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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>
Web Share Target API
</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c-common" class=
"remove"></script>
<script class='remove'>
var respecConfig = {
specStatus: "CG-DRAFT",
edDraftURI: "https://wicg.github.io/web-share-target/",
github: {
repoURL: "https://github.com/WICG/web-share-target/",
branch: "master"
},
shortName: "web-share-target",
processVersion: 2017,
wg: "Web Incubator Community Group",
wgURI: "https://wicg.io",
editors: [{
name: "Matt Giuca",
company: "Google Inc.",
companyURL: "https://google.com"
},
{
name: "Eric Willigers",
company: "Google Inc.",
companyURL: "https://google.com"
}],
otherLinks: [{
key: "Implementation status",
data: [{
value: "Chromium",
href: "https://www.chromestatus.com/features/5662315307335680"
}]
}],
localBiblio: {
"WebShare": {
id: "WebShare",
title: "Web Share API",
href: "https://wicg.github.io/web-share",
authors: [
"Matt Giuca",
],
status: "Draft Report",
publisher: "WICG"
}
}
};
</script>
</head>
<body>
<section id="abstract">
<p>
This specification defines an API that allows websites to declare
themselves as <a>web share targets</a>, which can receive shared
content from either the <a data-cite="!WebShare">Web Share API</a>, or
system events (e.g., shares from native apps).
</p>
<p>
This is a similar mechanism to <a data-cite=
"HTML#dom-navigator-registerprotocolhandler"><code>navigator.registerProtocolHandler</code></a>,
in that it works by registering the website with the user agent, to
later be <a>invoked</a> from another site or native application via the
user agent (possibly at the discretion of the user). The difference is
that <a data-cite=
"HTML#dom-navigator-registerprotocolhandler"><code>registerProtocolHandler</code></a>
registers the handler via a programmatic API, whereas a Web Share
Target is declared in the <a data-cite="!appmanifest">Web App
Manifest</a>, to be registered at a time of the user agent or user's
choosing.
</p>
</section>
<section id="sotd">
<p>
This is an early draft of the Web Share Target spec.
</p>
</section>
<section>
<h2>
Prerequisites
</h2>
<p>
In order to implement this API, it is REQUIRED that the user agent
implements <a data-cite="!appmanifest">Web App Manifest</a>. This spec
also re-uses some definitions from the <a data-cite="!WebShare">Web
Share API</a> spec. Implementation of that spec is NOT REQUIRED to
implement this one (but it is RECOMMENDED).
</p>
</section>
<section class="informative" data-link-for="WebAppManifest">
<h2>
Usage Example
</h2>
<p>
To register a site as a share target, a <a>share_target</a> entry is
added to the <a data-cite="appmanifest">Web App Manifest</a>, as shown:
</p>
<pre class="example json" title="manifest.webmanifest">
{
"name": "Includinator",
"share_target": {
"action": "share.html",
"params": {
"title": "name",
"text": "description",
"url": "link"
}
}
}
</pre>
<p>
The <a data-link-for="ShareTarget">params</a> keys correspond to the
key names in <a data-cite="WebShare#dom-sharedata">ShareData</a> from
[[!WebShare]], while the values are arbitrary names that will be used
as query parameters when the target is launched.
</p>
<p>
When a share takes place, if the user selects this share target, the
user agent opens a new browsing context at the <code>action</code> URL,
with query parameter values containing the shared data, just like an
HTML form submission.
</p>
<p>
For the purpose of this example, we assume the manifest is located at
<code>https://example.org/includinator/manifest.webmanifest</code>.
</p>
<pre class="example html" title="share.html">
<html>
<head>
<link rel="manifest" href="manifest.webmanifest">
</head>
<body>
<script>
window.addEventListener('load', () => {
var parsedUrl = new URL(window.location);
console.log('Title shared: ' + parsedUrl.searchParams.get('name'));
console.log('Text shared: ' + parsedUrl.searchParams.get('description'));
console.log('URL shared: ' + parsedUrl.searchParams.get('link'));
});
</script>
</body>
</html>
</pre>
<p>
If an incoming share contains the title "My News" and the URL
<code>http://example.com/news</code>, the user agent will open a new
window or tab and navigate to:
</p>
<pre>
https://example.org/includinator/share.html?name=My%20News&link=http%3A%2F%2Fexample.com%2Fnews
</pre>
<p>
The query parameters are populated with information from the
<a data-cite="WebShare#dom-sharedata"><code>ShareData</code></a> being
shared. If the ShareData contains no information for a given member,
the query parameter is omitted.
