Being a ReactiveCocoa newbie, I'm hoping for some advice with this:
I'm trying to create a dynamic form that contains multiple Field objects parsed from an XML file. Each Field can have muliple validation rules that will run against the Field's NSString *value param.
For the RAC part of the question-
inside each Field object, I want to bind BOOL completed to a signal that checks the Field's *value param against an array of rules. So far I've gotten here with my thinking:
#implementation Field
self = [super init];
if (self) {
RAC(self, completed) = [RACObserve(self, value) filter:^BOOL(NSString *fieldValue) {
NSLog(#"%s::self.completed = %d\n", sel_getName(_cmd), self.completed); // trying to watch the values here, with no luck
NSLog(#"%s::fieldValue = %#\n", sel_getName(_cmd), fieldValue); // same here, I'd like to be able to view the `*value` here but so far no luck
return [self validateCurrentValue]; // currently this method just checks value.length > 5
}];
}
return self;
The *value param has already been bound to my view model (successfully) and it gets updated each time a textfield changes.
What I'm looking for is a basic example or best-practice, the code above crashes when run so I know I'm missing something fundamental.
Thanks all
-filter: is simply passing values from RACObserve(self, value) through unchanged, but only if the block returns YES. So that means you're trying to set completed to values of whatever type value is. That's Probably Bad®.
But the good news is that you're really close!
Instead of filtering, you want to transform. You want to take every value and map it to something other thing. Namely whether that value passes validation. To do that, we use -map::
RAC(self, completed) = [RACObserve(self, value) map:^(NSString *fieldValue) {
return #([self validateCurrentValue]);
}];
Related
I have an app with a Share button. I want to customize what content is shared based on the activity type. For example, Messages might get an image and text, whereas AirDrop would just get a file.
I actually have this working perfectly, and the code I'm using has worked fine in every version of iOS through iOS 10. But I've realized I'm returning nil where I'm not supposed to, so I'm trying to figure out how to fix that.
I do something like this to set up my activity view controller:
JUNActivityProvider *fileProvider = [[JUNActivityProvider alloc] initWithPlaceholderItem:[NSObject new]];
fileProvider.objectID = objectID;
fileProvider.fileURL = fileURL;
JUNActivityProvider *textProvider = [[JUNActivityProvider alloc] initWithPlaceholderItem:[NSString new]];
textProvider.objectID = objectID;
...
UIActivityViewController *activityController = [[UIActivityViewController alloc]
initWithActivityItems:#[fileProvider,imageProvider,textProvider,urlProvider,printFormatter]
applicationActivities:nil];
Then in JUNActivityProvider, I have an item method that customizes the return value based on the activityType:
- (id)item {
if (self.fileURL) {
if ([self.activityType isEqualToString:UIActivityTypeAirDrop]) {
// Create the file
return url;
}
} else if ([self.placeholderItem isKindOfClass:[UIImage class]]) {
if ([self.activityType isEqualToString:UIActivityTypeAirDrop] == NO &&
[self.activityType isEqualToString:UIActivityTypeMail] == NO &&
[self.activityType isEqualToString:UIActivityTypePrint] == NO) {
// Create the image
return image;
}
} else if ([self.placeholderItem isKindOfClass:[NSString class]]) {
if ([self.activityType isEqualToString:UIActivityTypeMail]) {
return #"example one";
} else if ([self.activityType isEqualToString:UIActivityTypeMessage] ||
[self.activityType isEqualToString:UIActivityTypeCopyToPasteboard]) {
return #"example two";
}
}
return nil;
}
That return return nil at the end is the problem. It works fine and does exactly what I want—when it's nil that item isn't shared. The written documentation doesn't say that it must return a value, but the header file does:
- (nonnull id)item; // called on secondary thread when user selects an activity. you must subclass and return a non-nil value.
I don't want to risk a crash by returning nil when a nonnull value is expected, so I need to fix this. As far as I can tell my only option is to stop using UIActivityItemProvider, and instead implement the UIActivityItemSource protocol on my own. That protocol includes the method activityViewController:itemForActivityType:, which clearly states that you can return nil there:
May be nil if multiple items were registered for a single activity type, so long as one of the items returns an actual value.
