&quot;Leading&quot; does not always mean &quot;Left&quot;. 
For RTL-written languages (locales) leading edge of the object’s alignment rectangle will be located at the right side of the object.

Quote from [Auto Layout Guide](https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/AutolayoutPG/AutoLayoutConcepts/AutoLayoutConcepts.html#//apple_ref/doc/uid/TP40010853-CH14-SW1):
&gt;The attributes leading and trailing are the same as left and right for left-to-right languages such as English, but in a right-to-left environment such as Hebrew or Arabic, leading and trailing are the same as right and left. When you create constraints, leading and trailing are the default values. You should usually use leading and trailing to make sure your interface is laid out appropriately in all languages unless you’re making constraints that should remain the same regardless of language (such as the order of master and detail panes in a split view).

Recently I was reading implementation of .NET [Hashtable][1] and encountered piece of code that I don&#39;t understand. Part of the code is:

    int num3 = 0;
    int num4;
    do
    {
       num4 = this.version;
       bucket = bucketArray[index];
       if (++num3 % 8 == 0)
         Thread.Sleep(1);
    }
    while (this.isWriterInProgress || num4 != this.version);

The whole code is within `public virtual object this[object key]` of `System.Collections.Hashtable` (mscorlib Version=4.0.0.0).

The question is:

What is the reason of having `Thread.Sleep(1)` there?

  [1]: http://msdn.microsoft.com/en-us/library/system.collections.hashtable%28v=vs.110%29.aspx

Why there is a Thread.Sleep(1) in .NET internal Hashtable?

There are a lot of discussions about whether you should use `Activities` or `Fragments`.
For example:

 - [To-Fragment-or-not-to-Fragment][1]
 - [One Activity and all other Fragments][2]
 - [How many Activities vs. Fragments][3]
 - [Use Activity or Fragment in Android Application][4]

Most of the discussions I found were released before Android 4.2.   
With Android 4.2 Google invented [nested Fragments][5].

Therefore I actually don&#39;t see any reason anymore to use more than one `Activity`.

In the early stage of `Fragments` they were supposed to be used within Apps for supporting **Tablets** and **Smartphones** in a comfortable way at the same time. 

Thus for example you have a `ListView` which can open a detail `View` on click on an item. On a Smartphone we would replace the `ListView` and show the detailed `View` instead. Whereas a **Tablet** instead of replacing the List with the detail View can show both `Views` at the same time.

----------------------

Now with nested `Fragments` there are a lot of other possibilities. In case you want to use a single `Activity`, you could store general information in the `Activity` and every `Fragment` would have access to it. 

Besides this, `Fragments` who have nested `Fragments`, could also store information for their children `Fragments`. 

With `Fragments` I can easily reuse the `Views`, I can show more than one `Fragment` at the same time and I can easly form a dialog out of a `Fragment`. This all would take me probably not more than just some copy &amp; paste actions.

If I use `Activities` instead I seriously have to change a lot to get this done. 

------
I recently implemented an **Application** where I easily could use two `Fragment-ViewPager` to get things really beautiful and dynamic (Some kind of: Today&#39;s Information - Yesterday&#39;s Information). 
In my opinion `Fragments` make our life a way easier :) 

------

**Questions:**

 - **Why should I use more than one `Activity`?**

Could you provide any good example in which the usage of multiple `Activities` makes more sense instead of using `Fragments`?

 - **Are there any good examples where you don&#39;t have any choice but to use `Activities`?**

I think most of the bigger frameworks like **Maps**, **YouTube** and co already support `Fragments`. So we don&#39;t have to rely on `Activities`. 
Also is it quite easy to deal with `NavigationBar`, `TabHosts`, `ViewPager`, `ActionBar` in case you use `Fragments`. 


----------


**From Udacity:** 

Why not always create one Activity with lots of Fragments?

