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<li><em>KusonStack一站式可编程配置技术栈(Go): <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
<li><em>KCL 配置编程语言(Rust): <a href="https://github.com/KusionStack/KCLVM">https://github.com/KusionStack/KCLVM</a></em></li>
<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="105-包的匿名导入"><a class="header" href="#105-包的匿名导入">10.5. 包的匿名导入</a></h2>
<p>如果只是导入一个包而并不使用导入的包将会导致一个编译错误。但是有时候我们只是想利用导入包而产生的副作用它会计算包级变量的初始化表达式和执行导入包的init初始化函数§2.6.2。这时候我们需要抑制“unused import”编译错误我们可以用下划线<code>_</code>来重命名导入的包。像往常一样,下划线<code>_</code>为空白标识符,并不能被访问。</p>
<pre><code class="language-Go">import _ &quot;image/png&quot; // register PNG decoder
</code></pre>
<p>这个被称为包的匿名导入。它通常是用来实现一个编译时机制然后通过在main主程序入口选择性地导入附加的包。首先让我们看看如何使用该特性然后再看看它是如何工作的。</p>
<p>标准库的image图像包包含了一个<code>Decode</code>函数,用于从<code>io.Reader</code>接口读取数据并解码图像它调用底层注册的图像解码器来完成任务然后返回image.Image类型的图像。使用<code>image.Decode</code>很容易编写一个图像格式的转换工具,读取一种格式的图像,然后编码为另一种图像格式:</p>
<p><u><i>gopl.io/ch10/jpeg</i></u></p>
<pre><code class="language-Go">// The jpeg command reads a PNG image from the standard input
// and writes it as a JPEG image to the standard output.
package main
import (
&quot;fmt&quot;
&quot;image&quot;
&quot;image/jpeg&quot;
_ &quot;image/png&quot; // register PNG decoder
&quot;io&quot;
&quot;os&quot;
)
func main() {
if err := toJPEG(os.Stdin, os.Stdout); err != nil {
fmt.Fprintf(os.Stderr, &quot;jpeg: %v\n&quot;, err)
os.Exit(1)
}
}
func toJPEG(in io.Reader, out io.Writer) error {
img, kind, err := image.Decode(in)
if err != nil {
return err
}
fmt.Fprintln(os.Stderr, &quot;Input format =&quot;, kind)
return jpeg.Encode(out, img, &amp;jpeg.Options{Quality: 95})
}
</code></pre>
<p>如果我们将<code>gopl.io/ch3/mandelbrot</code>§3.3的输出导入到这个程序的标准输入它将解码输入的PNG格式图像然后转换为JPEG格式的图像输出图3.3)。</p>
<pre><code>$ go build gopl.io/ch3/mandelbrot
$ go build gopl.io/ch10/jpeg
$ ./mandelbrot | ./jpeg &gt;mandelbrot.jpg
Input format = png
</code></pre>
<p>要注意image/png包的匿名导入语句。如果没有这一行语句程序依然可以编译和运行但是它将不能正确识别和解码PNG格式的图像</p>
<pre><code>$ go build gopl.io/ch10/jpeg
$ ./mandelbrot | ./jpeg &gt;mandelbrot.jpg
jpeg: image: unknown format
</code></pre>
<p>下面的代码演示了它的工作机制。标准库还提供了GIF、PNG和JPEG等格式图像的解码器用户也可以提供自己的解码器但是为了保持程序体积较小很多解码器并没有被全部包含除非是明确需要支持的格式。image.Decode函数在解码时会依次查询支持的格式列表。每个格式驱动列表的每个入口指定了四件事情格式的名称一个用于描述这种图像数据开头部分模式的字符串用于解码器检测识别一个Decode函数用于完成解码图像工作一个DecodeConfig函数用于解码图像的大小和颜色空间的信息。每个驱动入口是通过调用image.RegisterFormat函数注册一般是在每个格式包的init初始化函数中调用例如image/png包是这样注册的</p>
<pre><code class="language-Go">package png // image/png
func Decode(r io.Reader) (image.Image, error)
func DecodeConfig(r io.Reader) (image.Config, error)
func init() {
const pngHeader = &quot;\x89PNG\r\n\x1a\n&quot;
image.RegisterFormat(&quot;png&quot;, pngHeader, Decode, DecodeConfig)
}
</code></pre>
<p>最终的效果是主程序只需要匿名导入特定图像驱动包就可以用image.Decode解码对应格式的图像了。</p>
<p>数据库包database/sql也是采用了类似的技术让用户可以根据自己需要选择导入必要的数据库驱动。例如</p>
<pre><code class="language-Go">import (
&quot;database/sql&quot;
_ &quot;github.com/lib/pq&quot; // enable support for Postgres
_ &quot;github.com/go-sql-driver/mysql&quot; // enable support for MySQL
)
db, err = sql.Open(&quot;postgres&quot;, dbname) // OK
db, err = sql.Open(&quot;mysql&quot;, dbname) // OK
db, err = sql.Open(&quot;sqlite3&quot;, dbname) // returns error: unknown driver &quot;sqlite3&quot;
</code></pre>
<p><strong>练习 10.1</strong> 扩展jpeg程序以支持任意图像格式之间的相互转换使用image.Decode检测支持的格式类型然后通过flag命令行标志参数选择输出的格式。</p>
<p><strong>练习 10.2</strong> 设计一个通用的压缩文件读取框架用来读取ZIParchive/zip和POSIX tararchive/tar格式压缩的文档。使用类似上面的注册技术来扩展支持不同的压缩格式然后根据需要通过匿名导入选择导入要支持的压缩格式的驱动包。</p>
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