Perfect. But here's the problem: activityViewController:itemForActivityType: is called on the main thread, which is causing problems with one of my items in particular. Here's a summary of what's happening:
I need to call some methods that run asynchronously. In order to deal with that I've tried using a dispatch semaphore. That keeps the method from returning until I've had a chance to set the return value.
Since activityViewController:itemForActivityType: is called on the main thread, that locks up while it's working.
I need to draw a UIView into an image. If I try to do that work on the main thread, nothing happens until the semaphore times out. But if I don't do it on the main thread, it crashes.
I'm at a loss for how to deal with this. Basically I need to keep the method from returning until I'm ready, but I can't lock up the main thread since I need to do some work there. This seems… impossible? Is there any way to make this work?
After filing an enhancement request I was just about to give up and settle on either returning nil or [NSNull null]. But then I realized there is absolutely a solution to this problem.
While UIActivityItemProvider includes a bunch of its own functionality, it still very much implements the UIActivityItemSource protocol. I knew that. What I hadn't considered is that this means I can just override activityViewController:itemForActivityType: and return nil there when it's appropriate.
So the last line of my item method now looks like this:
return self.placeholderItem;
You could also return [NSNull null] here, or really any object. I chose the placeholderItem because it seems a little safer—at the very least I know it's returning an object of the expected type, in case anything about the implementation ever changes.
Then all I have to do is add my own implementation of activityViewController:itemForActivityType: (where we are allowed to return nil):
- (nullable id)activityViewController:(UIActivityViewController *)activityViewController itemForActivityType:(UIActivityType)activityType {
id item = [super activityViewController:activityViewController itemForActivityType:activityType];
if ([item isEqual:self.placeholderItem]) return nil;
return item;
}
Just call super to get the item, return nil if it's something you don't want to include, or return the item if it is. Note that if your placeholderItem might ever be equal to something you actually do want to share, you will need to change this implementation a bit—but the same basic concept should work.
This question already has answers here:
Should you use 'isEqual' or '=='?
(2 answers)
Comparing objects in Obj-C
(4 answers)
Closed 8 years ago.
I am reading the Programming with Objective-C . In the section of Determining Equality of Objects , it says the following words:
- When dealing with objects, the == operator is used to test whether two separate pointers are pointing to the same object:
if (firstPerson == secondPerson) {
// firstPerson is the same object as secondPerson
}
- If you need to test whether two objects represent the same data, you need to call a method like isEqual:, available from NSObject:
if ([firstPerson isEqual:secondPerson]) {
// firstPerson is identical to secondPerson
}
I get confused about the differences between == and isEqual with the above explanation, does it mean firstPerson == secondPerson is an alternative of [firstPerson isEqual:secondPerson] ?
The definition of == is correct, it checks to see that they're pointing to the actual same pointer/memory address (ie. 0xffffff)
The key to understanding what you're asking is to think about what you mean by the word "equal". "equal" typically means, from the developer's point of view, that "these two objects contain the same data in the fields that I require for all practical purposes". You can have two user objects each with the same userID property but different times in lastUpdated - would you consider them equal? Depends on your use case. Most likely you would say yes because they're the same user. They were updated from the server at different times, so some fields differ, but for your implementation, they're equal.
In the case above, are they the same object? Definitely not. They point to different memory addresses. So == would be NO, whereas if you wrote your isEqual: method to check just the userID property, it would return YES
The definition of isEqual: is entirely up to the author of the class. isEqual: can be written to use == if you wanted. All you have to do, in your class, is to override the isEqual: method which is defined by the NSObject protocol.