</p>
<p>
How the handler deals with the shared data is at the handler's
discretion, and will generally depend on the type of app. Here are some
suggestions:
</p>
<ul>
<li>An email client might draft a new email, using <code>title</code>
as the subject of an email, with <code>text</code> and <code>url</code>
concatenated together as the body.
</li>
<li>A social networking app might draft a new post, ignoring
<code>title</code>, using <code>text</code> as the body of the message
and adding <code>url</code> as a link. If <code>text</code> is missing,
it might use <code>url</code> in the body as well. If <code>url</code>
is missing, it might scan <code>text</code> looking for a URL and add
that as a link.
</li>
<li>A text messaging app might draft a new message, ignoring
<code>title</code> and using <code>text</code> and <code>url</code>
concatenated together. It might truncate the text or replace
<code>url</code> with a short link to fit into the message size.
</li>
</ul>
</section>
<section data-link-for="WebAppManifest">
<h2>
Extension to the Web App Manifest
</h2>
<p>
The following IDL extends the <dfn data-cite=
"!appmanifest#dom-webappmanifest">WebAppManifest</dfn> dictionary.
</p>
<pre class="idl">
dictionary ShareTargetParams {
USVString title;
USVString text;
USVString url;
};
dictionary ShareTarget {
required USVString action;
DOMString method = "GET";
DOMString enctype = "application/x-www-form-urlencoded";
required ShareTargetParams params;
};
partial dictionary WebAppManifest {
ShareTarget share_target;
};
</pre>
<p>
The following steps are added to the <a data-cite=
"!appmanifest#dfn-extension-point">extension point</a> in the steps for
<a data-cite="!appmanifest#dfn-processing-a-manifest">processing a
manifest</a>:
</p>
<ol>
<li>Set <var>manifest</var>["<a>share_target</a>"] to the result of
running <a>post-processing the <code>share_target</code> member</a>
given <var>manifest</var>["<a>share_target</a>"],
<var>manifest</var>["<a data-cite=
"!appmanifest#dom-webappmanifest-scope"><code>scope</code></a>"], and
<var>manifest URL</var>.
</li>
</ol>
<section data-dfn-for="WebAppManifest">
<h3>
<code>share_target</code> member
</h3>
<p>
The <dfn>share_target</dfn> member of the manifest is a
<a>ShareTarget</a> dictionary that declares this application to be a
<a>web share target</a>, and describes how the application receives
share data.
</p>
<p>
A <dfn data-lt="web share targets">web share target</dfn> is a web
site in a <a data-cite="!SECURE-CONTEXTS#secure-context">secure
context</a> with a valid manifest containing a <a>share_target</a>
member. A web share target is a type of <a data-cite=
"!WebShare#dfn-share-target">share target</a>.
</p>
<div class="issue" data-number="27"></div>
<p>
The steps for <dfn>post-processing the <code>share_target</code>
member</dfn> is given by the following algorithm. The algorithm takes
a <a>ShareTarget</a> <var>share target</var>, a <a data-cite=
"!URL#concept-url">URL</a> <var>scope URL</var>, and a <a data-cite=
"!URL#concept-url">URL</a> <var>manifest URL</var>. This algorithm
returns a <a>ShareTarget</a> or <code>undefined</code>.
</p>
<ol>
<li>If <var>share target</var> is <code>undefined</code>, then return
<code>undefined</code>.
</li>
<li>If <var>share target</var>["<a data-link-for=
"ShareTarget">method</a>"] is neither an <a data-cite=
"!INFRA#ascii-case-insensitive">ASCII case-insensitive</a> match for
the strings <code>"GET"</code> nor <code>"POST"</code>,
<a data-cite="!appmanifest#dfn-issue-a-developer-warning">issue a
developer warning</a> that the method is not supported, and return
<code>undefined</code>.
</li>
<li>If <var>share target</var>["<a data-link-for=
"ShareTarget">method</a>"] is an <a data-cite=
"!INFRA#ascii-case-insensitive">ASCII case-insensitive</a> match for
the string <code>"POST"</code> and <var>share
target</var>["<a data-link-for="ShareTarget">enctype</a>"] is neither
an <a data-cite="!INFRA#ascii-case-insensitive">ASCII
case-insensitive</a> match for the strings
<code>"application/x-www-form-urlencoded"</code> nor
<code>"multipart/form-data"</code>, <a data-cite=
"!appmanifest#dfn-issue-a-developer-warning">issue a developer
warning</a> that the enctype is not supported, and return
<code>undefined</code>.
