After merging master to my working branch I got compiler error on the line, which wasn't be changed. The error looks like
id test;
[test count];
Multiple methods named 'count' found with mismatched result.
At first it looks clear, because compiler doesn't know which concrete type the "test" variable is. But I don't understand why it worked before.
If I create a new file this line works, assuming that is a NSArray's method. Why compiler doesn't show error in this case?
While showing error message, there is several possible receivers of count method are shown. (NSArray, NSDictionary, NSSet) Does it search all classes that can receive that message and show error if there are multiple?
I noticed that error occurs when I import "-Swift.h" file. How it depends?
Compiler doesn't cast or check your id type. It just provides you all possible selectors. You said that this issue connected with importing "-Swift.h" file. In this case check you Swift code, probably you have count function visible for Objective C which returns something else than Int.
Also, you can check the issue in Issue navigator, select it and it will show all count calls visible in Objective C. Check them all, most of them will return NSUInteger, but there should be one that returns something else, for example:
SWIFT_CLASS("_TtC3dev19YourClass")
#interface YourClass : NSObject
- (int32_t)count SWIFT_WARN_UNUSED_RESULT;
#end
Objective-C doesn't need to know the type of the receiver. At run-time, all objects are just id, and everything is dynamically dispatched. So any message can be sent to any object, no matter its type. (At run-time, objects are free to decide what to do with messages they don't understand. The most common thing to do is raise an exception and crash, but there are many kinds of objects that can handle arbitrary messages that don't map directly to method calls.)
There is a couple of technical details, however, that complicate this.
The ABI (application binary interface) defines different mechanisms for returning certain primitive types. As long as the value is "a word-sized integer," then it doesn't matter (this includes things like NSInteger and all pointers, which means by extension all objects). But on some processors, floats are returned in different registers than integers, and structs (like CGRect) might be returned in a variety of ways depending on their size. In order to write the necessary assembly language, the compiler has to know what kind of return value it will be.
ARC has added additional wrinkles that require that the compiler know a more about the type of the parameters (specifically whether they're objects or primitives), and whether there are any memory-management attributes that have to be considered.
The compiler doesn't really care what "real" type test is, as long as it can figure out the types and attributes of -count. So when dealing with an id value, it looks through every known selector it can see (i.e. every one defined in an included header or the current .m). It's fine if there are many of them on different classes, as long as they all agree. But if it can't find the selector at all, or if some of the interfaces disagree, then it can't compile the line of code.
As lobstah notes, you likely have a type somewhere in your Swift code that has an #objc method called count() or an #objc property named count that returns something other than Int (which maps to NSInteger, and so match the usual signature of -count). You'll need to fix that method, or you'll need to hide it from ObjC (for example, by adding #nonobjc).
Or much better: get rid of the id, and use its actual type. id is generally a bad idea in Cocoa, and is especially a bad idea if you're calling methods on it, since the compiler can't check that the object will respond and you may crash.
I am creating a set of API and some users have suggested that I use id type for a particular method that can accept custom object (defined by the API) or string instead of creating two versions. Is the use of id type in method a good or acceptable practice? Does Apple do it with their any of their API?
That would be very poor practice. If you're creating an API you need to retain full control, and allowing users to pass any object to your method at which point you would have to cast it to that object or string you mentioned could be fatal depending on what's passed. Creating two methods with different parameters is not only okay, but follows the tenets of polymorphism to the T.
Accepting id is not in itself good or bad practice. How much manual procedural if/then/else/if/then/else nonsense will you acquire? If quite a lot then something is wrong.
Put another way: if the conditional logic related to different kinds of object ends up being implicit, via the Objective-C dispatch mechanisms, then the design is good. If you end up impliedly reimplementing dynamic dispatch then you've gone completely wrong.
Apple does it frequently. Just off the top of my head there are:
as per Nikolai's comment, all the collection types: set, dictionary, array, etc.
anything that takes %# as a format specifier: NSLog, certain methods on NSString, etc.
anything that still uses an informal protocol.
anything in or semi-close to the runtime like key-value coding.
archiving and the user defaults.
anywhere that storage is offered for your own use — the hardy userInfo on NSTimer and the rest.
anywhere that target/action is used — all UIControls, the notification centre, etc.
As per my comment, suppose your custom class had this method:
- (NSData *)dataUsingEncoding:(NSStringEncoding)encoding
And suppose it were the only method being called by whomever is being passed either a string or your custom object. Then id would be the right choice, since you'd have in effect implemented an informal protocol, and the thing being passed an object genuinely doesn't care whether it's a string or not. The only contractual requirement is the informal protocol and the protocol is informal i.e. has no footprint on the type syntax.
