[DOC] move some component doc in sub pages

closes #354

doc/reference/props_validation.md

@@ -0,0 +1,103 @@
+# 🦉 Props Validation 🦉
+
+As an application becomes complex, it may be quite unsafe to define props in an informal way. This leads to two issues:
+
+- hard to tell how a component should be used, by looking at its code.
+- unsafe, it is easy to send wrong props into a component, either by refactoring a component, or one of its parents.
+
+A props type system solves both issues, by describing the types and shapes
+of the props. Here is how it works in Owl:
+
+- `props` key is a static key (so, different from `this.props` in a component instance)
+- it is optional: it is ok for a component to not define a `props` key.
+- props are validated whenever a component is created/updated
+- props are only validated in `dev` mode (see [config page](config.md#mode))
+- if a key does not match the description, an error is thrown
+- it validates keys defined in (static) `props`. Additional keys given by the
+  parent will cause an error.
+
+For example:
+
+```js
+class ComponentA extends owl.Component {
+    static props = ['id', 'url'];
+
+    ...
+}
+
+class ComponentB extends owl.Component {
+  static props = {
+    count: {type: Number},
+    messages: {
+      type: Array,
+      element: {type: Object, shape: {id: Boolean, text: 'string' }
+    },
+   date: Date,
+   combinedVal: [Number, Boolean]
+  };
+
+  ...
+}
+```
+
+- it is an object or a list of strings
+- a list of strings is a simplified props definition, which only lists the name
+  of the props. Also, if the name ends with `?`, it is considered optional.
+- all props are by default required, unless they are defined with `optional: true`
+  (in that case, validation is only done if there is a value)
+- valid types are: `Number, String, Boolean, Object, Array, Date, Function`, and all
+  constructor functions (so, if you have a `Person` class, it can be used as a type)
+- arrays are homogeneous (all elements have the same type/shape)
+
+For each key, a `prop` definition is either a boolean, a constructor, a list of constructors, or an object:
+
+- a boolean: indicate that the props exists, and is mandatory.
+- a constructor: this should describe the type, for example: `id: Number` describe
+  the props `id` as a number
+- a list of constructors. In that case, this means that we allow more than one
+  type. For example, `id: [Number, String]` means that `id` can be either a string
+  or a number.
+- an object. This makes it possible to have more expressive definition. The following sub keys are then allowed (but not mandatory):
+  - `type`: the main type of the prop being validated
+  - `element`: if the type was `Array`, then the `element` key describes the type of each element in the array. If it is not set, then we only validate the array, not its elements,
+  - `shape`: if the type was `Object`, then the `shape` key describes the interface of the object. If it is not set, then we only validate the object, not its elements,
+  - `validate`: this is a function which should return a boolean to determine if
+    the value is valid or not. Useful for custom validation logic.
+
+Examples:
+
+```js
+  // only the existence of those 3 keys is documented
+  static props = ['message', 'id', 'date'];
+```
+
+```js
+  // size is optional
+  static props = ['message', 'size?'];
+```
+
+```js
+  static props = {
+    messageIds: {type: Array, element: Number},  // list of number
+    otherArr: {type: Array},   // just array. no validation is made on sub elements
+    otherArr2: Array,   // same as otherArr
+    someObj: {type: Object},  // just an object, no internal validation
+    someObj2: {
+      type: Object,
+      shape: {
+        id: Number,
+        name: {type: String, optional: true},
+        url: String
+      ]},    // object, with keys id (number), name (string, optional) and url (string)
+    someFlag: Boolean,     // a boolean, mandatory (even if `false`)
+    someVal: [Boolean, Date],   // either a boolean or a date
+    otherValue: true,     // indicates that it is a prop
+    kindofsmallnumber: {
+      type: Number,
+      validate: n => (0 <= n && n <= 10)
+    },
+    size: {
+      validate:  e => ["small", "medium", "large"].includes(e)
+    },
+  };
+```