</li>
<li>Let <var>action</var> be the result of <a data-cite=
"!URL#concept-url-parser">parsing</a> the <a data-cite=
"!URL#concept-url">URL</a> <var>share
target</var>["<a data-link-for="ShareTarget">action</a>"], relative
to the <var>manifest URL</var> and with no encoding override. If
the result is failure, <a data-cite=
"!appmanifest#dfn-issue-a-developer-warning">issue a developer
warning</a> and return <code>undefined</code>.
</li>
<li>If <var>action</var> is not <a data-cite=
"!appmanifest#dfn-within-scope">within scope</a> of <var>scope
URL</var>, <a data-cite="!appmanifest#dfn-issue-a-developer-warning">
issue a developer warning</a> that <a data-link-for=
"ShareTarget">action</a> is outside of the <a data-cite=
"!appmanifest#dfn-navigation-scope">navigation scope</a> , and
return <code>undefined</code>.
</li>
<li>Set <var>share target</var>["<a data-link-for=
"ShareTarget">action</a>"] to <var>action</var>.
</li>
<li>Return <var>share target</var>.
</li>
</ol>
</section>
<section data-dfn-for="ShareTarget" data-link-for="ShareTarget">
<h3>
<code>ShareTarget</code> and its members
</h3>
<p>
The <dfn>ShareTarget</dfn> dictionary contains the following members.
</p>
<p>
The <dfn>action</dfn> member specifies the <a data-cite=
"!URL#concept-url">URL</a> for the <a data-link-for=
"web share targets">web share target</a>.
</p>
<p>
The <dfn>method</dfn> member specifies the HTTP request <a href=
"https://tools.ietf.org/html/rfc7231#section-4">method</a> for the
<a data-link-for="web share targets">web share target</a>.
</p>
<p>
The <dfn>enctype</dfn> member specifies how the share data is encoded
in the body of a <code>POST</code> request. It is ignored when
<a>method</a> is <code>"GET"</code>.
</p>
<p>
The <dfn>params</dfn> member contains a <a>ShareTargetParams</a>
dictionary.
</p>
</section>
<section data-dfn-for="ShareTargetParams" data-link-for=
"ShareTargetParams">
<h3>
<code>ShareTargetParams</code> and its members
</h3>
<p>
The <dfn>ShareTargetParams</dfn> dictionary contains the following
members.
</p>
<p>
The <dfn>title</dfn> member specifies the name of the query parameter
used for the title of the document being shared.
</p>
<p>
The <dfn>text</dfn> member specifies the name of the query parameter
used for the arbitrary text that forms the body of the message being
shared.
</p>
<p>
The <dfn>url</dfn> member specifies the name of the query parameter
used for the URL string referring to a resource being shared.
</p>
</section>
</section>
<section>
<h2>
Registration of web share targets
</h2>
<p>
How and when <a>web share targets</a> are "registered" is at the
discretion of the user agent and/or the end user. In fact,
"registration" is a user-agent-specific concept that is not formally
defined here; user agents are NOT REQUIRED to "register" web share
targets at all; they are only REQUIRED to provide some mechanism to
convey shared data to a web share target of the end user's choosing.
User agents MAY consider a web share target "registered" even if it is
not <a data-cite="!appmanifest#dfn-install">installed</a>.
</p>
<p>
The user agent MAY automatically register all <a>web share targets</a>
as the user visits the site, but it is RECOMMENDED that more discretion
is applied, to avoid overwhelming the user with the choice of a large
number of targets.
</p>
<div class="note">
<p>
Examples of registration strategies that user agents can employ are:
</p>
<ul>
<li>Only register a web share target once it is <a data-cite=
"appmanifest#dfn-install">installed</a>.
</li>
<li>Only register a web share target once the end user has used the
site for some period of time.
</li>
<li>Explicitly prompt the user to register a web share target
(perhaps using the same UI as <a data-cite=
"HTML#dom-navigator-registerprotocolhandler"><code>navigator.registerProtocolHandler</code></a>).
</li>
<li>Only register a web share target if it has a service worker.
</li>
</ul>
</div>
<p>
When presenting the end user with a list of <a>web share targets</a>,
the user agent MAY use an online service which has pre-indexed
manifests, and therefore show the user targets that they have never
visited or explicitly registered.
