Merge pull request #20 from dotkernel/issue-19

Added content to `docs/book/v4/core-features/exceptions.md`

Author: alexmerlin

docs/book/v4/core-features/exceptions.md

@@ -1,5 +1,123 @@
-# Exceptions [WIP]
+# Exceptions
 
-https://github.com/dotkernel/api/pull/269/files#
+## What are exceptions?
 
-src/App/src/Handler/ResponseTrait.php
+Exceptions are a powerful mechanism for handling errors and other exceptional conditions that may occur during the execution of a script.
+They provide a way to manage errors in a structured and controlled manner, separating error-handling code from regular code.
+
+## How we use exceptions?
+
+When it comes to handling exceptions, DotKernel API relies on the usage of easy-to-understand, problem-specific exceptions.
+
+Ou-of-the-box we provide the following custom exceptions:
+
+* `BadRequestException` thrown when:
+
+  1. client tries to create/update resource, but the data from the request is invalid/incomplete (example: client tries to create an account, but does not send the required `identity` field)
+
+* `ConflictException` thrown when:
+
+  1. resource cannot be created because a different resource with the same identifier already exists (example: cannot change existing user's identity because another user with the same identity already exists)
+  2. resource cannot change its state because it is already in the specified state (example: user cannot be activated because it is already active)
+
+* `ExpiredException` thrown when:
+
+  1. resource cannot be accessed because it expired (example: account activation link)
+  2. resource cannot be accessed because it has been consumed (example: one-time password)
+
+* `ForbiddenException` thrown when:
+
+  1. resource cannot be accessed by the authenticated client (example: client authenticated as regular user sends a `GET /admin` request)
+
+* `MethodNotAllowedException` thrown when:
+
+  1. client tries to interact with a resource via an invalid HTTP request method (example: client sends a `PATCH /avatar` request)
+
+* `NotFoundException` thrown when:
+
+  1. client tries to interact with a resource that does not exist on the server (example: client sends a `GET /resource-does-not-exist` request)
+
+* `UnauthorizedException` thrown when:
+
+  1. resource cannot be accessed because the client is not authenticated (example: unauthenticated client sends a `GET /admin` request)
+
+## How it works?
+
+During a request, if there is no uncaught exception DotKernel API will return a JSON response with the data provided by the handler that handled the request.
+
+Else, it will build and send a response based on the exception thrown:
+
+* `BadRequestException` will return a `400 Bad Request` response
+* `UnauthorizedException` will return a `401 Unauthorized` response
+* `ForbiddenException` will return a `403 Forbidden` response
+* `OutOfBoundsException` and `NotFoundException` will return a `404 Not Found` response
+* `MethodNotAllowedException` will return a `405 Method Not Allowed` response
+* `ConflictException` will return a `409 Conflict` response
+* `ExpiredException` will return a `410 Gone` response
+* `MailException`, `RuntimeException` and the generic `Exception` will return a `500 Internal Server Error` response
+
+## How to extend?
+
+In this example we will create a custom exception called `CustomException`, place it next to the already existing custom exceptions (you can use your preferred location) and finally return a custom HTTP status code when `CustomException` is encountered.
+
+### Step 1: Create exception file
+
+Navigate to the directory `src/App/src/Handler/Exception` and create a PHP class called `CustomException.php`.
+Open `CustomException.php` and add the following content:
+
+```php
+<?php
+
+declare(strict_types=1);
+
+namespace Api\App\Exception;
+
+use Exception;
+
+class CustomException extends Exception
+{
+}
+```
+
+Save and close the file.
+
+### Step 2: Use exception file
+
+Open the file `src/App/src/Handler/HomeHandler.php` and at the beginning of the `get` method, place the following code:
+
+```php
+throw new \Api\App\Exception\CustomException('some message');
+```
+
+Save and close the file.
+
+### Step 3: Test for failure
+
+Access your API's home page URL and make sure it returns `500 Internal Server Error` HTTP status code and the following content:
+
+```json
+{
+    "error": {
+        "messages": [
+            "some message"
+        ]
+    }
+}
+```
+
+### Step 4: Prepare for success
+
+Open the file `src/App/src/Handler/ResponseTrait.php` and locate the `handle` method.
+Insert the following lines of code before the first catch statement:
+
+```php
+        } catch (\Api\App\Exception\CustomException $exception) {
+            return $this->errorResponse($exception->getMessage(), StatusCodeInterface::STATUS_IM_A_TEAPOT);
+```
+
+Save and close the file.
+
+### Step 5: Test for success
+
+Again, access your API's home page URL, which should return the same content.
+Notice that this time it returns `418 I'm a teapot` HTTP status code.