 1. Increased Complexity
 2. Harder Intent handling
 3. Difficult to read, maintain and test
 4. Risk of tight coupling
 5. Security concerns

 


  [1]: http://andytsui.wordpress.com/2011/10/23/to-fragment-or-not-to-fragment/
  [2]: https://stackoverflow.com/questions/12154505/one-activity-and-all-other-fragments
  [3]: https://stackoverflow.com/questions/12363790/how-many-activities-vs-fragments
  [4]: https://stackoverflow.com/questions/18885311/use-activity-or-fragment-in-android-application?rq=1
  [5]: http://developer.android.com/about/versions/android-4.2.html#NestedFragments

To Fragment or not to Fragment - Nested Fragments against Activities. Why should I use more than one Activity?

What is the difference between `NSLayoutAttributeLeft` and `NSLayoutAttributeLeading` in iOS autolayouts?

Difference between NSLayoutAttributeLeft vs NSLayoutAttributeLeading

<p>What is the difference between <code>NSLayoutAttributeLeft</code> and <code>NSLayoutAttributeLeading</code> in iOS autolayouts?</p>


I&#39;m trying to execute a completion-block after my CAAnimation has finished. However, it seems that animation block is called before my animation completes. The animation still happens correctly though.

    [CATransaction begin];
    [self.view.layer addAnimation:self.dropAndBounceAnimation forKey:@&quot;appearance&quot;];
    [CATransaction setCompletionBlock:completionBlock];
    [CATransaction commit];

The dropAndBounceAnimation is a CAKeyFrameAnimation on position.y, with a fixed duration.

CATransaction completion being called immediately

I am getting an error after adding this code from parse.com:

    - (void)application:(UIApplication *)application
    didReceiveRemoteNotification:(NSDictionary *)userInfo
    fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
        if (application.applicationState == UIApplicationStateInactive) {
            [PFAnalytics trackAppOpenedWithRemoteNotificationPayload:userInfo];
        }
    }

I don&#39;t really understand what is going on, but I am getting this warning in the log:

&gt; You&#39;ve implemented -[&lt;UIApplicationDelegate&gt;
&gt; application:didReceiveRemoteNotification:fetchCompletionHandler:], but
&gt; you still need to add &quot;remote-notification&quot; to the list of your
&gt; supported UIBackgroundModes in your Info.plist.

I think adding in your plist file `UIBackgroundModes - remote-notification` would fix the problem,

But when I do that, it changes the words to the follow:

`Required Background Modes` -&gt; `App downloads content in response to push notifications` 

Which my app doesn&#39;t do, so I am confused as to why I am doing this in the first place.


Parse warning from didReceiveRemoteNotification:fetchCompletionHandler

I am a little confused about block usage in Objective-C. I currently use ARC and I have quite a lot of blocks in my app, currently always referring to `self` instead of its weak reference. May that be the cause of these blocks retaining `self` and keeping it from being dealloced ? The question is, should I always use a `weak` reference of `self` in a block ? 

    -(void)handleNewerData:(NSArray *)arr
    {
        ProcessOperation *operation =
        [[ProcessOperation alloc] initWithDataToProcess:arr
                                             completion:^(NSMutableArray *rows) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [self updateFeed:arr rows:rows];
            });
        }];
        [dataProcessQueue addOperation:operation];
    }

**ProcessOperation.h**

    @interface ProcessOperation : NSOperation
    {
        NSMutableArray *dataArr;
        NSMutableArray *rowHeightsArr;
        void (^callback)(NSMutableArray *rows);
    }

**ProcessOperation.m**

    -(id)initWithDataToProcess:(NSArray *)data completion:(void (^)(NSMutableArray *rows))cb{
        
        if(self =[super init]){
            dataArr = [NSMutableArray arrayWithArray:data];
            rowHeightsArr = [NSMutableArray new];
            callback = cb;
        }
        return self;
    }

    - (void)main {
        @autoreleasepool {
            ...
            callback(rowHeightsArr);
        }
    }


Always pass weak reference of self into block in ARC?

I am sending a zip file to server via HTTPREQUEST. What should be the [`Content-Type`][1] HTTP header *value* for this kind of file?

The file is a ZIP archive that contains images on type PNG.

Thanks 


  [1]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type

ZIP file content type for HTTP request

I have an app on app store, which is using an iOS Provisioning Profile (Distribution) which is expired.
This Profile contains Push Certificate that&#39;s also expired (and does not appear anymore in the portal).


**Question 1**: 
Is there a way to recreate the push certificate and then renew the profile?  (I still have the push certificate (expired) on my keychain)?

