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

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

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

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.

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.

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.