将URL中的图像保存为Blob对象。

huangapple
go评论186阅读模式
英文:

Save an image from url to blob

问题

我可以将一个来自URL的图像下载到文件中，如这里所示：

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"net/http"
  6. 	"net/url"
  7. 	"os"
  8. 	"strings"
  9. )
  11. var (
  12. 	fileName    string
  13. 	fullUrlFile string
  14. )
  16. func main() {
  17. 	fullUrlFile = "https://i.imgur.com/m1UIjW1.jpg"
  18. 	r, e := http.Get(fullUrlFile)
  19. 	if e != nil {
  20. 		panic(e)
  21. 	}
  22. 	defer r.Body.Close()
  23. 	buildFileName()
  24. 	// 创建目标文件
  25. 	f, e := os.Create(fileName) // "m1UIjW1.jpg";
  26. 	if e != nil {
  27. 		panic(e)
  28. 	}
  29. 	defer f.Close()
  30. 	// 将内容填充到目标文件中
  31. 	n, e := f.ReadFrom(r.Body)
  32. 	if e != nil {
  33. 		panic(e)
  34. 	}
  35. 	fmt.Println("文件大小：", n)
  36. }
  38. func buildFileName() {
  39. 	fileUrl, e := url.Parse(fullUrlFile)
  40. 	if e != nil {
  41. 		panic(e)
  42. 	}
  44. 	path := fileUrl.Path
  45. 	segments := strings.Split(path, "/")
  47. 	fileName = segments[len(segments)-1]
  48. 	println(fileName)
  49. }

但是在我的应用程序中，我希望直接将图像保存为blob到indexedDB中，在JavaScript中可以这样做：

  1. // 获取blob
  2. // 创建XHR
  3. var xhr = new XMLHttpRequest(),
  4.     blob;
  6. xhr.open("GET", "elephant.png", true);
  7. // 将responseType设置为blob
  8. xhr.responseType = "blob";
  10. xhr.addEventListener("load", function () {
  11.     if (xhr.status === 200) {
  12.         console.log("图像已检索");
  13.         
  14.         // 作为响应的文件
  15.         blob = xhr.response;
  17.         // 将接收到的blob放入IndexedDB
  18.         transaction.objectStore("elephants").put(blob, "image");
  19.     }
  20. }, false);
  21. // 发送XHR
  22. xhr.send();

我理解blob只是一个[]byte，所以我尝试将图像下载代码中从r.Body读取的内容转换为[]byte，如下所示：

  1. 	buf := new(bytes.Buffer)
  2. 	buf.ReadFrom(r.Body)
  3.     blob := bytes.NewReader(buf.Bytes())
  5.     dest, e := os.Create("test.jpg")
  6. 	n2, e := dest.ReadFrom(blob)
  7. 	fmt.Printf("字节数为：%d\n", n2)

但是它没有起作用！

问题是：我如何将从io.Reader读取的[]bytes转换为另一个可以保存在另一个目标中的io.Reader。

英文:

I can download an image from url to file as shown here:

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"net/http"
  6. 	"net/url"
  7. 	"os"
  8. 	"strings"
  9. )
  11. var (
  12. 	fileName    string
  13. 	fullUrlFile string
  14. )
  16. func main() {
  17. 	fullUrlFile = "https://i.imgur.com/m1UIjW1.jpg"
  18. 	r, e := http.Get(fullUrlFile)
  19. 	if e != nil {
  20. 		panic(e)
  21. 	}
  22. 	defer r.Body.Close()
  23. 	buildFileName()
  24. 	// Create distination
  25. 	f, e := os.Create(fileName) // "m1UIjW1.jpg"
  26. 	if e != nil {
  27. 		panic(e)
  28. 	}
  29. 	defer f.Close()
  30. 	// Fill distination with content
  31. 	n, e := f.ReadFrom(r.Body)
  32. 	if e != nil {
  33. 		panic(e)
  34. 	}
  35. 	fmt.Println("File size: ", n)
  36. }
  38. func buildFileName() {
  39. 	fileUrl, e := url.Parse(fullUrlFile)
  40. 	if e != nil {
  41. 		panic(e)
  42. 	}
  44. 	path := fileUrl.Path
  45. 	segments := strings.Split(path, "/")
  47. 	fileName = segments[len(segments)-1]
  48. 	println(fileName)
  49. }

But i my application, I want to save the image as blob directly from uel to indexedDB, in JavaScript this can be done as:

  1. // Get the blob
  2. // Create XHR
  3. var xhr = new XMLHttpRequest(),
  4.     blob;
  6. xhr.open("GET", "elephant.png", true);
  7. // Set the responseType to blob
  8. xhr.responseType = "blob";
  10. xhr.addEventListener("load", function () {
  11.     if (xhr.status === 200) {
  12.         console.log("Image retrieved");
  13.         
  14.         // File as response
  15.         blob = xhr.response;
  17.         // Put the received blob into IndexedDB
  18.         transaction.objectStore("elephants").put(blob, "image");
  19.     }
  20. }, false);
  21. // Send XHR
  22. xhr.send();

My understanding the the blob is nothing but a []byte, so I tried converting the r.Body read in the image downloading code to []byte as:

  1. 	buf := new(bytes.Buffer)
  2. 	buf.ReadFrom(r.Body)
  3.     blob := bytes.NewReader(buf.Bytes())
  5.     dest, e := os.Create("test.jpg")
  6. 	n2, e := dest.ReadFrom(blob)
  7. 	fmt.Printf("The number of bytes are: %d\n", n2)

But it did not work!