**Question 2**:
Do I need to re-submit the app to app store with a new profile that contains the new Push certificate?


Since the push certificate is expired, I can probably not send notifications to the existing users of the app.

Renew Push certificate and keep current App Store App working

Given a datum containing multiple data elements, such as an object or array, is it possible to set multiple attributes on a selection with a single value function?

E.g. something like:

    var data = [{ &#39;x&#39;: 10, &#39;y&#39;: 20, &#39;r&#39;: 5 }];
    d3.select(&#39;body&#39;).append(&#39;svg&#39;).selectAll(&#39;circle&#39;)
        .data(data)
        .enter().append(&#39;circle&#39;)
        .attr(&#39;cx cy r&#39;, function (d) {
            return [d.x, d.y, d.r];
        });

instead of:

    var data = [{ &#39;x&#39;: 10, &#39;y&#39;: 20, &#39;r&#39;: 5 }];
    d3.select(&#39;body&#39;).append(&#39;svg&#39;).selectAll(&#39;circle&#39;)
        .data(data)
        .enter().append(&#39;circle&#39;)
        .attr(&#39;cx&#39;, function (d) {
            return d.x;
        });
        .attr(&#39;cy&#39;, function (d) {
            return d.y;
        });
        .attr(&#39;r&#39;, function (d) {
            return d.r;
        });


How to set multiple attributes with one value function?

Here is my code

    N = namedtuple(&quot;N&quot;, [&#39;ind&#39;, &#39;set&#39;, &#39;v&#39;])
    def solve()
        items=[]
        stack=[]
        R = set(range(0,8))
        for i in range(0,8):
            items.append(N(i,R,8))		
            stack.append(N(0,R-set(range(0,1)),i))
        while(len(stack)&gt;0): 
            node = stack.pop()
            print node
            print items[node.ind]	
            items[node.ind].v = node.v

In the last line I cant set the `items[node.ind].v` value to `node.v` as I want, and am getting the error 

    &quot;AttributeError: can&#39;t set attribute&quot;

I don&#39;t know what&#39;s wrong but it must be something based on syntax as using statements like `node.v+=1` is also showing same error. I&#39;m new to Python, so please suggest a way to make the above change possible.

AttributeError: can&#39;t set attribute in python

I was going through the source code of a website and found this piece of code.

    &lt;meta name=&quot;viewport&quot; content=&quot;width=device-width, initial-scale=1.0, user-scalable=1.0, minimum-scale=1.0, maximum-scale=1.0&quot;&gt;

I want to know what this initial scale, user-scalable, minimum-scale, maximum-scale are and what does those values mean? And also tell me what all values they accepts.

What is initial scale, user-scalable, minimum-scale, maximum-scale attribute in meta tag?

How can I declare a static attribute in Python?

Here is written how I can declare a method: 
https://stackoverflow.com/questions/735975/static-methods-in-python


How to declare a static attribute in Python?

What is the most elegant way to determine if all attributes in a javascript object are either null or the empty string?  It should work for an arbitrary number of attributes.

    {&#39;a&#39;:null, &#39;b&#39;:&#39;&#39;} //should return true for this object
    {&#39;a&#39;:1, &#39;b&#39;:&#39;&#39;} //should return false for this object
    {&#39;a&#39;:0, &#39;b&#39;:1} //should return false
    {&#39;a&#39;:&#39;&#39;, &#39;b&#39;:&#39;&#39;} //should return true

Determining if all attributes on a javascript object are null or an empty string

Is it possible to drop all NOT NULL constraints from a table in one go?

I have a big table with a lot of NOT NULL constraints and I&#39;m searching for a solution that is faster than dropping them separately.

How to drop all NOT NULL constraints from a PostgreSQL table in one go

So I have two UILabels side by side in Storyboard. The second label should butt up against the right edge of the first one (trailing constraint of 1), but I also need the first label (the one on the left) to set it&#39;s width equal to it&#39;s content size unless it hits a max width. Visually:


------------------
|Label one text| |Label two text|

------------------

And I need the following constraints:

1) Label one should resize width wise unless it hits a max size.

2) Label two should always be butted up against the right edge of label one 

How do I set this up in Storyboard?

Auto Layout to dynamically size uilabel width

Here&#39;s the scenario: I&#39;ve written some code with a type signature and GHC complains could not deduce x ~ y for some `x` and `y`. You can usually throw GHC a bone and simply add the isomorphism to the function constraints, but this is a bad idea for several reasons:

  1. It does not emphasize understanding the code.
  2. You can end up with 5 constraints where one would have sufficed (for example, if the 5 are implied by one more specific constraint)
  3. You can end up with bogus constraints if you&#39;ve done something wrong or if GHC is being unhelpful

I just spent several hours battling case 3. I&#39;m playing with [`syntactic-2.0`][1], and I was trying to define a domain-independent version of `share`, similar to the version defined in [`NanoFeldspar.hs`][2].

I had this:

    {-# LANGUAGE GADTs, FlexibleContexts, TypeOperators #-}
    import Data.Syntactic
    
    -- Based on NanoFeldspar.hs
    data Let a where
        Let :: Let (a :-&gt; (a -&gt; b) :-&gt; Full b)

    share :: (Let :&lt;: sup,
              Domain a ~ sup,
              Domain b ~ sup,
              SyntacticN (a -&gt; (a -&gt; b) -&gt; b) fi) 
          =&gt; a -&gt; (a -&gt; b) -&gt; a
    share = sugarSym Let

and GHC `could not deduce (Internal a) ~ (Internal b)`, which is certainly not what I was going for. So either I had written some code I didn&#39;t intend to (which required the constraint), or GHC wanted that constraint due to some other constraints I had written.

It turns out I needed to add `(Syntactic a, Syntactic b, Syntactic (a-&gt;b))` to the constraint list, none of which imply `(Internal a) ~ (Internal b)`. I basically stumbled upon the correct constraints; I still don&#39;t have a systematic way to find them.

My questions are:

  1. Why did GHC propose that constraint? Nowhere in syntactic is there a constraint `Internal a ~ Internal b`, so where did GHC pull that from?
  2. In general, what techniques can be used to trace the origin of a constraint which GHC believes it needs? Even for constraints that I *can* discover myself, my approach is essentially brute forcing the offending path by physically writing down recursive constraints. This approach is basically going down an infinite rabbit hole of constraints and is about the least efficient method I can imagine.

  [1]: https://hackage.haskell.org/package/syntactic-2.0
  [2]: https://github.com/emilaxelsson/syntactic/blob/2.0/examples/NanoFeldspar.hs

Techniques for Tracing Constraints

I have a UIView which is placed on the screen via several constraints. Some of the constraints are owned by the superview, others are owned by other ancestors (e.g. perhaps the view property of a UIViewController).

I want to remove all of these old constraints, and place it somewhere new using new constraints.

How can I do this without creating an IBOutlet for every single constraint and having to remember which view owns said constraint?

To elaborate, the naive approach would be to create a bunch of IBOutlets for each of the constraints, and would then involve calling code such as:

    [viewA removeConstraint:self.myViewsLeftConstraint];
    [viewB removeConstraint:self.myViewsTopConstraint];
    [viewB removeConstraint:self.myViewsBottomConstraint];
    [self.view removeConstraint:self.myViewsRightConstraint];

The problem with this code is that even in the simplest case, I would need to create 2 IBOutlets. For complex layouts, this could easily reach 4 or 8 required IBOutlets. Furthermore, I would need to ensure that my call to remove the constraint is being called on the proper view. For example, imagine that `myViewsLeftConstraint` is owned by `viewA`. If I were to accidentally call `[self.view removeConstraint:self.myViewsLeftConstraint]`, nothing would happen.

**Note:** The method [constraintsAffectingLayoutForAxis][1] looks promising, but is intended for debugging purposes only.

----------

**Update:** Many of the answers I am receiving deal with `self.constraints`, `self.superview.constraints`, or some variant of those. These solutions won&#39;t work since those methods return only the constraints *owned* by the view, not the ones *affecting* the view.

