GCD to perform task in main thread

Question

I have a callback which might come from any thread  When I get this callback  then I would like to perform a certain task on the main thread   Do I need to check whether I already am on the main thread - or is there any penalty by not performing this check befora calling the code below   dispatch async dispatch get main queue             do work here

User · Accepted Answer

No  you do not need to check whether you   re already on the main thread  By dispatching the block to the main queue  you   re just scheduling the block to be executed serially on the main thread  which happens when the corresponding run loop is run   If you already are on the main thread  the behaviour is the same  the block is scheduled  and executed when the run loop of the main thread is run

User · Answer

No you don t need to check if you re in the main thread  Here is how you can do this in Swift   runThisInMainThread      - gt  Void in     runThisInMainThread      - gt  Void in            No problem          func runThisInMainThread block  dispatch block t        dispatch async dispatch get main queue    block      Its included as a standard function in my repo  check it out  https   github com goktugyil EZSwiftExtensions

User · Answer

For the asynchronous dispatch case you describe above  you shouldn t need to check if you re on the main thread   As Bavarious indicates  this will simply be queued up to be run on the main thread   However  if you attempt to do the above using a dispatch sync   and your callback is on the main thread  your application will deadlock at that point   I describe this in my answer here  because this behavior surprised me when moving some code from -performSelectorOnMainThread    As I mention there  I created a helper function   void runOnMainQueueWithoutDeadlocking void   block  void         if   NSThread isMainThread                 block              else               dispatch sync dispatch get main queue    block             which will run a block synchronously on the main thread if the method you re in isn t currently on the main thread  and just executes the block inline if it is   You can employ syntax like the following to use this   runOnMainQueueWithoutDeadlocking          Do stuff

User · Answer

As the other answers mentioned  dispatch async from the main thread is fine   However  depending on your use case  there is a side effect that you may consider a disadvantage  since the block is scheduled on a queue  it won t execute until control goes back to the run loop  which will have the effect of delaying your block s execution   For example   NSLog   before dispatch async    dispatch async dispatch get main queue           NSLog   inside dispatch async block main thread from main thread        NSLog   after dispatch async      Will print out   before dispatch async after dispatch async inside dispatch async block main thread from main thread   For this reason  if you were expecting the block to execute in-between the outer NSLog s  dispatch async would not help you

[objective-c] GCD to perform task in main thread

Examples related to objective-c

Examples related to grand-central-dispatch