Conversely, suppose your custom class had no methods in common with NSString and your code just looked like:
- (void)myMethod:(id)object
{
if([object isKindOfClass:[NSString class]])
[self myMethodOnString:object];
else
[self myMethodOnCustomClass:object];
}
Then id would be inappropriate. You're just obscuring what the method does and implicitly reproducing work that's built into the runtime anyway.
These two questions are quite common when we search it but yet I need to get a satisfying answer about both.When ever we search a difference between say subclass and a category we actually get definition of both not the difference.I went to an interview to a very good MNC working on iOS and I was encountered with these two questions and I gave almost all the answers I have read here but the interviewer was not satisfied.He stuck to his questions and was that-
Why do we needed category when we can use a subclass?
Why we needed blocks when we can use functions?
So please explain me what specific qualities blocks and category add in objective C that their counter part can't do.
First...
Just reading the documentation "Subclassing Notes" for NSString shows why creating categories is sometimes better than subclassing.
If you wanted to add a function -(void)reverseString (for instance) to NSString then subclassing it is going to be a massive pain in comparison to categories.
Second...
Blocks are useful for capturing scope and context. They can also be passed around. So you can pass a block into an asynchronous call which then may be passed elsewhere. TBH you don't care where the block is passed or where it is finally called from. The scope captured at the time of creating the block is captured too.
Yes, you can use methods too. But they both have different uses.
Your questions are a bit odd. It's like asking...
Why do hammers exist when we can just use wrenches?
You can't use subclassing when someone else is creating the objects. For instance, NSString is returned from hundreds of system APIs, and you can't change them to return MyImprovedString.
Functions split up the logic; blocks allow you to write it closer together. Like:
[thing doSomethingAndWhenFinishedDo: ^{ some_other_thing; }];
the same code written with functions would put the second part of the logic several lines away in the file. If you have a few nested scopes in your logic then blocks can really clean it up.
Why do we needed category when we can use a subclass?
Categories let you expand the API of existing classes without changing their type. Subclassing does the same thing but introduces a new type. Additionally subclassing lets you add state.
Why we needed blocks when we can use functions?
Block objects are a C-level syntactic and runtime feature. They are similar to standard C functions, but in addition to executable code they may also contain variable bindings to automatic (stack) or managed (heap) memory. A block can therefore maintain a set of state (data) that it can use to impact behavior when executed.
You can use blocks to compose function expressions that can be passed to API, optionally stored, and used by multiple threads. Blocks are particularly useful as a callback because the block carries both the code to be executed on callback and the data needed during that execution
Category : It is used if we want to add any method on a given class whose source is not known. This is basically used when we want to alter the behaviour of any Class.
For example : If we want to add a method on NSString to reverse a string we can go for categories.
Subclassing : If we want to modify state as well as behaviour of any class or override any methods to alter the behaviour of the parent class then we go for subclassing.
For example : We subclass UIView to alter its state and behaviour in our iOS code.
Reference :
When to use categories and when to use subclassing?
What is the difference between inheritance and Categories in Objective-C
We need new method but we don't need new class so we need category.
We need function but we don't need named function so we need block.
For the purposes of my project's UI, I am creating a general method in a category on UIViewController that sets up the UI for a navigation item. This particular navigation item has a yellow button corresponding to an action (save, ok, choose etc.) and a gray button (cancel, close)
- (void)configureAsSaveCancelIPadHeaderWithTarget:(id)theTarget actionForYellowButton:(SEL)selYellow actionForGrayButton:(SEL)selGray
I think I can make this method smaller like so:
- (void)configureAsSaveCancelIPadHeaderWithTarget:(id<PSaveCancelViewControllerNavigationBar>)theTarget
and have the target respond to a protocol.
The protocol would look like this:
#protocol PSaveCancelViewControllerNavigationBar <NSObject>
#required
- (void)save:(id)sender;
- (void)closeThisView:(id)sender;
#end
The #required keyword will only give a warning if those 2 methods are not implemented.