The question is: How can I convert []bytes read from an io.Reader to another io.Reader that I can save in another destination.

答案1

得分: 1

我找到了，它非常简单，可以使用以下两种方法之一：

方法一：

  1. 	var dest []byte
  2. 	blob := bytes.NewBuffer(dest)
  3. 	io.Copy(blob, r.Body)

方法二：

  1. 	var dest []byte
  2. 	blob := bytes.NewBuffer(dest)
  3. 	b, e := ioutil.ReadAll(r.Body)
  4. 	if e != nil {
  5. 		panic(e)
  6. 	}
  7. 	src := bytes.NewReader(b)
  8. 	io.Copy(blob, src)

在这两种情况下，一旦r.Body或者基于它创建的任何io.Writer被使用一次后，就会完全消耗掉。

现在我的代码变成了：

  1. package main
  3. import (
  4. 	"bytes"
  5. 	"fmt"
  6. 	"io"
  7. 	"net/http"
  8. 	"net/url"
  9. 	"os"
  10. 	"strings"
  11. )
  13. var (
  14. 	fileName    string
  15. 	fullUrlFile string
  16. )
  18. func main() {
  19. 	fullUrlFile = "https://i.imgur.com/m1UIjW1.jpg"
  20. 	r, e := http.Get(fullUrlFile)
  21. 	if e != nil {
  22. 		panic(e)
  23. 	}
  24. 	defer r.Body.Close()
  25. 	buildFileName()
  26. 	// 创建 blob
  27. 	var dest []byte
  28. 	blob := bytes.NewBuffer(dest)
  29. 	io.Copy(blob, r.Body)
  31.     add_to_database(blob)
  32. }
  34. func buildFileName() {
  35. 	fileUrl, e := url.Parse(fullUrlFile)
  36. 	if e != nil {
  37. 		panic(e)
  38. 	}
  40. 	path := fileUrl.Path
  41. 	segments := strings.Split(path, "/")
  43. 	fileName = segments[len(segments)-1]
  44. 	println(fileName)
  45. }
  47. func add_to_database(blob *bytes.Buffer){}

请注意，我只翻译了代码部分，其他内容不包括在内。

英文:

I found it, it was very simple, either using:

  1. 	var dest []byte
  2. 	blob := bytes.NewBuffer(dest)
  3. 	io.Copy(blob, r.Body)

or using:

  1. 	var dest []byte
  2. 	blob := bytes.NewBuffer(dest)
  3. 	b, e := ioutil.ReadAll(r.Body)
  4. 	if e != nil {
  5. 		panic(e)
  6. 	}
  7. 	src := bytes.NewReader(b)
  8. 	io.Copy(blob, src)

In both cases, once r.Body or any io.Writer created based on it, is valid for one time usage, after that it is consumed in full.

my code now became:

  1. package main
  3. import (
  4. 	"bytes"
  5. 	"fmt"
  6. 	"io"
  7. 	"net/http"
  8. 	"net/url"
  9. 	"os"
  10. 	"strings"
  11. )
  13. var (
  14. 	fileName    string
  15. 	fullUrlFile string
  16. )
  18. func main() {
  19. 	fullUrlFile = "https://i.imgur.com/m1UIjW1.jpg"
  20. 	r, e := http.Get(fullUrlFile)
  21. 	if e != nil {
  22. 		panic(e)
  23. 	}
  24. 	defer r.Body.Close()
  25. 	buildFileName()
  26. 	// Create blob
  27. 	var dest []byte
  28. 	blob := bytes.NewBuffer(dest)
  29. 	io.Copy(blob, r.Body)
  31.     add_to_database(blob)
  32. }
  33. func buildFileName() {
  34. 	fileUrl, e := url.Parse(fullUrlFile)
  35. 	if e != nil {
  36. 		panic(e)
  37. 	}
  39. 	path := fileUrl.Path
  40. 	segments := strings.Split(path, "/")
  42. 	fileName = segments[len(segments)-1]
  43. 	println(fileName)
  44. }
  46. func add_to_database(blob *bytes.Buffer){}

huangapple
  • 本文由 发表于 2021年11月18日 13:37:16
  • 转载请务必保留本文链接：https://go.coder-hub.com/70015043.html
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