[CONTEXT] adds context theory to the slides

go-training.slide

@@ -1600,6 +1600,50 @@ Tasks:
     Peek:solutions/concurrency3
 
 
+#----------------------------------------------
+* Context
+#----------------------------------------------
+* Context
+
+The context type carries deadlines, cancellation signals, across API boundaries and between processes.
+
+- Incoming requests to a server should create a Context.
+
+- Outgoing calls to servers should accept a Context.
+
+The Context should be the first parameter, typically named ctx:
+
+    func (s *service) SendEmail(ctx context.Context, recipientAddress string, body string) error {
+        ...
+    }
+
+#----------------------------------------------
+* Context
+
+.image examples/images/context.PNG  345 775
+#----------------------------------------------
+* Context
+
+Default context
+
+    ctx := context.Background()
+
+Context with cancel
+
+    ctx, cancel := context.WithCancel(context.Background())
+    defer cancel()
+
+Context with timeout
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5 * time.Second)
+	defer cancel()
+
+Context with assigned value
+
+	ip := "192.0.2.1"
+	ctx := context.WithValue(context.Background(), "userIP", ip)
+	ctx.Value("userIP")
+
 #----------------------------------------------
 * Generics
 #----------------------------------------------