Update dependencies.md

Author: samdark

book/dependencies.md

@@ -25,11 +25,80 @@ it doesn't care how these dependencies are satisfied.
 
 You can't eliminate dependencies altogether but you can make them more flexible.
 
-## Inversion of control
-
-
 ## Dependency injection
 
+There are two ways of re-using things in OOP: inheritance and composition.
+
+Inheritance is simple:
+
+```php
+class Cache
+{
+    public function getCachedValue($key)
+    {
+        // ..
+    }
+}
+
+class CachedWidget extends Cache
+{
+    public function render()
+    {
+        $output = parent::getCachedValue('cachedWidget');
+        if ($output !== null) {
+            return $output;
+        }
+        // ...        
+    }
+}
+```
+
+The issue here is that these two are becoming unnecessarily coupled or inter-dependent making them more fragile.
+
+Another way to handle it is composition:
+
+```php
+interface CacheInterface
+{
+    public function getCachedValue($key);
+}
+
+class Cache implements CacheInterface
+{
+    public function getCachedValue($key)
+    {
+        // ..
+    }
+}
+
+class CachedWidget
+{
+    private $cache;
+
+    public function __construct(CacheInterface $cache)
+    {
+        $this->cache = $cache;
+    }
+    
+    public function render()
+    {
+        $output = $this->cache('cachedWidget');
+        if ($output !== null) {
+            return $output;
+        }
+        // ...        
+    }
+}
+```
+
+In the above we've avoided unnecessary inheritance and used interface to reduce coupling. You can replace cache implementation without changing `CachedWidget` so it's becoming more stable.
+
+The process of getting an instance of `CacheInterface` into `CachedWidget` is called dependency injection. There are multiple ways to do so:
+
+- Constructor. Preferred for mandatory dependencies.
+- Setter. OK for optional dependencies.
+- Property. Potentially violates encapsulation.
+
 ## Dependency container
 
 Injecting basic dependencies is simple and easy. You're choosing a place where you don't care about dependencies, which is usually controller which you aren't going to unit-test ever, create instances of dependencies needed and pass these to dependent classes.
@@ -45,3 +114,5 @@ Additionally, lots of dependencies, such as certain third party API wrapper, are
 That's what dependency containers are for.
 
 See [official guide](http://www.yiiframework.com/doc-2.0/guide-concept-di-container.html) for more information about Yii's dependency container.
+
+## Inversion of control