better method retry implementation In practical it isn't necessary to passed err and current as parameter to finalErrorCallback(which the parameter of method retry) due to function closure. Hence some thing changed:

• modified method retry
• removed the member maxRetry from task object and ITask
• removed the member error from default plan's interface ICurrent

cancel plan's default info The task's info is designed to store the task special information so it can be used in callback as a member of current. With that in mind, the task info should be used in task instead of in plan. Hence the need to cancel the confusing support of plan's default info:

• modify IPlan, stream plan, function startTask, download plan, default plan

add parameter check to method save;
adapt the document and unit test to the change;