Enable canceling graph node's successors. (sogou#1583)

* Enable canceling graph node's successors.

* Update tutorial-11-graph_task.md

* Update tutorial-11-graph_task.md

Author: Barenboim

docs/en/tutorial-11-graph_task.md

@@ -84,6 +84,29 @@ Also, any of the following codes is legal and equivalent:
 }
 ~~~
 
+# Canceling successors
+
+In graph tasks, we extend SeriesWork's **cancel** operation. When the series of a graph node is canceled, the operation will apply on all it's successive nodes recursively. The **cancel** operation is usually used in a task's callback:
+~~~cpp
+int main()
+{
+    WFGraphTask *graph = WFTaskFactory::create_graph_task(graph_callback);
+    WFHttpTask *task = WFTaskFactory::create_http_task(url, 0, 0, [](WFHttpTask *t){
+        if (t->get_state() != WFT_STATE_SUCCESS)
+            series_of(t)->cancel();
+    });
+    WFGraphNode& a = graph->create_graph_node(task);
+    WFGraphNode& b = ...;
+    WFGraphNode& c = ...;
+    WFGraphNode& d = ...;
+    a-->b-->c;
+    b-->d;
+    graph->start();
+    ...
+}
+~~~
+In this case, when http task failed, nodes b, c, d will all be canceled, because the operation is recursive.
+
 # Data passing
 
 Because the tasks in a graph don't share a same series, there is no general method for passing data between graph nodes.

docs/tutorial-11-graph_task.md

@@ -85,6 +85,30 @@ WFGraphTask的create_graph_node接口，产生一个图节点并返回节点的
 接下来直接运行graph，或者把graph放入任务流中就可以运行啦，和一般的任务没有区别。  
 当然，把一个图任务变成另一个图的节点，也是完全正确的行为。
 
+# 取消后继节点
+
+在图任务里，我们扩展了series的cancel操作，这个操作会取消该节点的所有后继结点。  
+取消操作一般在节点任务的callback里执行，例如：
+~~~cpp
+int main()
+{
+    WFGraphTask *graph = WFTaskFactory::create_graph_task(graph_callback);
+    WFHttpTask *task = WFTaskFactory::create_http_task(url, 0, 0, [](WFHttpTask *t){
+        if (t->get_state() != WFT_STATE_SUCCESS)
+            series_of(t)->cancel();
+    });
+    WFGraphNode& a = graph->create_graph_node(task);
+    WFGraphNode& b = ...;
+    WFGraphNode& c = ...;
+    WFGraphNode& d = ...;
+    a-->b-->c;
+    b-->d;
+    graph->start();
+    ...
+}
+~~~
+注意取消后继节点的操作是递归的，这个例子里，如果http任务失败，b,c,d三个节点的任务都会被取消。
+
 # 数据传递
 
 图节点之间目前没有统一的数据传递方法，它们并不共享某一个series。因此，节点间数据传递需要用户解决。

src/factory/WFGraphTask.cc

@@ -39,6 +39,12 @@ WFGraphNode::~WFGraphNode()
 {
 	if (this->user_data)
 	{
+		if (series_of(this)->is_canceled())
+		{
+			for (WFGraphNode *node : this->successors)
+				series_of(node)->SeriesWork::cancel();
+		}
+
 		for (WFGraphNode *node : this->successors)
 			node->WFCounterTask::count();
 	}