flutter中的几种图片加载方式

图片加载对应的有本地和网络之别，其中囊括了原生和插件，现在介绍点常用的：

• 本地图片加载：new Image.asset("images/tupian.png")
  全路径填写，并在pubspec.yaml文件中注册

• 网络图片加载：new Image.network( “https://cssccs”,fit: BoxFit.fill,width: 70,)
  可以加载GIF

• 网络图片加载：FadeInImage.memoryNetwork有默认占位图和淡入效果
  https://pub.dartlang.org/packages/transparent_image

FadeInImage.memoryNetwork(
            image: 'https://xxx/lake.jpg',
            placeholder: kTransparentImage/* 透明图片 */,）

• 网络图片加载：FadeInImage.assetNetwork 用本地图做占位图

FadeInImage.assetNetwork(
        image: 'https://xxx/lake.jpg',
        placeholder: 'images/birds.gif' /* 指定gif */,)),
    );

• 图片缓存：cached_network_image
  https://pub.dartlang.org/packages/cached_network_image

new CachedNetworkImage(
        fit: BoxFit.fill,
        width: 30,
        height:40,
        imageUrl:  avatar,
        placeholder: (context, url) => new ProgressView(),
        errorWidget: (context, url, error) => new Icon(Icons.error),
      );

共同拥有的相关部分属性;

1. alignment → AlignmentGeometry
   How to align the image within its bounds

2. centerSlice → Rect
   The center slice for a nine-patch image

3. color → Color
   If non-null, this color is blended with each image pixel using colorBlendMode.

4. colorBlendMode → BlendMode
   Used to combine（组合） color with this image

5. excludeFromSemantics → bool
   Whether to exclude this image from semantics.

6. filterQuality → FilterQuality
   Used to set the FilterQuality of the image

7. fit → BoxFit
   How to inscribe the image into the space allocated during layout

8. matchTextDirection → bool
   Whether to paint the image in the direction of the TextDirection.

9. gaplessPlayback → bool
   Whether to continue showing the old image (true), or briefly show nothing (false), when the image provider changes

10. image → ImageProvider
    The image to display.

11. repeat → ImageRepeat
    . How to paint any portions(边缘) of the layout bounds not covered by the image.

12. semanticLabel → String
    A Semantic description of the image.

13. width、height → double