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1# Schematics
2
3> A scaffolding library for the modern web.
4
5## Description
6
7Schematics are generators that transform an existing filesystem. They can create files, refactor existing files, or move files around.
8
9What distinguishes Schematics from other generators, such as Yeoman or Yarn Create, is that schematics are purely descriptive; no changes are applied to the actual filesystem until everything is ready to be committed. There is no side effect, by design, in Schematics.
10
11# Glossary
12
13| Term | Description |
14| -------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
15| **Schematics** | A generator that executes descriptive code without side effects on an existing file system. |
16| **Collection** | A list of schematics metadata. Schematics can be referred by name inside a collection. |
17| **Tool** | The code using the Schematics library. |
18| **Tree** | A staging area for changes, containing the original file system, and a list of changes to apply to it. |
19| **Rule** | A function that applies actions to a `Tree`. It returns a new `Tree` that will contain all transformations to be applied. |
20| **Source** | A function that creates an entirely new `Tree` from an empty filesystem. For example, a file source could read files from disk and create a Create Action for each of those. |
21| **Action** | An atomic operation to be validated and committed to a filesystem or a `Tree`. Actions are created by schematics. |
22| **Sink** | The final destination of all `Action`s. |
23| **Task** | A Task is a way to execute an external command or script in a schematic. A Task can be used to perform actions such as installing dependencies, running tests, or building a project. A Task is created by using the `SchematicContext` object and can be scheduled to run before or after the schematic `Tree` is applied. |
24
25# Tooling
26
27Schematics is a library, and does not work by itself. A [reference CLI](https://github.com/angular/angular-cli/blob/main/packages/angular_devkit/schematics_cli/bin/schematics.ts) is available on this repository, and is published on NPM at [@angular-devkit/schematics-cli](https://www.npmjs.com/package/@angular-devkit/schematics-cli). This document explains the library usage and the tooling API, but does not go into the tool implementation itself.
28
29The tooling is responsible for the following tasks:
30
311. Create the Schematic Engine, and pass in a Collection and Schematic loader.
321. Understand and respect the Schematics metadata and dependencies between collections. Schematics can refer to dependencies, and it's the responsibility of the tool to honor those dependencies. The reference CLI uses NPM packages for its collections.
331. Create the Options object. Options can be anything, but the schematics can specify a JSON Schema that should be respected. The reference CLI, for example, parses the arguments as a JSON object and validates it with the Schema specified by the collection.
341. Schematics provides some JSON Schema formats for validation that tooling should add. These validate paths, html selectors and app names. Please check the reference CLI for how these can be added.
351. Call the schematics with the original Tree. The tree should represent the initial state of the filesystem. The reference CLI uses the current directory for this.
361. Create a Sink and commit the result of the schematics to the Sink. Many sinks are provided by the library; FileSystemSink and DryRunSink are examples.
371. Output any logs propagated by the library, including debugging information.
38
39The tooling API is composed of the following pieces:
40
41## Engine
42
43The `SchematicEngine` is responsible for loading and constructing `Collection`s and `Schematics`. When creating an engine, the tooling provides an `EngineHost` interface that understands how to create a `CollectionDescription` from a name, and how to create a `SchematicDescription`.
44
45# Schematics (Generators)
46
47Schematics are generators and part of a `Collection`.
48
49## Collection
50
51A Collection is defined by a `collection.json` file (in the reference CLI). This JSON defines the following properties:
52
53| Prop Name | Type | Description |
54| ----------- | -------- | --------------------------- |
55| **name** | `string` | The name of the collection. |
56| **version** | `string` | Unused field. |
57
58## Schematic
59
60# Operators, Sources and Rules
61
62A `Source` is a generator of a `Tree`; it creates an entirely new root tree from nothing. A `Rule` is a transformation from one `Tree` to another. A `Schematic` (at the root) is a `Rule` that is normally applied on the filesystem.
63
64## Operators
65
66`FileOperator`s apply changes to a single `FileEntry` and return a new `FileEntry`. The result follows these rules:
67
681. If the `FileEntry` returned is null, a `DeleteAction` will be added to the action list.
691. If the path changed, a `RenameAction` will be added to the action list.
701. If the content changed, an `OverwriteAction` will be added to the action list.
71
72It is impossible to create files using a `FileOperator`.