</p>
<div class="issue" data-number="26"></div>
</section>
<section>
<h2>
Handling incoming shares
</h2>
<p>
A <a>web share target</a> is <dfn data-lt=
"invoke|invocation">invoked</dfn> when the end user is sharing some
data intended for a generic application, and indicates that specific
web share target as the receiver of the data.
</p>
<p>
It is not specified where the data comes from, or how the end user
indicates the web share target as the receiver. However, one possible
source is a call to <a data-cite=
"!WebShare#dom-navigator-share"><code>navigator.share</code></a> in the
same user agent.
</p>
<div class="note">
<p>
Examples of other possible sources of a <a>web share target</a>
invocation are:
</p>
<ul>
<li>From the built-in UI of the user agent (<i>e.g.</i>, the end user
picks "Share" from a browser's menu, to share the current page title
as "<code>title</code>" and the current page URL as
"<code>url</code>").
</li>
<li>A share action triggered from a native application (via a
proprietary share system), followed by the end user choosing a web
share target as the receiver.
</li>
</ul>
</div>
<section>
<h3>
Obtaining a <code>ShareData</code>
</h3>
<p>
When a <a>web share target</a> is <a>invoked</a>, the data MAY be in
an unspecified format. The user agent MUST first convert the data
into a <a data-cite=
"!WebShare#dom-sharedata"><code>ShareData</code></a> dictionary, if
it is not already, by mapping to the fields of <code>ShareData</code>
from equivalent concepts in the host system. If the source was a call
to <a data-cite=
"!WebShare#dom-navigator-share"><code>navigator.share</code></a>, the
user agent SHOULD use the <a data-cite=
"!WebShare#dom-sharedata"><code>ShareData</code></a> argument
unmodified (but this is not always possible, as it might have to
round-trip through some other format in a lossy manner). The user
agent MAY employ heuristics to map the data onto the
<code>ShareData</code> fields as well as possible.
</p>
<p class="note">
For example, the host share system may not have a dedicated URL
field, but a convention that both plain text and URLs are sometimes
transmitted in a "text" field. This is the case on Android. The user
agent can check whether all or part of the "text" field is a
<a data-cite="URL#valid-url-string">valid URL string</a>, and if so,
move that part of the "text" field to the <a data-cite=
"!WebShare#dom-sharedata"><code>ShareData</code></a>'s <a data-cite=
"!WebShare#dom-sharedata-url"><code>url</code></a> field.
</p>
</section>
<section data-link-for="WebAppManifest">
<h3>
Launching the web share target
</h3>
<p>
When <a>web share target</a> having <a>WebAppManifest</a>
<var>manifest</var> at <a data-cite="!URL#concept-url">URL</a>
<var>manifest URL</var> is <a>invoked</a> with <a data-cite=
"!WebShare#dom-sharedata"><code>ShareData</code></a> <var>data</var>,
run the following steps:
</p>
<ol>
<li>Let <var>url</var> be a copy of
<var>manifest</var>["<a>share_target</a>"]["<a data-link-for=
"ShareTarget">action</a>"].
</li>
<li>Let <var>entry list</var> be a new empty list of name-value
tuples.
</li>
<li>For each <var>member</var> in the sequence «
<code>"title"</code>, <code>"text"</code>, <code>"url"</code> »,
<ol>
<li>Let <var>name</var> be the value of
<var>manifest</var>["<a>share_target</a>"]["<a data-link-for=
"ShareTarget">params</a>"][<var>member</var>].
</li>
<li>If <var>name</var> is <code>undefined</code> or an empty
string, then continue.
</li>
<li>Let <var>value</var> be the value of
<var>data</var>[<var>member</var>].
</li>
<li>If <var>value</var> is <code>undefined</code>, then continue.
</li>
<li>
<a data-cite="!INFRA#list-append">Append</a> to <var>entry
list</var> a tuple with <var>name</var> and <var>value</var>.
</li>
</ol>
</li>
<li>Let <var>header list</var> be a new empty <a data-cite=
"!FETCH/#concept-header-list">header list</a>.
</li>
<li>Let <var>method</var> be
<var>manifest</var>["<a>share_target</a>"]["<a data-link-for=
"ShareTarget">method</a>"].
</li>
<li>Let <var>enctype</var> be
<var>manifest</var>["<a>share_target</a>"]["<a data-link-for=
"ShareTarget">enctype</a>"].
</li>
<li>If <var>method</var> is <code>"GET"</code>:
<ol>
<li>Let <var>query</var> be the result of running the
<a data-cite=
"!URL#concept-urlencoded-serializer"><code>application/x-www-form-urlencoded</code>
serializer</a> with <var>entry list</var> and no encoding
override.