Question
Is it considered a good pattern to assert in the configureAsSaveCancelIPadHeaderWithTarget: method if the target contains those two methods? Like so:
- (void)configureAsSaveCancelIPadHeaderWithTarget:(id<PSaveCancelViewControllerNavigationBar>)theTarget
{
NSAssert([theTarget respondsToSelector:#selector(save:)], #"The provided target must implement the PSaveCancelViewControllerNavigationBar protocol and have the methods defined in that protocol.");
NSAssert([theTarget respondsToSelector:#selector(closeThisView:)], #"The provided target must implement the PSaveCancelViewControllerNavigationBar protocol and have the methods defined in that protocol.");
I will definitely call those two methods later (save, closeThisView) and so I must make sure that the class that calls this method has them implemented.
It all depends on how 'safe' you want to make things. Just because your parameter specifies that a protocol is required doesn't actually mean that the passed instance implements that protocol. All the compiler requires is for you to promise that it does when calling (a cast).
Generally, if you're writing all of the code then it is relatively 'safe' to just use the protocol and not check at runtime.
If other people are using the code, and in particular if you are releasing the code as a library or something like that then checking becomes much more prudent as you can't make any assumptions about what other people are going to do. In this case it is much better to fail early.
No, it’s pointless and extra-wordy. You’ve declared in -configureAsSaveCancelIPadHeaderWithTarget: that you are only accepting an object that implements your protocol, so you are going to REALLY TRY HARD to bone yourself, it’s going to work.
You could be infinitely “safe” about checking if every object STILL responds to messages they say they respond to, but all the extra wordiness just makes your code hard to read, hard to change, slower, and gives you more chances to introduce bugs.
Less code is better code.
I have heard people state that method swizzling is a dangerous practice. Even the name swizzling suggests that it is a bit of a cheat.
Method Swizzling is modifying the mapping so that calling selector A will actually invoke implementation B. One use of this is to extend behavior of closed source classes.
Can we formalise the risks so that anyone who is deciding whether to use swizzling can make an informed decision whether it is worth it for what they are trying to do.
E.g.
Naming Collisions: If the class later extends its functionality to include the method name that you have added, it will cause a huge manner of problems. Reduce the risk by sensibly naming swizzled methods.
I think this is a really great question, and it's a shame that rather than tackling the real question, most answers have skirted the issue and simply said not to use swizzling.
Using method sizzling is like using sharp knives in the kitchen. Some people are scared of sharp knives because they think they'll cut themselves badly, but the truth is that sharp knives are safer.
Method swizzling can be used to write better, more efficient, more maintainable code. It can also be abused and lead to horrible bugs.
Background
As with all design patterns, if we are fully aware of the consequences of the pattern, we are able to make more informed decisions about whether or not to use it. Singletons are a good example of something that's pretty controversial, and for good reason — they're really hard to implement properly. Many people still choose to use singletons, though. The same can be said about swizzling. You should form your own opinion once you fully understand both the good and the bad.
Discussion
Here are some of the pitfalls of method swizzling:
Method swizzling is not atomic
Changes behavior of un-owned code
Possible naming conflicts
Swizzling changes the method's arguments
The order of swizzles matters
Difficult to understand (looks recursive)
Difficult to debug
These points are all valid, and in addressing them we can improve both our understanding of method swizzling as well as the methodology used to achieve the result. I'll take each one at a time.
Method swizzling is not atomic
I have yet to see an implementation of method swizzling that is safe to use concurrently1. This is actually not a problem in 95% of cases that you'd want to use method swizzling. Usually, you simply want to replace the implementation of a method, and you want that implementation to be used for the entire lifetime of your program. This means that you should do your method swizzling in +(void)load. The load class method is executed serially at the start of your application. You won't have any issues with concurrency if you do your swizzling here. If you were to swizzle in +(void)initialize, however, you could end up with a race condition in your swizzling implementation and the runtime could end up in a weird state.
Changes behavior of un-owned code
This is an issue with swizzling, but it's kind of the whole point. The goal is to be able to change that code. The reason that people point this out as being a big deal is because you're not just changing things for the one instance of NSButton that you want to change things for, but instead for all NSButton instances in your application. For this reason, you should be cautious when you swizzle, but you don't need to avoid it altogether.
Think of it this way... if you override a method in a class and you don't call the super class method, you may cause problems to arise. In most cases, the super class is expecting that method to be called (unless documented otherwise). If you apply this same thought to swizzling, you've covered most issues. Always call the original implementation. If you don't, you're probably changing too much to be safe.
