{"id":2429,"date":"2017-09-04T00:19:10","date_gmt":"2017-09-04T04:19:10","guid":{"rendered":"http:\/\/www.ikriv.com\/blog\/?p=2429"},"modified":"2017-09-04T00:19:10","modified_gmt":"2017-09-04T04:19:10","slug":"tasktimer-how-to-execute-task-on-a-timer","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=2429","title":{"rendered":"TaskTimer: how to execute task on a timer"},"content":{"rendered":"

Over the week-end I created the\u00a0TaskTimer<\/code> class that allows to execute code on timer in an async<\/code> method:<\/p>\n

static<\/span>\u00a0async<\/span>\u00a0Task<\/span>\u00a0DoStuffOnTimer()\r\n{\r\n\u00a0\u00a0\u00a0\u00a0using<\/span>\u00a0(var<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).Start())\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0foreach<\/span>\u00a0(var<\/span>\u00a0task\u00a0in<\/span>\u00a0timer)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0await<\/span>\u00a0task;\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0DoStuff();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/pre>\n
Source code:\u00a0https:\/\/github.com\/ikriv\/tasktimer<\/a><\/p>\n
Nuget package:\u00a0https:\/\/www.nuget.org\/packages\/TaskTimer\/1.0.0<\/a><\/p>\n
Motivation<\/strong><\/p>\n
To my astonishment, I have found that .NET Task Library does not (seem to) have support for executing a task on timer, similar to Observable.Interval()<\/a><\/code>. I needed to execute an activity every second in my async\u00a0<\/code>method,\u00a0and the suggestions I found on StackOverflow<\/a> were quite awful: from using Task.Delay()<\/code> to using a dedicated thread and\u00a0Thread.Sleep()<\/code>.<\/p>\n
Task.Delay()<\/code>\u00a0is good enough in many cases, but it has a problem. Consider this code:<\/p>\n
static<\/span>\u00a0async<\/span>\u00a0Task<\/span>\u00a0UseTaskDelay()\r\n{\r\n\u00a0\u00a0\u00a0\u00a0while<\/span>\u00a0(true<\/span>)\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0DoSomething();\u00a0\/\/\u00a0takes\u00a0300ms\u00a0on\u00a0average<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0await<\/span>\u00a0Task<\/span>.Delay(1000);\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/pre>\n
One execution of the loop takes 1300ms on average, instead of \u00a01000ms we wanted. So, on every execution we will accumulate an additional delay of 300ms. If\u00a0DoSomething()<\/code> takes different time from iteration to iteration, our activity will not only be late, but it will also be irregular. One could, of course, use real-time clock to adjust the delay after every execution, but this is quite laborious and hard to encapsulate.<\/p>\n
Using TaskTimer<\/strong><\/p>\n
Browser for “TaskTimer” package on NuGet and add it to your project. Alternatively, you can get the code from GitHub. The code requires .NET 4.5 or above.<\/p>\n
You will need to choose the timer period, and the initial delay, or absolute start time:<\/p>\n
\/\/\u00a0period=1000ms,\u00a0starts\u00a0immediately<\/span>\r\nvar<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).Start();\r\n\r\n\/\/\u00a0period=1\u00a0second,\u00a0starts\u00a0immediately<\/span>\r\nvar<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(TimeSpan<\/span>.FromSeconds(1)).Start();\r\n\r\n\/\/\u00a0period=1000ms,\u00a0starts\u00a0300ms\u00a0from\u00a0now<\/span>\r\nvar<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).Start(300);\r\n\r\n\/\/\u00a0period=1000ms,\u00a0starts\u00a05\u00a0minutes\u00a0from\u00a0now<\/span>\r\nvar<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).Start(TimeSpan<\/span>.FromMinutes(5));\r\n\r\n\/\/\u00a0period=1000ms,\u00a0starts\u00a0at\u00a0next\u00a0UTC\u00a0midnight<\/span>\r\nvar<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).StartAt(DateTime<\/span>.UtcNow.Date.AddDays(1));<\/pre>\n
The returned object is a disposable enumberable.<\/em>\u00a0You must call Dispose()<\/code> on it to properly disposer of the underlying timer object. The enumerable iterates over an infinite series of tasks.\u00a0\u00a0Task number N becomes completed after the timer ticks N times. \u00a0The enumerable can be enumerated only once.<\/p>\n
Avoiding infinite loop<\/strong><\/p>\n
The enumerable returned by the timer is infinite, so you cannot get all the task at once.<\/p>\n
\/\/\u00a0won't\u00a0work;\u00a0this\u00a0would\u00a0require\u00a0an\u00a0array\u00a0of\u00a0infinite\u00a0length<\/span>\r\nvar<\/span>\u00a0tasks\u00a0=\u00a0timer.ToArray();<\/pre>\n
You can use foreach<\/code> on the timer, but that would be an infinite loop:<\/p>\n
using<\/span>\u00a0(var<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).Start())\r\n{\r\n\u00a0\u00a0\u00a0\u00a0\/\/\u00a0infinite\u00a0loop<\/span>\r\n\u00a0\u00a0\u00a0\u00a0foreach<\/span>\u00a0(var<\/span>\u00a0task\u00a0in<\/span>\u00a0timer)\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0await<\/span>\u00a0task;\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0DoSomethingUseful();\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/pre>\n
If you know the number of iterations in advance, use Take()<\/code> method:<\/p>\n
\/\/\u00a0loop\u00a0executes\u00a010\u00a0times<\/span>\r\nforeach<\/span>\u00a0(var<\/span>\u00a0task\u00a0in<\/span>\u00a0timer.Take(10))\r\n{\r\n\u00a0\u00a0\u00a0\u00a0await<\/span>\u00a0task;\r\n\u00a0\u00a0\u00a0\u00a0DoSomethingUseful();\r\n}<\/pre>\n
If you don’t, you can use a cancellation token:<\/p>\n
async<\/span>\u00a0Task<\/span>\u00a0Tick(CancellationToken<\/span>\u00a0token)\r\n{\r\n\u00a0\u00a0\u00a0\u00a0using<\/span>\u00a0(var<\/span>\u00a0timer\u00a0=\u00a0new<\/span>\u00a0TaskTimer<\/span>(1000).CancelWith(token).Start())\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0try<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0foreach<\/span>\u00a0(var<\/span>\u00a0task\u00a0in<\/span>\u00a0timer)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0await<\/span>\u00a0task;\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0DoSomethingUseful();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0catch<\/span>\u00a0(TaskCanceledException<\/span>)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Console<\/span>.WriteLine(\"Timer\u00a0Canceled\"<\/span>);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/pre>\n
About Disposable Enumerable<\/strong><\/p>\n
I initially implemented the timer sequence as generator method using the yield<\/code> keyword. You can see this in the GitHub history:<\/p>\n
public<\/span>\u00a0IEnumerable<\/span><Task<\/span>>\u00a0StartOnTimer(...)\r\n{\r\n\u00a0\u00a0\u00a0\u00a0using<\/span>\u00a0(var<\/span>\u00a0taskTimer\u00a0=\u00a0new<\/span>\u00a0TaskTimerImpl<\/span>(...))\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0while<\/span>\u00a0(true<\/span>)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0yield<\/span>\u00a0return<\/span>\u00a0taskTimer.NextTask();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/pre>\n
So, I decided to switch to a schema where the timer gets started immediately upon calling of the Start()<\/code> method. In reactive terms, TaskTimer<\/code> is a hot observable. To stop the timer, one needs to dispose the return value of the Start()<\/code> method, hence the disposable enumerable. To use the timer properly, one should put a foreach<\/code> loop inside a using<\/code> block like in the examples, above.<\/p>\n
Also, it is only possible to iterate over the timer once. The following will not work:<\/p>\n
using<\/span> (var<\/span> timer = new<\/span> TaskTimer<\/span>(1000).Start())\r\n{\r\n    \/\/ loop executes 10 times<\/span>\r\n    foreach<\/span> (var<\/span> task in<\/span> timer.Take(10))\r\n    {\r\n        await<\/span> task;\r\n        DoSomethingUseful();\r\n    }\r\n\r\n    \/\/ execute 20 more times -- WON'T WORK! Throws InvalidOperationException!<\/span>\r\n    foreach<\/span> (var<\/span> task in<\/span> timer.Take(20)) ...\r\n}\r\n<\/pre>\n
It should be possible to implement a timer with multiple iteration semantics, but I decided it was not worth the complexity.<\/p>\n
Conclusion<\/b><\/p>\n
It’s a shame Task Library does not provide timer object out of the box. I hope I was able to fill the gap, at least to some extent. In the process I ran into interesting problems with the way IEnumerable<\/code>, IEnumerator<\/code> and IDisposable<\/code> intersect, this is a fascinating world. I hope you do enjoy the result!<\/p>\n","protected":false},"excerpt":{"rendered":"
Over the week-end I created the\u00a0TaskTimer class that allows to execute code on timer in an async method: static\u00a0async\u00a0Task\u00a0DoStuffOnTimer() { \u00a0\u00a0\u00a0\u00a0using\u00a0(var\u00a0timer\u00a0=\u00a0new\u00a0TaskTimer(1000).Start()) \u00a0\u00a0\u00a0\u00a0{ \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0foreach\u00a0(var\u00a0task\u00a0in\u00a0timer) \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0{ \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0await\u00a0task; \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0DoStuff(); \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0} \u00a0\u00a0\u00a0\u00a0} } Source […]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"Layout":"","footnotes":""},"categories":[3,4],"tags":[],"class_list":["entry","author-ikriv","post-2429","post","type-post","status-publish","format-standard","category-dotnet","category-hack"],"_links":{"self":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2429","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2429"}],"version-history":[{"count":1,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2429\/revisions"}],"predecessor-version":[{"id":2432,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2429\/revisions\/2432"}],"wp:attachment":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2429"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2429"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2429"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
The trouble with this approach was that calling the Start()<\/code> method did not actually start the timer: the timer would be started only when one begins to iterate<\/i> over it. Furthermore, each iteration will create a new timer. I found this too confusing. I had enough trouble understanding similar behavior in observables <\/a> to inflict this on the users of my own class.<\/p>\n