To clarify the problem with these solutions, consider this view hierarchy:

 - Grandfather
   - Father
     - Me
         - Son
         - Daughter
     - Brother
   - Uncle

Now imagine we create the following constraints, and always attach them to their nearest common ancestor:

 - C0: Me: same top as Son (owned by Me)
 - C1: Me: width = 100 (owned by Me)
 - C2: Me: same height as Brother (owned by Father)
 - C3: Me: same top as Uncle (owned by Grandfather)
 - C4: Me: same left as Grandfather (owned by Grandfather)
 - C5: Brother: same left as Father (owned by Father)
 - C6: Uncle: same left as Grandfather (owned by Grandfather)
 - C7: Son: same left as Daughter (owned by Me)

Now imagine we want to remove all constraints affecting `Me`. Any proper solution should remove `[C0,C1,C2,C3,C4]` and nothing else.

If I use `self.constraints` (where self is Me), I will get `[C0,C1,C7]`, since those are the only constraints owned by Me. Obviously it wouldn&#39;t be enough to remove this since it is missing `[C2,C3,C4]`. Furthermore, it is removing `C7` unnecessarily.

If I use `self.superview.constraints` (where self is Me), I will get `[C2,C5]`, since those are the constraints owned by Father. Obviously we cannot remove all these since `C5` is completely unrelated to `Me`.

If I use `grandfather.constraints`, I will get `[C3,C4,C6]`. Again, we cannot remove all of these since `C6` should remain intact.

The brute force approach is to loop over each of the view&#39;s ancestors (including itself), and seeing if `firstItem` or `secondItem` are the view itself; if so, remove that constraint. This will lead to a correct solution, returning `[C0,C1,C2,C3,C4]`, and only those constraints.

However, I&#39;m hoping there is a more elegant solution than having to loop through the entire list of ancestors.

  [1]: https://developer.apple.com/library/ios/documentation/uikit/reference/uiview_class/uiview/uiview.html#//apple_ref/occ/instm/UIView/constraintsAffectingLayoutForAxis:

Remove all constraints affecting a UIView

I have a `UITextField` that I want to enlarge its width when tapped on. I set up the constraints and made sure the constraint on the left has the lower priority then the one that I am trying to animate on the right side.

Here is the code that I am trying to use.

```swift
// move the input box
UIView.animateWithDuration(10.5, animations: {
    self.nameInputConstraint.constant = 8
    }, completion: {
        (value: Bool) in
        println(&quot;&gt;&gt;&gt; move const&quot;)
})
```

This works, but it seems to just happen instantly and there doesn&#39;t seem to be any movement. I tried to set it 10 seconds to make sure I wasn&#39;t missing anything, but I got the same results.

`nameInputConstraint` is the name of the constraint that I control dragged to connect into my class from IB.

trying to animate a constraint in swift

I have spent two days trying out the various solutions for [Mixed and Pure Autolayout approaches][1] to achieve what was a trivial scrollview setup prior to autolayout, and it&#39;s now official - I must be too stupid. I am setting this up mostly in Storyboard (well, it&#39;s just the way it is).

So here&#39;s my plea for help.

Viewtree:

    UIView
    -UIView
    -UIView
    ..-UIScrollview
    ...-UIButton
    ...-UIButton
    ...-UIButton

The buttons are supposed to scroll horizontally (left to right and vice versa). Can someone *please* let me know how to set the constraints to achieve this using pure Autolayout???

--

I have tried the mixed approach, like so:

    UIView
    - UIView
    - UIView
    ..-UIScrollview
    ...-UIView (contentview)
    ....-UIButton
    ....-UIButton
    ....-UIButton

...and setting fixed width and height constraints for the `contentview` and the `translatesAutoresizingMaskIntoConstraints` settings as per Apple&#39;s TechNote. The buttons and scrollview are set up using constraints. This gets the scrollview scrolling (yay) but alas, it scrolls too far! As far as I can tell, the scroll width is somehow doubled from what I set the contentview at???!!!???

I tried the pure autolayout approach as well, both with `contentview` and without. **All** the views are `translatesAutoresizingMaskIntoConstraints=NO`, except for `self.view`. The buttons have fixed width/height constraints, and are pinned to all four edges of the scrollview. Nothing scrolls.

So I am totally baffled why I can&#39;t get it to work correctly. Any help is much appreciated, and if you need any other info, please ask!

