Add a method to the @@images view to render a img tag with srcset

README.rst

@@ -12,13 +12,14 @@ and BLOBs to be configured in zope.conf.
 
 plone.supermodel handlers are registered.
 
-See the ``usage.rst`` doctest for more details.
+See the `image handling section of Plone documentation <https://6.docs.plone.org/classic-ui/images.html#all-image-scales-in-the-srcset>`_ to learn how to
+use the features provided by this package.
 
 
 Source Code
 ===========
 
- Note: This packages is licensed under a *BSD license*. 
+ Note: This packages is licensed under a *BSD license*.
  Please do not add dependencies on GPL code!
 
 Contributors please read the document `Process for Plone core's development <https://docs.plone.org/develop/coredev/docs/index.html>`_

news/170.feature

@@ -0,0 +1,2 @@
+Add a srcset method to the @@images view
+[erral]

plone/namedfile/scaling.py

@@ -739,6 +739,71 @@ def picture(
             fieldname=fieldname,
         ).prettify()
 
+    def srcset(
+        self,
+        fieldname=None,
+        scale_in_src="huge",
+        sizes="",
+        alt=_marker,
+        css_class=None,
+        title=_marker,
+        **kwargs,
+    ):
+        if fieldname is None:
+            try:
+                primary = IPrimaryFieldInfo(self.context, None)
+            except TypeError:
+                return
+            if primary is None:
+                return  # 404
+            fieldname = primary.fieldname
+
+        original_width, original_height = self.getImageSize(fieldname)
+
+        storage = getMultiAdapter(
+            (self.context, functools.partial(self.modified, fieldname)),
+            IImageScaleStorage,
+        )
+
+        srcset_urls = []
+        for width, height in self.available_sizes.values():
+            if width <= original_width:
+                scale = storage.pre_scale(
+                    fieldname=fieldname, width=width, height=height, mode="scale"
+                )
+                extension = scale["mimetype"].split("/")[-1].lower()
+                srcset_urls.append(
+                    f'{self.context.absolute_url()}/@@images/{scale["uid"]}.{extension} {scale["width"]}w'
+                )
+        attributes = {}
+        if title is _marker:
+            attributes["title"] = self.context.Title()
+        elif title:
+            attributes["title"] = title
+        if alt is not _marker:
+            attributes["alt"] = alt
+
+        if css_class is not None:
+            attributes["class"] = css_class
+
+        attributes.update(**kwargs)
+
+        attributes["sizes"] = sizes
+
+        srcset_string = ", ".join(srcset_urls)
+        attributes["srcset"] = srcset_string
+
+        if scale_in_src not in self.available_sizes:
+            for key, (width, height) in self.available_sizes.items():
+                if width <= original_width:
+                    scale_in_src = key
+                    break
+
+        scale = self.scale(fieldname=fieldname, scale=scale_in_src)
+        attributes["src"] = scale.url
+
+        return _image_tag_from_values(*attributes.items())
+
 
 class NavigationRootScaling(ImageScaling):
     @lazy_property

plone/namedfile/test.pt

@@ -42,6 +42,10 @@
              i18n:name=""
              i18n:translate=""
           >picture tags</a>,
+          <a href="#srcset"
+             i18n="name"
+             i18n:translate=""
+          >img tag with srcset</a>,
           <a href="#stored"
              i18n:name=""
              i18n:translate=""
@@ -198,6 +202,26 @@
           </p>
         </section>
 
+        <section class="section"
+                 id="srcset"
+        >
+          <h2 i18n:translate="">img with srcset attributes</h2>
+          <p i18n:translate="">
+      srcset allows the browser to select the correct image, depending on the space the image has on a page.
+          </p>
+          <p i18n:translate="msg_images_test_srcset">
+      To do so, the @@images view provides a srcset method, that will output the full srcset of this image, using all available image scales. It has as required parameter the value of the sizes attribute that the user of this method has to provide and will be output as is in the generated HTML.
+          </p>
+          <p>
+            <img tal:replace="structure python:images.srcset('image', sizes='(min-width: 1400px) 550px, 90vw')" />
+          </p>
+          <p>
+            <code>
+              <img tal:replace="python:images.srcset('image', sizes='(min-width: 1400px) 550px, 90vw')" />
+            </code>
+          </p>
+        </section>
+
         <section class="section"
                  id="stored"
         >

plone/namedfile/tests/test_scaling.py

@@ -813,6 +813,117 @@ def testOversizedHighPixelDensityScale(self):
         self.assertEqual(len(foo.srcset), 1)
         self.assertEqual(foo.srcset[0]["scale"], 2)
 