</li>
<li>Set <var>url</var>'s <a data-cite=
"!URL#concept-url-query">query</a> component to <var>query</var>.
</li>
<li>Let <var>body</var> be null.
</li>
</ol>
</li>
<li>Otherwise, if <var>method</var> is <code>"POST"</code> and <var>
enctype</var> is <code>"application/x-www-form-urlencoded"</code>:
<ol>
<li>Let <var>body</var> be the result of running the
<a data-cite="!URL#concept-urlencoded-serializer"><code>application/x-www-form-urlencoded</code>
serializer</a> with <var>entry list</var> and no encoding
override.
</li>
<li>Set <var>body</var> to the result of <a data-cite=
"!Encoding#utf-8-encode">encoding</a> <var>body</var>.
</li>
<li>
<a data-cite="!FETCH/#concept-header-list-append">Append</a>
<code>"Content-Type"</code>/<code>"application/x-www-form-urlencoded"</code>
to <var>header list</var>.
</li>
</ol>
</li>
<li>Otherwise, if <var>method</var> is <code>"POST"</code> and <var>
enctype</var> is <code>"multipart/form-data"</code>:
<ol>
<li>Let <var>body</var> be the result of running the
<a data-cite="!HTML#multipart/form-data-encoding-algorithm"><code>
multipart/form-data</code> encoding algorithm</a> with
<var>entry list</var> and <a data-cite="!UTF-8">UTF-8</a>
encoding.
</li>
<li>Let <var>MIME type</var> be the concatenation of the string
<code>"multipart/form-data;"</code>, a U+0020 SPACE character,
the string <code>"boundary="</code>, and the <a data-cite=
"!HTML#multipart/form-data-boundary-string"><code>multipart/form-data</code>
boundary string</a> generated by the <a data-cite=
"!HTML#multipart/form-data-encoding-algorithm"><code>multipart/form-data</code>
encoding algorithm</a>.
</li>
<li>
<a data-cite="!FETCH/#concept-header-list-append">Append</a>
<code>"Content-Type"</code>/<var>MIME type</var> to <var>header
list</var>.
</li>
</ol>
</li>
<li>Let <var>browsing context</var> be the result of creating a
<a data-cite="!HTML#creating-a-new-browsing-context">new</a>
<a data-cite="!HTML#top-level-browsing-context">top-level browsing
context</a>.
</li>
<li>
<a data-cite="!HTML#navigate">Navigate</a> <var>browsing
context</var> to a new <a data-cite=
"!FETCH/#concept-request">request</a> whose method is
<var>method</var>, url is <var>url</var>, header list is
<var>header list</var>, and body is <var>body</var>.
</li>
</ol>
<p class="note">
This algorithm assumes that <var>manifest</var> has had the
<a>post-processing the <code>share_target</code> member</a> algorithm
run on it and still has a <a>share_target</a> afterwards.
</p>
</section>
</section>
<section class="informative">
<h2>
Security and privacy considerations
</h2>
<ul>
<li>Invoking a <a>web share target</a> means potentially sending
private user data to a third-party website. Implementors are to take
every precaution that the user understands which party the data is
being sent to (<i>e.g.</i>, communicating the web share target's
origin), before running the <a>invocation</a> algorithm.
</li>
<li>Of particular concern is spoofing: a web share target presenting
itself as a different party (by setting its name and icon accordingly).
</li>
<li>The spoofing risk is heightened if web share targets are chosen
from an online index, rather than a set of targets that the end user
has explicitly installed or registered.
</li>
<li>The requirement that the web share target be a <a data-cite=
"SECURE-CONTEXTS#secure-context">secure context</a> is to prevent
private user data from being transmitted to a party that does not
control the origin in question, or in clear text over the network.
</li>
</ul>
</section>
<section class="appendix informative">
<h2>
Acknowledgments
</h2>
<p>
Thanks to the <a data-cite="web-intents">Web Intents</a> team, who laid
the groundwork for the web app interoperability use cases. In
particular, <a href="https://paul.kinlan.me/">Paul Kinlan</a>, who did
a lot of early advocacy for Web Share and Web Share Target.
</p>
<p>
Thanks to Connie Pyromallis, who wrote an early draft of this spec, and
helped design and prototype the API.
</p>
<p>
Thanks to Alex Russell and David Baron, for their feedback on early
drafts of this spec.
</p>
</section>
</body>
</html>