docs: Mixins documentation

docs/source/index.rst

@@ -33,5 +33,6 @@ Contents
    item-validation
    settings
    actions
+   mixins
    changelog
 

docs/source/mixins.rst

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+.. _mixins:
+
+======
+Mixins
+======
+
+What's a Mixin in Python?
+-------------------------
+
+A mixin is a special kind of multiple inheritance. There are two main situations where mixins are used:
+
+* You want to provide a lot of optional features for a class.
+* You want to use one particular feature in a lot of different classes.
+
+That's a little definition, but if you want more info you can see that `StackOverFlow post`_, or this `blog`_
+
+.. _`StackOverFLow post`: https://stackoverflow.com/questions/533631/what-is-a-mixin-and-why-are-they-useful
+.. _`blog`: https://easyaspython.com/mixins-for-fun-and-profit-cb9962760556
+
+With Spidermon can use four mixins: `StatsMonitorMixin`_, `ValidationMonitorMixin`_, `SpiderMonitorMixin`_, `JobMonitorMixin`_ mixins
+
+.. _`StatsMonitorMixin`:
+
+StatsMonitorMixin 
+-----------------
+
+`StatsMonitorMixin code`_
+
+.. _`StatsMonitorMixin code`: https://github.com/scrapinghub/spidermon/blob/master/spidermon/contrib/monitors/mixins/stats.py
+
+
+We have an example in the next example: 
+
+.. code-block:: python
+
+    # monitors.py
+    # (...other monitors...)
+
+    @monitors.name('Item validation')
+    class ItemValidationMonitor(Monitor, StatsMonitorMixin):
+
+        @monitors.name('No item validation errors')
+        def test_no_item_validation_errors(self):
+            validation_errors = getattr(
+                self.stats, 'spidermon/validation/fields/errors', 0
+            )
+            self.assertEqual(
+                validation_errors,
+                0,
+                msg='Found validation errors in {} fields'.format(
+                    validation_errors)
+            )
+
+
+In this example, the `ItemValidationMonitor` have inheritance of `Monitor` class. The `Monitor` class have the data property
+and in this data property is an `stats` dict. Then, if we don't want to use the mixin, we can do something like:
+
+.. code-block:: python
+
+    @monitors.name('Item validation')
+    class ItemValidationMonitor(Monitor):
+
+        @monitors.name('No item validation errors')
+        def test_no_item_validation_errors(self):
+            validation_errors = getattr(
+                self.data.stats, 'spidermon/validation/fields/errors', 0
+            )
+            ...
+            ...
+            ...
+
+So, why use mixin? Mixin is used to provide optional features for a class, for this example, we can use the mixin to create another class with the property `data.stats` but if is not configured, an exception will rise.
+
+
+.. _`JobMonitorMixin`:
+
+JobMonitorMixin
+---------------
+
+`JobMonitorMixin code`_
+
+.. _`JobMonitorMixin code`: https://github.com/scrapinghub/spidermon/blob/master/spidermon/contrib/monitors/mixins/job.py
+
+This is similar to `StatsMonitorMixin`_. If the object don't have the property `data.job`, it will raise an exception
+
+.. _`SpiderMonitorMixin`:
+
+SpiderMonitorMixin
+------------------
+
+`SpiderMonitorMixin code`_
+
+.. _`SpiderMonitorMixin code`: https://github.com/scrapinghub/spidermon/blob/master/spidermon/contrib/monitors/mixins/spider.py
+
+This class use `StatsMonitorMixin`_ and `JobMonitorMixin`_ . `SpiderMonitorMixin` add the `crawler`, `spider` and `responses` property for be use like the examples above.
+
+This mixin create an `_response` property that is an object for `ResponsesInfo` class. This class has the stats for the response, you can get the number of all codes for requests, 
+informational, successfuls, redirections, bad requests, internal server errors, others and errors.
+
+.. _`ValidationMonitorMixin`:
+
+ValidationMonitorMixin
+----------------------
+
+`ValidationMonitorMixin code`_
+
+.. _`ValidationMonitorMixin code`: https://github.com/scrapinghub/spidermon/blob/master/spidermon/contrib/monitors/mixins/validation.py
+
+This class use `StatsMonitorMixin`_ and add the `_validation` property.