**UPDATED Screenshot with solution -**
buttonZ constraints:

![enter image description here][5]

**EDIT @ Jamie Forrest**
So the solution turns out to be the wrong trailing constraint on the last button. Instead of 6441, the value I had set was negative, -6441. The tricky thing is, that when setting the value in storyboard, there are two options in the Pin toolbar:

![enter image description here][6]

The Current Canvas Value is negative (leading to no scroll), and the option below is positive (activating scroll). This means I&#39;m not stupid but at least half-blind I guess. *Although, to my defense, isn&#39;t it somewhat disturbing that XCode doesn&#39;t show an error for the &quot;incorrect&quot; setting?*

**EDITED AGAIN**
Now this is funny... changing the trailing value from -6441 (no scroll) to 6441 enabled scroll. But my old friend the &quot;too much contentsize&quot; was back, leading to a content size twice as large as what it should be! The solution to get the correct content scroll was to set the trailing constraint to ZERO! This is not obvious when working in Storyboard but looking at @Infinity James&#39; code, it is what it should be.

  [1]: https://developer.apple.com/library/ios/technotes/tn2154/_index.html#//apple_ref/doc/uid/DTS40013309
  [2]: http://i.stack.imgur.com/fKRRR.png
  [3]: http://i.stack.imgur.com/zLqG7.png
  [4]: http://i.stack.imgur.com/wsDZI.png
  [5]: http://i.stack.imgur.com/fdkm8.png
  [6]: http://i.stack.imgur.com/18dfp.png

How can I use Autolayout to set constraints on my UIScrollview?

Short version: 

I am having a problem with auto layout top layout guide when used in conjunction with custom transition and `UINavigationController` in iOS7. Specifically, the constraint between the top layout guide and the text view is not being honored. Has anyone encountered this issue?

---

Long version:

I have a scene which has unambiguously define constraints (i.e. top, bottom, left and right) that renders a view like so:

![right][1]

But when I use this with a custom transition on the navigation controller, the top constraint to the top layout guide seems off and it renders is as follows, as if the top layout guide was at the top of the screen, rather than at the bottom of the navigation controller:

![wrong][2]

It would appear that the &quot;top layout guide&quot; with the navigation controller is getting confused when employing the custom transition. The rest of the constraints are being applied correctly. And if I rotate the device and rotate it again, everything is suddenly rendered correctly, so it does not appear to be not a matter that the constraints are not defined properly. Likewise, when I turn off my custom transition, the views render correctly.

Having said that, `_autolayoutTrace` is reporting that the `UILayoutGuide` objects suffer from `AMBIGUOUS LAYOUT`, when I run:

    (lldb) po [[UIWindow keyWindow] _autolayoutTrace]

But those layout guides are always reported as ambiguous whenever I look at them even though I&#39;ve ensured that there are no missing constraints (I&#39;ve done the customary selecting of view controller and choosing &quot;Add missing constraints for view controller&quot; or selecting all of the controls and doing the same for them).

In terms of how precisely I&#39;m doing the transition, I&#39;ve specified an object that conforms to `UIViewControllerAnimatedTransitioning` in the `animationControllerForOperation` method:

    - (id&lt;UIViewControllerAnimatedTransitioning&gt;)navigationController:(UINavigationController *)navigationController
                                      animationControllerForOperation:(UINavigationControllerOperation)operation
                                                   fromViewController:(UIViewController*)fromVC
                                                     toViewController:(UIViewController*)toVC
    {
        if (operation == UINavigationControllerOperationPush)
            return [[PushAnimator alloc] init];
    
        return nil;
    }

And

    @implementation PushAnimator
    
    - (NSTimeInterval)transitionDuration:(id &lt;UIViewControllerContextTransitioning&gt;)transitionContext
    {
        return 0.5;
    }
    
    - (void)animateTransition:(id&lt;UIViewControllerContextTransitioning&gt;)transitionContext
    {
        UIViewController* toViewController   = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
        UIViewController* fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    
        [[transitionContext containerView] addSubview:toViewController.view];
        CGFloat width = fromViewController.view.frame.size.width;
    
        toViewController.view.transform = CGAffineTransformMakeTranslation(width, 0);
    
        [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
            fromViewController.view.transform = CGAffineTransformMakeTranslation(-width / 2.0, 0);
            toViewController.view.transform = CGAffineTransformIdentity;
        } completion:^(BOOL finished) {
            fromViewController.view.transform = CGAffineTransformIdentity;
            [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
        }];
    }

    @end

I&#39;ve also done a rendition of the above, setting the `frame` of the view rather than the `transform`, with the same result.