Possible naming conflicts
Naming conflicts are an issue all throughout Cocoa. We frequently prefix class names and method names in categories. Unfortunately, naming conflicts are a plague in our language. In the case of swizzling, though, they don't have to be. We just need to change the way that we think about method swizzling slightly. Most swizzling is done like this:
#interface NSView : NSObject
- (void)setFrame:(NSRect)frame;
#end
#implementation NSView (MyViewAdditions)
- (void)my_setFrame:(NSRect)frame {
// do custom work
[self my_setFrame:frame];
}
+ (void)load {
[self swizzle:#selector(setFrame:) with:#selector(my_setFrame:)];
}
#end
This works just fine, but what would happen if my_setFrame: was defined somewhere else? This problem isn't unique to swizzling, but we can work around it anyway. The workaround has an added benefit of addressing other pitfalls as well. Here's what we do instead:
#implementation NSView (MyViewAdditions)
static void MySetFrame(id self, SEL _cmd, NSRect frame);
static void (*SetFrameIMP)(id self, SEL _cmd, NSRect frame);
static void MySetFrame(id self, SEL _cmd, NSRect frame) {
// do custom work
SetFrameIMP(self, _cmd, frame);
}
+ (void)load {
[self swizzle:#selector(setFrame:) with:(IMP)MySetFrame store:(IMP *)&SetFrameIMP];
}
#end
While this looks a little less like Objective-C (since it's using function pointers), it avoids any naming conflicts. In principle, it's doing the exact same thing as standard swizzling. This may be a bit of a change for people who have been using swizzling as it has been defined for a while, but in the end, I think that it's better. The swizzling method is defined thusly:
typedef IMP *IMPPointer;
BOOL class_swizzleMethodAndStore(Class class, SEL original, IMP replacement, IMPPointer store) {
IMP imp = NULL;
Method method = class_getInstanceMethod(class, original);
if (method) {
const char *type = method_getTypeEncoding(method);
imp = class_replaceMethod(class, original, replacement, type);
if (!imp) {
imp = method_getImplementation(method);
}
}
if (imp && store) { *store = imp; }
return (imp != NULL);
}
#implementation NSObject (FRRuntimeAdditions)
+ (BOOL)swizzle:(SEL)original with:(IMP)replacement store:(IMPPointer)store {
return class_swizzleMethodAndStore(self, original, replacement, store);
}
#end
Swizzling by renaming methods changes the method's arguments
This is the big one in my mind. This is the reason that standard method swizzling should not be done. You are changing the arguments passed to the original method's implementation. This is where it happens:
[self my_setFrame:frame];
What this line does is:
objc_msgSend(self, #selector(my_setFrame:), frame);
Which will use the runtime to look up the implementation of my_setFrame:. Once the implementation is found, it invokes the implementation with the same arguments that were given. The implementation it finds is the original implementation of setFrame:, so it goes ahead and calls that, but the _cmd argument isn't setFrame: like it should be. It's now my_setFrame:. The original implementation is being called with an argument it never expected it would receive. This is no good.
There's a simple solution — use the alternative swizzling technique defined above. The arguments will remain unchanged!
The order of swizzles matters
The order in which methods get swizzled matters. Assuming setFrame: is only defined on NSView, imagine this order of things:
[NSButton swizzle:#selector(setFrame:) with:#selector(my_buttonSetFrame:)];
[NSControl swizzle:#selector(setFrame:) with:#selector(my_controlSetFrame:)];
[NSView swizzle:#selector(setFrame:) with:#selector(my_viewSetFrame:)];
What happens when the method on NSButton is swizzled? Well most swizzling will ensure that it's not replacing the implementation of setFrame: for all views, so it will pull up the instance method. This will use the existing implementation to re-define setFrame: in the NSButton class so that exchanging implementations doesn't affect all views. The existing implementation is the one defined on NSView. The same thing will happen when swizzling on NSControl (again using the NSView implementation).
When you call setFrame: on a button, it will therefore call your swizzled method, and then jump straight to the setFrame: method originally defined on NSView. The NSControl and NSView swizzled implementations will not be called.
But what if the order were:
[NSView swizzle:#selector(setFrame:) with:#selector(my_viewSetFrame:)];
[NSControl swizzle:#selector(setFrame:) with:#selector(my_controlSetFrame:)];
[NSButton swizzle:#selector(setFrame:) with:#selector(my_buttonSetFrame:)];
Since the view swizzling takes place first, the control swizzling will be able to pull up the right method. Likewise, since the control swizzling was before the button swizzling, the button will pull up the control's swizzled implementation of setFrame:. This is a bit confusing, but this is the correct order. How can we ensure this order of things?
Again, just use load to swizzle things. If you swizzle in load and you only make changes to the class being loaded, you'll be safe. The load method guarantees that the super class load method will be called before any subclasses. We'll get the exact right order!
