What is the most robust way to force a UIView to redraw

Question

I have a UITableView with a list of items  Selecting an item pushes a viewController that then proceeds to do the following  from method viewDidLoad I fire off a URLRequest for data that is required by on of my subviews - a UIView subclass with drawRect overridden  When the data arrives from the cloud I start building my view hierarchy  the subclass in question gets passed the data and it s drawRect method now has everything it needs to render   But   Because I don t call drawRect explicitly - Cocoa-Touch handles that - I have no way of informing Cocoa-Touch that I really  really want this UIView subclass to render  When  Now would be good   I ve tried  myView setNeedsDisplay   This kinda works sometimes  Very spotty   I ve be wrestling with this for hours and hours  Could someone who please provide me with a rock solid  guaranteed approach to forcing a UIView re-render   Here is the snippet of code that feeds data to the view      Create the subview self chromosomeBlockView      ChromosomeBlockView alloc  initWithFrame frame  autorelease       Set some properties self chromosomeBlockView sequenceString       self sequenceString  self chromosomeBlockView nucleotideBases      self nucleotideLettersDictionary      Insert the view in the view hierarchy  self containerView          addSubview self chromosomeBlockView    self containerView bringSubviewToFront self chromosomeBlockView       A vain attempt to convince Cocoa-Touch that this view is worthy of being displayed  -   self chromosomeBlockView setNeedsDisplay     Cheers  Doug

User · Answer

The money-back guaranteed  reinforced-concrete-solid way to force a view to draw synchronously  before returning to the calling code  is to configure the CALayer s interactions with your UIView subclass   In your UIView subclass  create a displayNow   method that tells the layer to    set course for display    then to    make it so      Swift       Redraws the view s contents immediately      Serves the same purpose as the display method in GLKView  public func displayNow         let layer   self layer     layer setNeedsDisplay       layer displayIfNeeded       Objective-C       Redraws the view s contents immediately      Serves the same purpose as the display method in GLKView  -  void displayNow       CALayer  layer   self layer       layer setNeedsDisplay        layer displayIfNeeded       Also implement a draw    CALayer  in  CGContext  method that ll call your private internal drawing method  which works since every UIView is a CALayerDelegate    Swift       Called by our CALayer when it wants us to draw          in compliance with the CALayerDelegate protocol   override func draw   layer  CALayer  in context  CGContext        UIGraphicsPushContext context      internalDraw self bounds      UIGraphicsPopContext       Objective-C       Called by our CALayer when it wants us to draw          in compliance with the CALayerDelegate protocol   -  void drawLayer  CALayer   layer inContext  CGContextRef context       UIGraphicsPushContext context        self internalDrawWithRect self bounds       UIGraphicsPopContext        And create your custom internalDraw    CGRect  method  along with fail-safe draw    CGRect    Swift       Internal drawing method  naming s up to you  func internalDraw   rect  CGRect            FILLIN  Custom drawing code goes here           Use  UIGraphicsGetCurrentContext    where necessary          For compatibility  if something besides our display method asks for draw  override func draw   rect  CGRect        internalDraw rect      Objective-C       Internal drawing method  naming s up to you  -  void internalDrawWithRect  CGRect rect           FILLIN  Custom drawing code goes here           Use  UIGraphicsGetCurrentContext    where necessary          For compatibility  if something besides our display method asks for draw  -  void drawRect  CGRect rect        self internalDrawWithRect rect       And now just call myView displayNow   whenever you really-really need it to draw  such as from a CADisplayLink callback     Our displayNow   method will tell the CALayer to displayIfNeeded    which will synchronously call back into our draw    in   and do the drawing in internalDraw      updating the visual with what s drawn into the context before moving on     This approach is similar to  RobNapier s above  but has the advantage of calling displayIfNeeded   in addition to setNeedsDisplay    which makes it synchronous   This is possible because CALayers expose more drawing functionality than UIViews do    layers are lower-level than views and designed explicitly for the purpose of highly-configurable drawing within the layout  and  like many things in Cocoa  are designed to be used flexibly  as a parent class  or as a delegator  or as a bridge to other drawing systems  or just on their own    Proper usage of the CALayerDelegate protocol makes all this possible   More information about the configurability of CALayers can be found in the Setting Up Layer Objects section of the Core Animation Programming Guide

User · Answer

The guaranteed  rock solid way to force a UIView to re-render is  myView setNeedsDisplay   If you re having trouble with that  you re likely running into one of these issues    You re calling it before you actually have the data  or your -drawRect  is over-caching something  You re expecting the view to draw at the moment you call this method  There is intentionally no way to demand  draw right now this very second  using the Cocoa drawing system  That would disrupt the entire view compositing system  trash performance and likely create all kinds of artifacting  There are only ways to say  this needs to be drawn in the next draw cycle     If what you need is  some logic  draw  some more logic   then you need to put the  some more logic  in a separate method and invoke it using -performSelector withObject afterDelay  with a delay of 0  That will put  some more logic  after the next draw cycle  See this question for an example of that kind of code  and a case where it might be needed  though it s usually best to look for other solutions if possible since it complicates the code    If you don t think things are getting drawn  put a breakpoint in -drawRect  and see when you re getting called  If you re calling -setNeedsDisplay  but -drawRect  isn t getting called in the next event loop  then dig into your view hierarchy and make sure you re not trying to outsmart is somewhere  Over-cleverness is the  1 cause of bad drawing in my experience  When you think you know best how to trick the system into doing what you want  you usually get it doing exactly what you don t want

User · Answer

I had a problem with a big delay between calling setNeedsDisplay and drawRect   5 seconds   It turned out I called setNeedsDisplay in a different thread than the main thread  After moving this call to the main thread the delay went away   Hope this is of some help

User · Answer

I had the same problem  and all the solutions from SO or Google didn t work for me  Usually  setNeedsDisplay does work  but when it doesn t    I ve tried calling setNeedsDisplay of the view just every possible way from every possible threads and stuff - still no success  We know  as Rob said  that       this needs to be drawn in the next draw cycle     But for some reason it wouldn t draw this time  And the only solution I ve found is calling it manually after some time  to let anything that blocks the draw pass away  like this   dispatch time t popTime   dispatch time DISPATCH TIME NOW                                            int64 t  0 005   NSEC PER SEC    dispatch after popTime  dispatch get main queue      void         viewToRefresh setNeedsDisplay         It s a good solution if you don t need the view to redraw really often  Otherwise  if you re doing some moving  action  stuff  there is usually no problems with just calling setNeedsDisplay   I hope it will help someone who is lost there  like I was

User · Answer

Well I know this might be a big change or even not suitable for your project  but did you consider not performing the push until you already have the data  That way you only need to draw the view once and the user experience will also be better - the push will move in already loaded   The way you do this is in the UITableView didSelectRowAtIndexPath you asynchronously ask for the data  Once you receive the response  you manually perform the segue and pass the data to your viewController in prepareForSegue  Meanwhile you may want to show some activity indicator  for simple loading indicator check https   github com jdg MBProgressHUD

[cocoa-touch] What is the most robust way to force a UIView to redraw?

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