The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic
-- atomic
was added as a keyword in recent versions of llvm/clang).
Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _
prepended to their name to prevent accidental direct access).
With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic
, no such guarantees are made. Thus, nonatomic
is considerably faster than "atomic".
What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.
Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.
Adding to this:
atomicity
of a single property also cannot guarantee thread safety when multiple dependent properties are in play.
Consider:
@property(atomic, copy) NSString *firstName;
@property(atomic, copy) NSString *lastName;
@property(readonly, atomic, copy) NSString *fullName;
In this case, thread A could be renaming the object by calling setFirstName:
and then calling setLastName:
. In the meantime, thread B may call fullName
in between thread A's two calls and will receive the new first name coupled with the old last name.
To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName
while the dependent properties are being updated.
There's no way to set the vertical-align on a UILabel
, but you can get the same effect by changing the label's frame. I've made my labels orange so you can see clearly what's happening.
Here's the quick and easy way to do this:
[myLabel sizeToFit];
If you have a label with longer text that will make more than one line, set numberOfLines
to 0
(zero here means an unlimited number of lines).
myLabel.numberOfLines = 0;
[myLabel sizeToFit];
Longer Version
I'll make my label in code so that you can see what's going on. You can set up most of this in Interface Builder too. My setup is a View-Based App with a background image I made in Photoshop to show margins (20 points). The label is an attractive orange color so you can see what's going on with the dimensions.
- (void)viewDidLoad
{
[super viewDidLoad];
// 20 point top and left margin. Sized to leave 20 pt at right.
CGRect labelFrame = CGRectMake(20, 20, 280, 150);
UILabel *myLabel = [[UILabel alloc] initWithFrame:labelFrame];
[myLabel setBackgroundColor:[UIColor orangeColor]];
NSString *labelText = @"I am the very model of a modern Major-General, I've information vegetable, animal, and mineral";
[myLabel setText:labelText];
// Tell the label to use an unlimited number of lines
[myLabel setNumberOfLines:0];
[myLabel sizeToFit];
[self.view addSubview:myLabel];
}
Some limitations of using sizeToFit
come into play with center- or right-aligned text. Here's what happens:
// myLabel.textAlignment = NSTextAlignmentRight;
myLabel.textAlignment = NSTextAlignmentCenter;
[myLabel setNumberOfLines:0];
[myLabel sizeToFit];
The label is still sized with a fixed top-left corner. You can save the original label's width in a variable and set it after sizeToFit
, or give it a fixed width to counter these problems:
myLabel.textAlignment = NSTextAlignmentCenter;
[myLabel setNumberOfLines:0];
[myLabel sizeToFit];
CGRect myFrame = myLabel.frame;
// Resize the frame's width to 280 (320 - margins)
// width could also be myOriginalLabelFrame.size.width
myFrame = CGRectMake(myFrame.origin.x, myFrame.origin.y, 280, myFrame.size.height);
myLabel.frame = myFrame;
Note that sizeToFit
will respect your initial label's minimum width. If you start with a label 100 wide and call sizeToFit
on it, it will give you back a (possibly very tall) label with 100 (or a little less) width. You might want to set your label to the minimum width you want before resizing.
Some other things to note:
Whether lineBreakMode
is respected depends on how it's set. NSLineBreakByTruncatingTail
(the default) is ignored after sizeToFit
, as are the other two truncation modes (head and middle). NSLineBreakByClipping
is also ignored. NSLineBreakByCharWrapping
works as usual. The frame width is still narrowed to fit to the rightmost letter.
Mark Amery gave a fix for NIBs and Storyboards using Auto Layout in the comments:
If your label is included in a nib or storyboard as a subview of the view
of a ViewController that uses autolayout, then putting your sizeToFit
call into viewDidLoad
won't work, because autolayout sizes and positions the subviews after viewDidLoad
is called and will immediately undo the effects of your sizeToFit
call. However, calling sizeToFit
from within viewDidLayoutSubviews
will work.
My Original Answer (for posterity/reference):
This uses the NSString
method sizeWithFont:constrainedToSize:lineBreakMode:
to calculate the frame height needed to fit a string, then sets the origin and width.
Resize the frame for the label using the text you want to insert. That way you can accommodate any number of lines.
CGSize maximumSize = CGSizeMake(300, 9999);
NSString *dateString = @"The date today is January 1st, 1999";
UIFont *dateFont = [UIFont fontWithName:@"Helvetica" size:14];
CGSize dateStringSize = [dateString sizeWithFont:dateFont
constrainedToSize:maximumSize
lineBreakMode:self.dateLabel.lineBreakMode];
CGRect dateFrame = CGRectMake(10, 10, 300, dateStringSize.height);
self.dateLabel.frame = dateFrame;
Best Answer
You want
visibleViewController
: