Add download link support

[bdjnk]

Describe the bug

Generate an a tag with the download attribute set and the href attribute pointing to a blob: URL. For example the following.

<a download="hello.txt" href='#' id="link">Download</a>
<script>
const blob = new Blob(["Hello, world!"], {type: 'text/plain'});
link.href = URL.createObjectURL(blob);
</script>

(example source)

Clicking such a link in any modern browser will initiate download of the linked resource. In Tauri, nothing happens.

To Reproduce

1. Generate an a tag with the download attribute set and the href attribute pointing to a blob: URL.
2. Click on the link generated in step 1.
3. Observe nothing happens

Expected behavior

Download of the linked resource should initiate.

Platform and Versions (required):

Operating System - Arch Linux, version Unknown X64

Node.js environment
  Node.js - 16.5.0
  @tauri-apps/cli - 1.0.0-beta.6
  @tauri-apps/api - 1.0.0-beta.5

Global packages
  npm - 7.17.0
  yarn - 1.22.10

Rust environment
  rustc - 1.53.0
  cargo - 1.53.0

App directory structure
/node_modules
/public
/src
/.git
/src-tauri

App
  tauri.rs - 1.0.0-beta.5
  build-type - bundle
  CSP - default-src blob: data: filesystem: ws: http: https: 'unsafe-eval' 'unsafe-inline' 'self' img-src: 'self'
  distDir - ../public
  devPath - http://localhost:3001

Additional context

If a valid navigable URL (eg https://duckduckgo.com/) is used as the href of an a tag with the download attribute set, clicking the resultant link navigates to the given URL instead of downloading the content there.

[bdjnk]

@lemarier, can you explain why this is a good first issue? I'm struggling to figure how to approach it.

[lemarier]

@lemarier, can you explain why this is a good first issue? I'm struggling to figure how to approach it.

It's been tagged as a good first issue mainly because I think it's a good way to jump in the Tauri stack and play with all dependencies (webview, javascript, etc..) and it affects all operating systems.

But I do agree it's quite complex, especially on macOS.

There are some references for WKWebview;

• https://bugs.webkit.org/show_bug.cgi?id=216918#c4
• Download of blob url is not working with elem.click() mozilla-mobile/firefox-ios#8635
• https://developer.apple.com/documentation/webkit/wkdownload?language=objc
• https://www.gfrigerio.com/download-files-in-a-wkwebview/

[wusyong]

Looks like webkitgtk has API for it, but the rust crate doesn't provide any method other than constructor.
And webview2 only has experimental function for it.

• https://docs.rs/webkit2gtk/0.14.0/webkit2gtk/struct.Download.html
• https://github.com/MicrosoftEdge/WebView2Feedback/blob/master/specs/.md

[nothingismagick]

@wusyong - think we'd have time to pick this back up again?

[lucasfernog]

I believe this is handled by #530 @nothingismagick

[atlanticaccent]

With #530 merged I believe this issue has been addressed completely. Notably, it includes examples for handling "normal" downloads and blob downloads, which happen to be different enough to require separate solutions.

[lucasfernog]

Nice! Let's try to add this to Tauri v1.3.