scottr.org

Asynchronous Networking with NSURLSession
iOS 7 introduces an entire new networking stack for developers,
NSURLSession. Generally it makes the common tasks simple, and the hard
tasks possible. It also drives the new background fetch capabilities as well.

Like my earlier post, offloading your networking from the main
thread is a simple way to help keep an app responsive. Doing this with
NSURLSession is actually quite easy.
```
NSURL *url = [NSURL urlWithString:@"http://iosdevelopmentjournal.com"];
NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithURL:url
                                                     completionHandler:
                          ^(NSData *data, NSURLResponse *response, NSError *error) {
    if (data) {
        // Do stuff with the data
    } else {
        NSLog(@"Failed to fetch %@: %@", url, error);
    }
}];
[task resume];
```
This will run the networking off of the main thread, and the callback will come from a system network queue. Generally this is what we want. If you want to have control over what queue the callback comes from, or to be able to handle authentication requests, you should create your own NSURLSession and delegate.
_spr_

September 18, 2013

Posts

networking, nsurlsession
Verifying UIKit is Running on the Main Thread

As previously mentioned: to get the UI to update, you need to run your
UIKit configuring code on the main thread. Peter Stienberger has posted a
great snippet of code that raises an assertion whenever you attempt to
access UIKit code from off of the main thread. Simply add the file to your
project (and swap out the PSPDFLogError with an NSLog) and build and run.
Everytime you call UIKit code off of the main thread, it’ll crash your app.
Great for finding bugs.

_spr_

June 1, 2013

Posts

gcd, uikit
Alignment Options in Auto Layout
In OS X Lion and iOS 6 Apple added a new way to layout views, Auto Layout. This is a very powerful system that will handle a lot of
the work and overhead of arranging your views. A well defined layout can make many headaches disapear entirely. However, it is a bit…quirky…to say the least.[^1]

I recommend that you read the guide to the visual format language before going on.

This post will discuss the various options that you can set in
[constraintsWithVisualFormat:options:metrics:views:][]. These options affect how the views described in the format string are to be aligned with each other.

Throughout this post, we’ll be discussing code from a sample project I’ve
created, called AutoLayoutAlignment, available in github.

What Options Do

In the constraintsWithVisualFormat:options:metrics:views:, the options
parameter is a bitmask that allows you define how the views in the format
string are aligned with each other. For example, the string
@"V:[first][second][third]", only tells you that the three views should be vertically stacked. It does not say what to do if the views are of different widths.

You could handle this by putting in a horizontal set of constraints for each
view, but this would become quickly tedious. Instead you can pass in an option to tell the layout engine how to poistion them relative to each other.

The Options

There are three sets of options: ones that apply to vertical constraints, ones
that apply to horizontal constraints and ones that adjust the direction
horizontal constraints apply. They are all part of [NSLayoutFormatOptions][]

Vertical Constraint Options
- NSLayoutFormatAlignAllLeft – This aligns all of the views on their left edge.
- NSLayoutFormatAlignAllRight – This aligns all of the views on their right edge.
- NSLayoutFormatAlignAllLeading – This aligns all of the views on the edge that is the start of text in the current locale (English: left, Hebrew: right).
- NSLayoutFormatAlignAllTrailing – This aligns all of the views on the edge that is the end of text in the current locale (English: right, Hebrew: left).
- NSLayoutFormatAlignAllCenterX – This aligns all of the views by setting their center’s X value equal to each other.
Of particular note is the leading and trailing alignments. Generally speaking
you probably want to use these instead of left and right, to allow your UI to
adjust for languages that read in the opposite direction than yours.

Horizontal Constraint Options
- NSLayoutFormatAlignAllTop – This aligns all of the views on their top edge.
- NSLayoutFormatAlignAllBottom – This aligns all of the views on their bottom edge.
- NSLayoutFormatAlignAllCenterY – This aligns all of the views by settings their center’s Y value equal to each other.
- NSLayoutFormatAlignAllBaseline – This lines up the views according to their baseline, which for text based views is the bottom of text that does not drop down (like g, p, j, etc). For non-text views it is the same as the bottom edge. When lining up text with each other it tends to look visually appealing to
  align on the baseline, but may not be what you want with a mixed set of views.
Direction Constraint Options

These can be or’d (|) into another option you pick for horizontal constraints.
- NSLayoutFormatDirectionLeadingToTrailing – The format string is read in assuming that the leading view in the string should be on the leading side of the display. Once again, based on the locale in use.
- NSLayoutFormatDirectionLeftToRight – The format string is read in assuming that the leading view in the string should be on the left side of the display.
- NSLayoutFormatDirectionRightToLeft – The format string is read in assuming that the leading view in the string should be on the right side of the display.
The default is NSLayoutFormatDirectionLeadingToTrailing and is generally what you want to use.

How They Apply

Go ahead and launch the example app, and you’ll quickly see how these alignment options work.

At the bottom center are the visual format strings being used in the two large
views on screen. The alignment settings are defined in the popover presented from the bottom left button. Try adjusting the settings for the alignment options, you should notice the text labels moving around their parent views, and hopefully see just how these work.

