补充 DropdownMenu 类的注释

app/src/main/java/net/micode/notes/ui/DropdownMenu.java

@@ -27,9 +27,16 @@ import android.widget.PopupMenu.OnMenuItemClickListener;
 
 import net.micode.notes.R;
 
+/**
+ * 下拉菜单的数据类
+ * 并封装了一个下拉菜单内容选定事件监听器
+ */
 public class DropdownMenu {
+    // 菜单按钮
     private Button mButton;
+    // 弹出菜单
     private PopupMenu mPopupMenu;
+    // 菜单
     private Menu mMenu;
 
     public DropdownMenu(Context context, Button button, int menuId) {
@@ -44,17 +51,17 @@ public class DropdownMenu {
             }
         });
     }
-
+    // 绑定监听器
     public void setOnDropdownMenuItemClickListener(OnMenuItemClickListener listener) {
         if (mPopupMenu != null) {
             mPopupMenu.setOnMenuItemClickListener(listener);
         }
     }
-
+    // 根据id获取MenuItem
     public MenuItem findItem(int id) {
         return mMenu.findItem(id);
     }
-
+    // 设置菜单按钮的文本
     public void setTitle(CharSequence title) {
         mButton.setText(title);
     }