占位符和网络图片淡入

从内存加载占位符
In-Memory

当使用默认的 Image widget 显示图片时，你可能会注意到图片只是在加载完后直接显示到屏幕上，用户可能会觉得这看起来不舒服。

When displaying images using the default Image widget, you might notice they simply pop onto the screen as they’re loaded. This might feel visually jarring to your users.

此外，如果可以先展示占位符，待图片加载完成后淡入显示图片不是很酷么？可以使用 Flutter 自带的 FadeInImage widget 来实现这个功能。

Instead, wouldn’t it be nice to display a placeholder at first, and images would fade in as they’re loaded? Use the FadeInImage widget for exactly this purpose.

FadeInImage 适用于任何类型的图片：内存中的，本地存储的，抑或是网络上的。

FadeInImage works with images of any type: in-memory, local assets, or images from the internet.

从内存加载占位符

In-Memory

本例将使用 transparent_image 包来实现一个简单的透明占位符。

In this example, use the transparent_image package for a simple transparent placeholder.

FadeInImage.memoryNetwork(
  placeholder: kTransparentImage,
  image: 'https://picsum.photos/250?image=9',
);

完整样例

Complete example

import 'package:flutter/material.dart';
import 'package:transparent_image/transparent_image.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final title = 'Fade in images';

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Stack(
          children: <Widget>[
            Center(child: CircularProgressIndicator()),
            Center(
              child: FadeInImage.memoryNetwork(
                placeholder: kTransparentImage,
                image: 'https://picsum.photos/250?image=9',
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Fading In Image Demo

从本地存储加载占位符

From asset bundle

也可以考虑使用本地资源作为占位符。首先将资源添加到项目的 pubspec.yaml 文件中（更多细节请参阅文档: 添加资源和图片）：

You can also consider using local assets for placeholders. First, add the asset to the project’s pubspec.yaml file (for more details, see Adding assets and images):

 flutter:
   assets:
+    - assets/loading.gif

然后使用 FadeInImage.assetNetwork() 构造函数：

Then, use the FadeInImage.assetNetwork() constructor:

FadeInImage.assetNetwork(
  placeholder: 'assets/loading.gif',
  image: 'https://picsum.photos/250?image=9',
);

完整样例

Complete example

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final title = 'Fade in images';

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: FadeInImage.assetNetwork(
            placeholder: 'assets/loading.gif',
            image: 'https://picsum.photos/250?image=9',
          ),
        ),
      ),
    );
  }
}

Asset fade-in