There is an additional setting in the app that is not an
NSLayoutFormatOptions item. The “Apply Option to Superview as Well” setting adds an additional constraint to these views. It applies a constraint that connects the top or leading view to the superview using the equivalent
[NSLayoutAttribute][] by using
[constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:].

It is important to note how the views cease to move around if you disable the
superview connecting constraint. Even though the superview is reference in the format string as |, the reference is only used for positioning purposes, and alignment options do not apply. If you do not set up the alignment to the
superview, the layout is considered ambiguous and the layout engine can’t
perfectly apply it. We’ll discuss later what ambiguity is, and how to fix it.
For now, take note that there is a button that will log to the console the
trace that will show what views on screen are ambiguous.

Conclusion

Hopefully you now understand what the options argument is for, and how you can use it to align views when defining them in the visual format string.
Please send questions my way, either via email (spr (at) iosdevelopmentjournal (dot) com) or on twitter/app.net at _spr_.

Keep an eye out for future auto layout posts that will cover issues like
multi-line labels, creating sufficient layouts, and handling dynamic view
creation.

[^1]: I will not be tackling interface builder’s use of auto layout. I do not use interface builder heavily, and its quirks are a whole different story.
_spr_

April 22, 2013

Posts

auto-layout, uikit
Swipe to Delete with Multiple Selection
The Problem:

(Solution found via StackOverflow)

You have a UITableView that has
allowsMultipleSelectionDuringEditing set to YES, since you’d like to
allows for bulk deletes or other actions. Prior to adding bulk editing support
you also could perform a swipe-to-delete when not in editing mode. However,
you’ve just noticed that swipe-to-delete no longer works. And you want both.

The Solution:

So it turns out that you can’t have allowsMultipleSelectionDuringEditing
active and have swipe-to-delete work at the same time. So what you need to do
is only enable allowsMultipleSelectionDuringEditing when the table view is in
editing mode.

This is achieved by a simple addition to your UITableViewController
sub-class.
```
- (void)setEditing:(BOOL)editing animated:(BOOL)animated {
    [super setEditing:editing animated:animated];
    self.tableView.allowsMultipleSelectionDuringEditing = editing;
}
```
And with that swipe-to-delete works, and so does bulk editing!
_spr_

April 22, 2013

Posts

uitableview
Don’t Reference View Controller Views in init
This is more of a note to my future self:

Don’t reference a view controller’s view in an init method.

Why not? Well, let me tell you what I did.

The Setup

Custom UITableViewController subclass. Has a few custom variables that
store useful data. An array of things to display, some basic text, etc. These
all get set in a custom init method: initWithTitle:Items:. All very simple.
These variables then get used in viewDidLoad to setup the table view, etc.

The Problem
```
[self.tableview registerClass:[UITableViewCell class] forCellReuseIdentifier:@"CELLID"]
```
Running that in my init function results in viewDidLoad running before I had
set up the other variables. Which meant viewDidLoad was running before any of
its variables were initialized correctly. Needless to say, this resulted in
some strange behavior.

The Solution

Stop referencing the view in the init method.
_spr_

March 21, 2013

Posts

mistakes, uiviewcontroller
MPMoviePlayerController Going Fullscreen Creates A Blackout

The Problem:

(Solution found via StackOverflow)

You’ve added an MPMoviePlayerController to one of your view controller’s
child views. You hit play, it starts playing the video. Then you hit the
fullscreen button. It fills the screen, and suddenly everything goes black, you
can’t get any controls, and all that happens is the audio keeps playing.

So what went wrong?

What Happened

So my best guess is that the view that contains the MPMoviePlayerController
view is a temporary one in your view controller. Something that you get rid of
when the view controller’s view stops being displayed. So what you decided was
to remove the view from the view controller (removeFromSuperview) when the
view controller is no long displayed.

The obvious place to do that is in viewWillDisappear:, right? It
consolidates all these actions into one place, and happens automatically.

The problem? When the movie goes fullscreen, your view disappears and
viewWillDisappear: is called.[^1] MPMovePlayerController doesn’t work well
when its view gets removed from the view hierarchy. Whoops.

The Solution

I solved this by moving my view destroying code into a new method and out of
viewWillDisappear:. In my case all the times I wanted the view to be removed
are times when I’m triggering viewWillDisappear: manually with a call to
pushViewController:animated: to the navigationController. So right
above that call, I added a call to my new method.

If you have situations where you don’t control the transition that causes your
view to be destroyed, you might want to add a BOOL to safegaurd the view from
being destroyed if video playback is happening.

[^1]: You might run into this problem transitioning from iOS 5 to iOS 6. Apple fixed a bug where viewWillDisappear: wasn’t being called for the fullscreen transition. So this doesn’t happen in iOS 5, but does in iOS 6.

_spr_

March 16, 2013

Posts

media, uiviewcontroller

Asynchronous Networking with NSURLSession

Verifying UIKit is Running on the Main Thread

Alignment Options in Auto Layout

What Options Do

The Options

Vertical Constraint Options

Horizontal Constraint Options

Direction Constraint Options

How They Apply

Conclusion

Swipe to Delete with Multiple Selection

The Problem:

The Solution:

Don’t Reference View Controller Views in init

The Setup

The Problem

The Solution

MPMoviePlayerController Going Fullscreen Creates A Blackout

The Problem:

What Happened

The Solution