Difficult to understand (looks recursive)
Looking at a traditionally defined swizzled method, I think it's really hard to tell what's going on. But looking at the alternative way we've done swizzling above, it's pretty easy to understand. This one's already been solved!
Difficult to debug
One of the confusions during debugging is seeing a strange backtrace where the swizzled names are mixed up and everything gets jumbled in your head. Again, the alternative implementation addresses this. You'll see clearly named functions in backtraces. Still, swizzling can be difficult to debug because it's hard to remember what impact the swizzling is having. Document your code well (even if you think you're the only one who will ever see it). Follow good practices, and you'll be alright. It's not harder to debug than multi-threaded code.
Conclusion
Method swizzling is safe if used properly. A simple safety measure you can take is to only swizzle in load. Like many things in programming, it can be dangerous, but understanding the consequences will allow you use it properly.
1 Using the above defined swizzling method, you could make things thread safe if you were to use trampolines. You would need two trampolines. At the start of the method, you would have to assign the function pointer, store, to a function that spun until the address to which store pointed to changed. This would avoid any race condition in which the swizzled method was called before you were able to set the store function pointer. You would then need to use a trampoline in the case where the implementation isn't already defined in the class and have the trampoline lookup and call the super class method properly. Defining the method so it dynamically looks up the super implementation will ensure that the order of swizzling calls does not matter.
First I will define exactly what I mean by method swizzling:
Re-routing all calls that were originally sent to a method (called A) to a new method (called B).
We own Method B
We dont own method A
Method B does some work then calls method A.
Method swizzling is more general than this, but this is the case I am interested in.
Dangers:
Changes in the original class. We dont own the class that we are swizzling. If the class changes our swizzle may stop working.
Hard to maintain. Not only have you got to write and maintain the swizzled method. you have to write and maintain the code that preforms the swizzle
Hard to debug. It is hard to follow the flow of a swizzle, some people may not even realise the swizzle has been preformed. If there are bugs introduced from the swizzle (perhaps dues to changes in the original class) they will be hard to resolve.
In summary, you should keep swizzling to a minimum and consider how changes in the original class might effect your swizzle. Also you should clearly comment and document what you are doing (or just avoid it entirely).
It's not the swizzling itself that's really dangerous. The problem is, as you say, that it's often used to modify the behavior of framework classes. It's assuming that you know something about how those private classes work that's "dangerous." Even if your modifications work today, there's always a chance that Apple will change the class in the future and cause your modification to break. Also, if many different apps do it, it makes it that much harder for Apple to change the framework without breaking a lot of existing software.
Used carefully and wisely, it can lead to elegant code, but usually, it just leads to confusing code.
I say that it should be banned, unless you happen to know that it presents a very elegant opportunity for a particular design task, but you need to clearly know why it applies well to the situation, and why alternatives do not work elegantly for the situation.
Eg, one good application of method swizzling is isa swizzling, which is how ObjC implements Key Value Observing.
A bad example might be relying on method swizzling as a means of extending your classes, which leads to extremely high coupling.
Although I have used this technique, I would like to point out that:
It obfuscates your code because it can cause un-documented, though desired, side effects. When one reads the code he/she may be unaware of the side effect behavior that is required unless he/she remembers to search the code base to see if it has been swizzled. I'm not sure how to alleviate this problem because it is not always possible to document every place where the code is dependent upon the side effect swizzled behavior.
It can make your code less reusable because someone who finds a segment of code which depends upon the swizzled behavior that they would like to use elsewhere cannot simply cut and paste it into some other code base without also finding and copying the swizzled method.
I feel that the biggest danger is in creating many unwanted side effects, completely by accident. These side effects may present themselves as 'bugs' which in turn lead you down the wrong path to find the solution. In my experience, the danger is illegible, confusing, and frustrating code. Kind of like when someone overuses function pointers in C++.
You may end up with odd looking code like
- (void)addSubview:(UIView *)view atIndex:(NSInteger)index {
//this looks like an infinite loop but we're swizzling so default will get called
[self addSubview:view atIndex:index];
from actual production code related to some UI magic.
Method swizzling can be very helpful is in unit testing.
It allows you to write a mock object and have that mock object used instead of the real object. Your code to remain clean and your unit test has predictable behavior. Let's say you want to test some code that uses CLLocationManager. Your unit test could swizzle startUpdatingLocation so that it would feed a predetermined set of locations to your delegate and your code would not have to change.