2021-03-04flutter图片适配

enum BoxFit {
  /// Fill the target box by distorting the source's aspect ratio.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fill.png)
  fill,

  /// As large as possible while still containing the source entirely within the
  /// target box.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_contain.png)
  contain,

  /// As small as possible while still covering the entire target box.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_cover.png)
  cover,

  /// Make sure the full width of the source is shown, regardless of
  /// whether this means the source overflows the target box vertically.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fitWidth.png)
  fitWidth,

  /// Make sure the full height of the source is shown, regardless of
  /// whether this means the source overflows the target box horizontally.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fitHeight.png)
  fitHeight,

  /// Align the source within the target box (by default, centering) and discard
  /// any portions of the source that lie outside the box.
  ///
  /// The source image is not resized.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_none.png)
  none,

  /// Align the source within the target box (by default, centering) and, if
  /// necessary, scale the source down to ensure that the source fits within the
  /// box.
  ///
  /// This is the same as `contain` if that would shrink the image, otherwise it
  /// is the same as `none`.
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_scaleDown.png)
  scaleDown,
}