I&#39;ve also tried manually make sure that the constraints are re-applied by calling `layoutIfNeeded`. I&#39;ve also tried `setNeedsUpdateConstraints`, `setNeedsLayout`, etc.

Bottom line, has anyone successfully married custom transition of navigation controller with constraints that use top layout guide?

  [1]: http://i.stack.imgur.com/9h6bP.png
  [2]: http://i.stack.imgur.com/Hz5Pd.png

Navigation controller top layout guide not honored with custom transition

I know that the auto layout chain consists in basically 3 different process.

 1. updating constraints
 2. layout views (here is where we get calculation of frames)
 3. display

What&#39;s is not totally clear to me is the inner difference between `-setNeedsLayout` and `-setNeedsUpdateConstraints`. From Apple Docs:&lt;br&gt; 

[**setNeedsLayout**][1]


&gt; Call this method on your application’s main thread when you want to
&gt; adjust the layout of a view’s subviews. This method makes a note of
&gt; the request and returns immediately. Because this method does not
&gt; force an immediate update, but instead waits for the next update
&gt; cycle, you can use it to invalidate the layout of multiple views
&gt; before any of those views are updated. This behavior allows you to
&gt; consolidate all of your layout updates to one update cycle, which is
&gt; usually better for performance.
&gt;  &lt;br&gt;

[**setNeedsUpdateConstraints**][2]


&gt; When a property of your custom view changes in a way that would impact
&gt; constraints, you can call this method to indicate that the constraints
&gt; need to be updated at some point in the future. The system will then
&gt; call updateConstraints as part of its normal layout pass. Updating
&gt; constraints all at once just before they are needed ensures that you
&gt; don’t needlessly recalculate constraints when multiple changes are
&gt; made to your view in between layout passes.


When I want to animate a view after modifying a constraint and animate the changes I usually call for instance:

    [UIView animateWithDuration:1.0f delay:0.0f usingSpringWithDamping:0.5f initialSpringVelocity:1 options:UIViewAnimationOptionCurveEaseInOut animations:^{
            [self.modifConstrView setNeedsUpdateConstraints];
            [self.modifConstrView layoutIfNeeded];
        } completion:NULL];

I&#39;ve found out that if I use `-setNeedsLayout` instead of `-setNeedsUpdateConstraints` everything work as expected, but if I change `-layoutIfNeeded` with `-updateConstraintsIfNeeded`, the animation won&#39;t happen.&lt;br&gt;
I&#39;ve tried to make my own conclusion:

 - `-updateConstraintsIfNeeded` only update constraints but doesn&#39;t force the layout to come into the process, thus original frames are still preserved
 - `-setNeedsLayout` calls also `-updateContraints` method

So when is ok to use one instead of the other? and about the layout methods, do I need to call them on the view that has a change in a constraint or on the parent view?


  [1]: https://developer.apple.com/reference/uikit/uiview/1622601-setneedslayout
  [2]: https://developer.apple.com/reference/uikit/uiview/1622450-setneedsupdateconstraints

setNeedsLayout vs. setNeedsUpdateConstraints and layoutIfNeeded vs updateConstraintsIfNeeded

I&#39;m been smashing my head against the wall with this for last 3 or 4 hours and I can&#39;t seem to figure it out. I have a UIViewController with a full screen UITableView inside of it (there&#39;s some other stuff on the screen, which is why I can&#39;t use a UITableViewController) and I want to get my tableHeaderView to resize with autolayout. Needless to say, it&#39;s not cooperating.

See screenshot below.

![enter image description here][1]


  [1]: http://i.stack.imgur.com/x0QoU.png


Because the overviewLabel (e.g. the &quot;List overview information here.&quot; text) has dynamic content, I&#39;m using autolayout to resize it and it&#39;s superview. I&#39;ve got everything resizing nicely, except for the tableHeaderView, which is right below Paralax Table View in the hiearchy. 

