Merge pull request #85 from sivaschenko/patch-1

Proposal: Allow class attribute for referenceBlock type in layout xml

design-documents/class-attibute-for-referenceBlock-type-in-layout-xml.md

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+# Allow class attribute for referenceBlock type in layout xml
+
+## Problem
+
+It is **impossible to substitute a block class** and/or customize block method(s) implementation **on a specific page keeping layout configuration of the block** (position, sorting in the layout, child blocks, arguments).
+
+Currently available block class substitution approaches do not resolve the problem:
+
+| Currently available approach                                                      | Why the approach does not resolve the problem  |
+| --------------------------------------------------------------------------------- |:----------------------------------------------:|
+| Introduce a preference for block class in di.xml                                  | Block is substituted globally, not only on the desired page |
+| Remove the existing block from the layout and declare a new one                   | Layout configuration related to this block is lost |
+| Use plugins                                                                       | Block functionality is affected for all instances, not only on the desired page |
+| Use plugins that change method functionality only for specific page/layout handle | This approach is imperative, implicit, prone to mistakes, bugs and implementation inconsistency, it is not compatible with layout updates | 
+
+## Solution
+
+Allow the `class` attribute of `blockReferenceType` in `lib/internal/Magento/Framework/View/Layout/etc/elements.xsd`
+
+See: https://github.com/magento/magento2/pull/18933
+
+### Pros
+
+* Possible to substitute a block class for a specific page easily and in declarative way preserving layout configuration of the block
+
+### Cons
+
+* Allows developer to substitute a block class with a class that is not compatible with a template
+
+## POC
+
+POC is implemented in public pull reqeust: https://github.com/magento/magento2/pull/18933