书写（TextDirection）方向有关

 

 

demo：

Container(
      width: 300,
      height: 100,
      color: Color.fromARGB(255, 66, 165, 245),
      child: new Text(
        "Flutter Cheatsheet",
        textDirection: TextDirection.ltr,
        style: TextStyle(

        ),
      ),
      alignment: AlignmentDirectional.topStart,
    );

二真机上的效果：