The only way I&#39;ve found to resize that header view is programatically, with the following code:

    CGRect headerFrame = self.headerView.frame;
    headerFrame.size.height = headerFrameHeight;
    self.headerView.frame = headerFrame;
    [self.listTableView setTableHeaderView:self.headerView];

In this case, headerFrameHeight is a manual calculation of the tableViewHeader height as follows *(innerHeaderView is the white area, or the second &quot;View&quot;, headerView is tableHeaderView)*: 

    CGFloat startingY = self.innerHeaderView.frame.origin.y + self.overviewLabel.frame.origin.y;
    CGRect overviewSize = [self.overviewLabel.text
                           boundingRectWithSize:CGSizeMake(290.f, CGFLOAT_MAX)
                           options:NSStringDrawingUsesLineFragmentOrigin
                           attributes:@{NSFontAttributeName: self.overviewLabel.font}
                           context:nil];
    CGFloat overviewHeight = overviewSize.size.height;
    CGFloat overviewPadding = ([self.overviewLabel.text length] &gt; 0) ? 10 : 0; // If there&#39;s no overviewText, eliminate the padding in the overall height.
    CGFloat headerFrameHeight = ceilf(startingY + overviewHeight + overviewPadding + 21.f + 10.f);

The manual calculation works, but it&#39;s clunky and prone to error if things change in the future. What I want to be able to do is have the tableHeaderView auto-resize based on the provided constraints, like you can anywhere else. But for the life of me, I can&#39;t figure it out.

There&#39;s several posts on SO about this, but none are clear and ended up confusing me more. Here&#39;s a few:

- https://stackoverflow.com/questions/14637376/auto-layout-on-uitableviewheader

- https://stackoverflow.com/questions/18281691/auto-layout-for-tableheaderview

- https://stackoverflow.com/questions/16471846/is-it-possible-to-use-autolayout-with-uitableviews-tableheaderview

- https://stackoverflow.com/questions/19005446/table-header-view-height-is-wrong-when-using-auto-layout-ib-and-font-sizes


It doesn&#39;t really make sense to change the translatesAutoresizingMaskIntoConstraints property to NO, since that just causes errors for me and doesn&#39;t make sense conceptually anyway.

Any help would really be appreciated!

**EDIT 1:**
Thanks to TomSwift&#39;s suggestion, I was able to figure it out. Instead of manually calculating the height of the overview, I can have it calculated for me as follows and then re-set the tableHeaderView as before.

    [self.headerView setNeedsLayout];
    [self.headerView layoutIfNeeded];
    CGFloat height = [self.innerHeaderView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height + self.innerHeaderView.frame.origin.y; // adding the origin because innerHeaderView starts partway down headerView.
    
    CGRect headerFrame = self.headerView.frame;
    headerFrame.size.height = height;
    self.headerView.frame = headerFrame;
    [self.listTableView setTableHeaderView:self.headerView];

**Edit 2:** As others have noted, the solution posted in Edit 1 doesn&#39;t seem to work in viewDidLoad. It does, however, seem to work in viewWillLayoutSubviews. Example code below:
    
    // Note 1: The variable names below don&#39;t match the variables above - this is intended to be a simplified &quot;final&quot; answer.
    // Note 2: _headerView was previously assigned to tableViewHeader (in loadView in my case since I now do everything programatically).
    // Note 3: autoLayout code can be setup programatically in updateViewConstraints.
    - (void)viewWillLayoutSubviews {
        [super viewWillLayoutSubviews];
        
        [_headerWrapper setNeedsLayout];
        [_headerWrapper layoutIfNeeded];
        CGFloat height = [_headerWrapper systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
        
        CGRect headerFrame = _headerWrapper.frame;
        headerFrame.size.height = height;
        _headerWrapper.frame = headerFrame;
        _tableView.tableHeaderView = _headerWrapper;
    }

Content Type	Original Author	Original Content on Stackoverflow
Question	Raj iOS	View Question on Stackoverflow
Solution 1 - Ios	Stanislav Borzenko	View Answer on Stackoverflow

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