+    def testImgSrcSet(self):
+        """test rendered srcset values"""
+        self.scaling.available_sizes = {
+            "huge": (1600, 65536),
+            "great": (1200, 65536),
+            "larger": (1000, 65536),
+            "large": (800, 65536),
+            "teaser": (600, 65536),
+            "preview": (400, 65536),
+            "mini": (200, 65536),
+            "thumb": (128, 128),
+            "tile": (64, 64),
+            "icon": (32, 32),
+            "listing": (16, 16),
+        }
+        tag = self.scaling.srcset("image", sizes="50vw")
+        base = self.item.absolute_url()
+        expected = f"""<img title="foo" sizes="50vw" srcset="{base}/@@images/image-200-....png 200w, {base}/@@images/image-128-....png 128w, {base}/@@images/image-64-....png 64w, {base}/@@images/image-32-....png 32w, {base}/@@images/image-16-....png 16w" src="{base}/@@images/image-1600-....png".../>"""
+        self.assertTrue(_ellipsis_match(expected, tag.strip()))
+
+    def testImgSrcSetCustomSrc(self):
+        """test that we can select a custom scale in the src attribute"""
+        self.scaling.available_sizes = {
+            "huge": (1600, 65536),
+            "great": (1200, 65536),
+            "larger": (1000, 65536),
+            "large": (800, 65536),
+            "teaser": (600, 65536),
+            "preview": (400, 65536),
+            "mini": (200, 65536),
+            "thumb": (128, 128),
+            "tile": (64, 64),
+            "icon": (32, 32),
+            "listing": (16, 16),
+        }
+        tag = self.scaling.srcset("image", sizes="50vw", scale_in_src="mini")
+        base = self.item.absolute_url()
+        expected = f"""<img title="foo" sizes="50vw" srcset="{base}/@@images/image-200-....png 200w, {base}/@@images/image-128-....png 128w, {base}/@@images/image-64-....png 64w, {base}/@@images/image-32-....png 32w, {base}/@@images/image-16-....png 16w" src="{base}/@@images/image-200-....png".../>"""
+        self.assertTrue(_ellipsis_match(expected, tag.strip()))
+
+    def testImgSrcSetInexistentScale(self):
+        """test that when requesting an inexistent scale for the src attribute
+        we provide the biggest scale we can produce
+        """
+        self.scaling.available_sizes = {
+            "huge": (1600, 65536),
+            "great": (1200, 65536),
+            "larger": (1000, 65536),
+            "large": (800, 65536),
+            "teaser": (600, 65536),
+            "preview": (400, 65536),
+            "mini": (200, 65536),
+            "thumb": (128, 128),
+            "tile": (64, 64),
+            "icon": (32, 32),
+            "listing": (16, 16),
+        }
+        tag = self.scaling.srcset(
+            "image", sizes="50vw", scale_in_src="inexistent-scale-name"
+        )
+        base = self.item.absolute_url()
+        expected = f"""<img title="foo" sizes="50vw" srcset="{base}/@@images/image-200-....png 200w, {base}/@@images/image-128-....png 128w, {base}/@@images/image-64-....png 64w, {base}/@@images/image-32-....png 32w, {base}/@@images/image-16-....png 16w" src="{base}/@@images/image-200-....png".../>"""
+        self.assertTrue(_ellipsis_match(expected, tag.strip()))
+
+    def testImgSrcSetCustomTitle(self):
+        """test passing a custom title to the srcset method"""
+        self.scaling.available_sizes = {
+            "huge": (1600, 65536),
+            "great": (1200, 65536),
+            "larger": (1000, 65536),
+            "large": (800, 65536),
+            "teaser": (600, 65536),
+            "preview": (400, 65536),
+            "mini": (200, 65536),
+            "thumb": (128, 128),
+            "tile": (64, 64),
+            "icon": (32, 32),
+            "listing": (16, 16),
+        }
+        tag = self.scaling.srcset("image", sizes="50vw", title="My Custom Title")
+        base = self.item.absolute_url()
+        expected = f"""<img title="My Custom Title" sizes="50vw" srcset="{base}/@@images/image-200-....png 200w, {base}/@@images/image-128-....png 128w, {base}/@@images/image-64-....png 64w, {base}/@@images/image-32-....png 32w, {base}/@@images/image-16-....png 16w" src="{base}/@@images/image-1600-....png".../>"""
+        self.assertTrue(_ellipsis_match(expected, tag.strip()))
+
+    def testImgSrcSetAdditionalAttributes(self):
+        """test that additional parameters are output as is, like alt, loading, ..."""
+        self.scaling.available_sizes = {
+            "huge": (1600, 65536),
+            "great": (1200, 65536),
+            "larger": (1000, 65536),
+            "large": (800, 65536),
+            "teaser": (600, 65536),
+            "preview": (400, 65536),
+            "mini": (200, 65536),
+            "thumb": (128, 128),
+            "tile": (64, 64),
+            "icon": (32, 32),
+            "listing": (16, 16),
+        }
+        tag = self.scaling.srcset(
+            "image",
+            sizes="50vw",
+            alt="This image shows nothing",
+            css_class="my-personal-class",
+            title="My Custom Title",
+            loading="lazy",
+        )
+        base = self.item.absolute_url()
+        expected = f"""<img title="My Custom Title" alt="This image shows nothing" class="my-personal-class" loading="lazy" sizes="50vw" srcset="{base}/@@images/image-200-....png 200w, {base}/@@images/image-128-....png 128w, {base}/@@images/image-64-....png 64w, {base}/@@images/image-32-....png 32w, {base}/@@images/image-16-....png 16w" src="{base}/@@images/image-1600-....png".../>"""
+        self.assertTrue(_ellipsis_match(expected, tag.strip()))
+
 
