[RFC] 0004 Simple Build Extensibility

Author: RandomByte

rfcs/0004-simple-build-extensibility.md

@@ -0,0 +1,158 @@
+- Start Date: 2018-08-28
+- RFC PR: [#](https://github.com/SAP/ui5-tooling/pull/)
+- Issue: -
+- Affected components
+    + [x] [ui5-builder](https://github.com/SAP/ui5-builder)
+    + [ ] [ui5-server](https://github.com/SAP/ui5-server)
+    + [ ] [ui5-cli](https://github.com/SAP/ui5-cli)
+    + [ ] [ui5-fs](https://github.com/SAP/ui5-fs)
+    + [x] [ui5-project](https://github.com/SAP/ui5-project)
+    + [ ] [ui5-logger](https://github.com/SAP/ui5-logger)
+
+# RFC 0004 Simple Build Extensibility
+## Summary
+Add a feature for basic customization of how a single UI5 project is being built.
+
+## Motivation
+Currently the UI5 build is only capable of building UI5 projects of types "application" and "library" with a fixed set of tasks to be executed.
+
+A UI5 project (for example a library) may want to add build steps. For this, an extensibility mechanism is needed.
+
+While multiple UI5 projects may require the same kind of "customized" build, easy reuse capabilities are not in focus of this RFC. [RFC 0001](https://github.com/SAP/ui5-tooling/pull/4) focuses more on that.
+
+This shall be a preliminary solution to allow for basic extensibility and to learn about the different needs and use cases in that area before proceeding with [RFC 0001](https://github.com/SAP/ui5-tooling/pull/4) or similar concepts.
+
+## Detailed design
+### Configuration
+In a projects `ui5.yaml`, a new configuration option should be added to define additional tasks that shall be executed at a specific time during the build process of a project. This configuration shall only affect the project it belongs to. The build process of any of the other projects (e.g. project dependencies) shall be unaffected by this configuration.
+
+A task may require certain tasks to be executed before and after it. This shall be configurable in a simple but less generic way. See [RFC 0001](https://github.com/SAP/ui5-tooling/pull/4) for a concept of a more generic handling.
+
+A project configuration might look like this:
+```yaml
+specVersion: "0.1"
+type: library
+metadata:
+    name: my.library
+builder:
+    customTasks:
+    - name: babel
+      beforeTask: generateComponentPreload
+    - name: generateMarkdownFiles
+      afterTask: uglify
+      configuration:
+        color: blue
+```
+
+When building "my.library", this will execute the custom task *babel* before the "standard" task *generateComponentPreload* and *generateMarkdownFiles* after *uglify*. This means that for example *generateComponentPreload* and all following tasks can work with the resources created or modified by the *babel* task.
+
+### Generic handling of extension
+**This section is partially equal to what is outlined in [RFC 0001](https://github.com/SAP/ui5-tooling/blob/rfc-type-ext/rfcs/0001-type-extensibility.md#generic-handling-of-extension).**
+
+Custom task implementations have similar characteristics than other possible "extensions" of the UI5 Build and Development Tooling. Examples for other extensions include "Shims" (see RFC 0002), server middlewares and translators.
+
+Therefore a somewhat generic concept for dealing with extensions is needed.
+
+To separate "UI5 Projects" (i.e. things that represent UI5-artifacts for the browser) from tooling specific things like "extensions", an additional attribute "kind" is added to the ui5.yaml.
+
+A custom task (a.k.a. "task extension") will consist of at least a ui5.yaml defining it as an extension and a JavaScript implementation.
+
+#### Example task extension
+**`ui5.yaml`**:
+```yaml
+specVersion: "0.1"
+kind: extension
+type: task
+metadata:
+    name: generateMarkdownFiles
+task:
+    path: generateMarkdownFiles.js
+```
+
+**`generateMarkdownFiles.js`**:
+```js
+const markdownGenerator = require("./markdownGenerator");
+
+module.exports = function({workspace, options}) {
+    return workspace.byGlob("**/*.txt")
+        .then((textResources) => {
+            return markdownGenerator({
+                resources: textResources
+            });
+        })
+        .then((markdownResources) => {
+            return Promise.all(markdownResources.map((resource) => {
+                return workspace.write(resource);
+            }));
+        });
+};
+```
+
+#### Collecting and applying task extensions
+A task extension might be a standalone module or part of a project.
+
+If the extension is part of a project, the single `ui5.yaml` for the above example might look like this:
+
+```yaml
+specVersion: "0.1"
+kind: project
+type: library
+metadata:
+    name: my.library
+builder:
+    customTasks:
+    - name: generateMarkdownFiles
+      afterTask: uglify
+      configuration:
+        color: blue
+----
+specVersion: "0.1"
+kind: extension
+type: task
+metadata:
+    name: generateMarkdownFiles
+task:
+    path: generateMarkdownFiles.js
+```
+
+In this case the extension is no dependency of any kind but automatically collected and processed with the processing of the project.
+
+The AbstractBuilder will detect the custom task configuration of the project my.library and inject the tasks into the build execution.
+
+
+### Task implementation
+A custom task implementation needs to return a function with the following signature (written in JSDoc):
+
+```js
+/**
+ * Custom task example
+ *
+ * @param {Object} parameters Parameters
+ * @param {DuplexCollection} parameters.workspace DuplexCollection to read and write files
+ * @param {AbstractReader} parameters.dependencies Reader or Collection to read dependency files
+ * @param {Object} parameters.options Options
+ * @param {string} parameters.options.projectName Project name
+ * @param {string} [parameters.options.configuration] Task configuration if given in ui5.yaml
+ * @returns {Promise<undefined>} Promise resolving with undefined once data has been written
+ */
+module.exports = function({workspace, options}) {
+    // [...]
+};
+```
+
+## How we teach this
+- Documentation about how to implement custom tasks
+- Explanation of the task/processor concept
+
+## Drawbacks
+Custom task configurations might break with future changes to the Application- and LibraryBuilder due to renaming or reordering of the standard tasks.
+
+## Alternatives
+There are ways to consume (and thereby possibly adapt) the existing tooling through its API via taskrunners such as grunt or gulp, or using a custom node.js script. But this offers only limited possibilities, especially when it comes to building transient dependencies.
+
+[RFC 0001](https://github.com/SAP/ui5-tooling/pull/4) may offer a more generic way to tackle this but requires additional concept and evaluation work. This RFC (0004) should not prevent the implementation of RFC 0001 in the future.
+
+## Unresolved questions
+- Detailed task signature
+    + Should the whole `project` object be handed over to custom tasks?
+