Add Type Factory fromJson() method

docs/index.md

@@ -114,6 +114,22 @@ $note = Type::create($array);
 
 ```
 
+You may create a type from a JSON string with the `fromJson()` method.
+JSON must have a `type` key.
+
+```php
+use ActivityPhp\Type;
+
+$json = '
+{
+    "type": "Note",
+    "content": "A content for my note"
+}';
+
+$note = Type::fromJson($json);
+
+```
+
 When a property does not exist, an Exception is thrown in strict mode.
 You can define 3 different behaviours:
 

src/ActivityPhp/Type.php

@@ -88,6 +88,27 @@ public static function create($type, array $attributes = [])
         return $class;
     }
 
+    /**
+     * Create an activitystream type from a JSON string
+     */
+    public static function fromJson(string $json): AbstractObject
+    {
+        $data = json_decode($json, true);
+
+        if (json_last_error() === JSON_ERROR_NONE
+            && is_array($data)
+        ) {
+            return self::create($data);
+        }
+
+        throw new Exception(
+            sprintf(
+                "An error occurred during the JSON decoding.\n '%s'",
+                $json
+            )
+        );
+    }
+
     /**
      * Add a custom type definition
      * It overrides defined types

tests/ActivityPhp/Type/FactoryTest.php

@@ -295,4 +295,42 @@ public function testCopyChaining()
             $original->id
         );  
     }
+
+    /**
+     * Test creating a type from a JSON string
+     */
+    public function testFromJson()
+    {
+        $json = '{"type":"Note","content":"A content for my note"}';
+
+        $note = Type::fromJson($json);
+
+        $this->assertEquals(
+            $json,
+            $note->toJson()
+        );          
+    }
+
+    /**
+     * Test creating a type from a malformed JSON string
+     */
+    public function testFromJsonMalformedJsonString()
+    {
+        $this->expectException(Exception::class);
+
+        $json = '{';
+        $note = Type::fromJson($json);
+    }
+
+    /**
+     * Test creating a type from a JSON string which does not contains
+     * an array.
+     */
+    public function testFromJsonNotAnArray()
+    {
+        $this->expectException(Exception::class);  
+
+        $json = '"OK"';
+        $note = Type::fromJson($json);
+    }
 }