73
74## Provided Operators
75
76The Schematics library provides multiple `Operator` factories by default that cover basic use cases:
77
78| FileOperator | Description |
79| -------------------------------- | -------------------------------------------------------------------- |
80| `contentTemplate<T>(options: T)` | Apply a content template (see the [Templating](#templating) section) |
81| `pathTemplate<T>(options: T)` | Apply a path template (see the [Templating](#templating) section) |
82
83## Provided Sources
84
85The Schematics library additionally provides multiple `Source` factories by default:
86
87| Source | Description |
88| -------------------------------------- | -------------------------------------------------------------------------------------------------------------------------- |
89| `empty()` | Creates a source that returns an empty `Tree`. |
90| `source(tree: Tree)` | Creates a `Source` that returns the `Tree` passed in as argument. |
91| `url(url: string)` | Loads a list of files from the given URL and returns a `Tree` with the files as `CreateAction` applied to an empty `Tree`. |
92| `apply(source: Source, rules: Rule[])` | Apply a list of `Rule`s to a source, and return the resulting `Source`. |
93
94## Provided Rules
95
96The schematics library also provides `Rule` factories by default:
97
98| Rule | Description |
99| ------------------------------------------- | -------------------------------------------------------------------------------------- |
100| `noop()` | Returns the input `Tree` as is. |
101| `chain(rules: Rule[])` | Returns a `Rule` that's the concatenation of other `Rule`s. |
102| `forEach(op: FileOperator)` | Returns a `Rule` that applies an operator to every file of the input `Tree`. |
103| `move(root: string)` | Moves all the files from the input to a subdirectory. |
104| `merge(other: Tree)` | Merge the input `Tree` with the other `Tree`. |
105| `contentTemplate<T>(options: T)` | Apply a content template (see the Template section) to the entire `Tree`. |
106| `pathTemplate<T>(options: T)` | Apply a path template (see the Template section) to the entire `Tree`. |
107| `template<T>(options: T)` | Apply both path and content templates (see the Template section) to the entire `Tree`. |
108| `filter(predicate: FilePredicate<boolean>)` | Returns the input `Tree` with files that do not pass the `FilePredicate`. |
109
110# Templating
111
112As referenced above, some functions are based upon a file templating system, which consists of path and content templating.
113
114The system operates on placeholders defined inside files or their paths as loaded in the `Tree` and fills these in as defined in the following, using values passed into the `Rule` which applies the templating (i.e. `template<T>(options: T)`).
115
116## Path Templating
117
118| Placeholder | Description |
119| ----------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
120| `__variable__` | Replaced with the value of `variable`. |
121| `__variable@function__` | Replaced with the result of the call `function(variable)`. Can be chained to the left (`__variable@function1@function2__ ` etc). |
122
123## Content Templating
124
125| Placeholder | Description |
126| ------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- |
127| `<%= expression %>` | Replaced with the result of the call of the given expression. This only supports direct expressions, no structural (for/if/...) JavaScript. |
128| `<%- expression %>` | Same as above, but the value of the result will be escaped for HTML when inserted (i.e. replacing '<' with '\&lt;') |
129| `<% inline code %>` | Inserts the given code into the template structure, allowing to insert structural JavaScript. |
130| `<%# text %>` | A comment, which gets entirely dropped. |
131
132# Examples
133
134## Simple
135
136An example of a simple Schematics which creates a "hello world" file, using an option to determine its path:
137
138```typescript
139import { Tree } from '@angular-devkit/schematics';
140
141export default function MySchematic(options: any) {
142 return (tree: Tree) => {
143 tree.create(options.path + '/hi', 'Hello world!');
144 return tree;
145 };
146}
147```
148
149A few things from this example:
150
1511. The function receives the list of options from the tooling.
1521. It returns a [`Rule`](src/engine/interface.ts#L73), which is a transformation from a `Tree` to another `Tree`.
153
154## Templating
155
156A simplified example of a Schematics which creates a file containing a new Class, using an option to determine its name:
157
158```typescript
159// files/__name@dasherize__.ts
160
161export class <%= classify(name) %> {
162}
163```
164
165```typescript
166// index.ts
167
168import { strings } from '@angular-devkit/core';
169import {
170 Rule,
171 SchematicContext,
172 SchematicsException,
173 Tree,
174 apply,
175 branchAndMerge,
176 mergeWith,
177 template,
178 url,
179} from '@angular-devkit/schematics';
180import { Schema as ClassOptions } from './schema';
181
182export default function (options: ClassOptions): Rule {
183 return (tree: Tree, context: SchematicContext) => {
184 if (!options.name) {
185 throw new SchematicsException('Option (name) is required.');
186 }
187
188 const templateSource = apply(url('./files'), [
189 template({
190 ...strings,
191 ...options,
192 }),
193 ]);
194
195 return branchAndMerge(mergeWith(templateSource));
196 };
197}
198```
199
200Additional things from this example:
201
2021. `strings` provides the used `dasherize` and `classify` functions, among others.
2031. The files are on-disk in the same root directory as the `index.ts` and loaded into a `Tree`.
2041. Then the `template` `Rule` fills in the specified templating placeholders. For this, it only knows about the variables and functions passed to it via the options-object.
2051. Finally, the resulting `Tree`, containing the new file, is merged with the existing files of the project which the Schematic is run on.