 class ImageTraverseTests(unittest.TestCase):
 

plone/namedfile/usage.rst

@@ -403,57 +403,3 @@ We will test this with a dummy request, faking traversal::
     'text/plain'
     >>> request.response.getHeader('Content-Disposition')
     "attachment; filename*=UTF-8''test.txt"
-
-
-Image scales
-------------
-
-This package can handle the creation, storage, and retrieval of arbitrarily
-sized scaled versions of images stored in NamedImage or NamedBlobImage fields.
-
-Image scales are accessed via an @@images view that is available for any item
-providing ``plone.namedfile.interfaces.IImageScaleTraversable``.  There are
-several ways that you may reference scales from page templates.
-
-1. for full control you may do the tag generation explicitly::
-
-     <img tal:define="images context/@@images;
-                      thumbnail python: images.scale('image', width=64, height=64);"
-          tal:condition="thumbnail"
-          tal:attributes="src thumbnail/url;
-                          width thumbnail/width;
-                          height thumbnail/height" />
-
-   This would create an up to 64 by 64 pixel scaled down version of the image
-   stored in the "image" field.  It also allows for passing in additional
-   parameters supported by the ``scaleImage`` function from ``plone.scale``,
-   e.g. ``mode`` or ``quality``.
-
-   .. _`plone.scale`: https://pypi.org/project/plone.scale/
-
-2. for automatic tag generation with extra parameters you would use::
-
-     <img tal:define="images context/@@images"
-          tal:replace="structure python: images.tag('image',
-                       width=1200, height=800, mode='contain')" />
-
-3. It is possible to access scales via predefined named scale sizes, rather
-   than hardcoding the dimensions every time you access a scale.  The scale
-   sizes are found via calling a utility providing
-   ``plone.namedfile.interfaces.IAvailableSizes``, which should return a dict of
-   scale name => (width, height).  A scale called 'mini' could then be accessed
-   like this::
-
-     <img tal:define="images context/@@images"
-          tal:replace="structure python: images.tag('image', scale='mini')" />
-
-   This would use the predefined scale size "mini" to determine the desired
-   image dimensions, but still allow to pass in extra parameters.
-
-4. a convenience short-cut for option 3 can be used::
-
-     <img tal:replace="structure context/@@images/image/mini" />
-
-5. and lastly, the short-cut can also be used to render the unscaled image::
-
-     <img tal:replace="structure context/@@images/image" />