If you have a custom object, use isEqual: to define what your definition of equal is. In the example of a user object, you might define:
- (BOOL)isEqual:(id)otherObject {
if ([otherObject isKindOfClass:[self class]]) {
MyClass *otherObjectAfterCast = (MyClass*)otherObject;
if ([otherObjectAfterCast.userID isEqualToString:self.userID])
return YES;
}
return NO;
}
Technically you'd probably want to use caseInsensitiveCompare: or something like that but you get the drift...
isEqual: can also be used to trigger other methods - in the case of NSString - calling isEqual: when both operands are strings results in a call to isEqualToString: - which is why the documentation recommends calling isEqualToString: if you know they're both strings, since it's a bit faster.
So, isEqual: is whatever you make of it, or whatever the class author has defined it to be.
This is also a pretty clear definition in the docs (for once lol): NSObject Protocol Reference
Hope this helps! Let me know if you need any further clarification.
NSString *string1 = [[NSString alloc] initWithString:#"some string"];
NSString *string2 = [[NSString alloc] initWithString:#"some string"];
NSString *string3 = string2;
BOOL equal1 = (string1 == string2); // NO
BOOL equal2 = [string1 isEqual:string2]; // YES
BOOL equal3 = (string2 == string3); // YES
BOOL equal4 = [string2 isEqualToString:string3]; // YES
The simple version is this.
== tells you if the pointers are the same object or not.
The isEqual: family of methods do something different.
They tell you if the objects at the other end of the pointers are effectively the same based on some criteria such as the properties or ivars holding equal values or whatever logic is implemented in the method used. They may or may not be the exact same object.
I have a project at https://github.com/niklassaers/NJSNotificationCenter with so far only two unit tests. One of them runs, one of them runs 60% of the time. The remaining 40% of the time, it will fail because my NSMutableValue contains a nil value, even though I have never put in a nil value (nor should that be possible)
The problem arises here:
- (void) addObserver:(id)observer selector:(SEL)aSelector name:(NSString *)aName object:(id)anObject priority:(NSInteger)priority {
NJSNotificationKey *key = [[NJSNotificationKey alloc] initWithObserver:observer name:aName object:anObject];
NSLog(#"Key is: %p", key);
key.priority = priority;
NJSNotificationValue *value = [[NJSNotificationValue alloc] initWithSelector:aSelector];
NSAssert(value, #"Value cannot be nil!");
#synchronized(observers) {
observers[key] = value;
NSLog(#"Key: %p\tValue: %p\t%#", key, value, observers);
if(observers[key] == nil)
NSLog(#"This can't be!");
}
}
I make a key, it is not nil, I make a value, it is not nil, I add it to my dictionary and get it back from the dictionary, but now it is nil! This makes no sense to me.
I have wrapped every access to observers (a local instance variable) in a #synchronized block just in case there was any other threading going on (there isn't).
Please check out my code (BSD license) and have a look at it, and help me understand how this can be. If you'd like, I'd love to pair program on this with you, I'm #niklassaers on Twitter
You haven't implemented hash.
https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/Collections/Articles/Dictionaries.html#//apple_ref/doc/uid/20000134-SW8
Keys must implement the hash and isEqual: methods because a dictionary
uses a hash table to organize its storage and to quickly access contained
objects
The dictionary is copying your key object and storing that - when it tried to lookup the original key object, it does not find it because the hash values do not match.
I'm trying to save some data into database using CoreData so I created Entity named 'Client' with some attributes. Two of them are 'city' and 'post_code', both of String type. I also created Client class extending NSManagedObjects and I wrote some methods there.
-(void) setCity: (NSString*) city
{
[self setValue:city forKey:#"city"];
}
-(NSString*) getCity
{
return [self valueForKey:#"city"];
}
-(void) setPostCode: (NSString*) postCode
{
[self setValue:postCode forKey:#"post_code"];
}
-(NSString*) getPostCode
{
return [self valueForKey:#"post_code"];
}
getPostCode and setPostCode work as I expected but calling setCity or getCity is causing problems. Appication stops and method is looping in thread as you can see on screenshot.
Full size image
This is how I call those methods
if([databaseResult count] > 0)
c = [databaseResult objectAtIndex:0];
else
c = [NSEntityDescription insertNewObjectForEntityForName:#"Client" inManagedObjectContext:context];
[c setPostCode:[jsonData valueForKey:#"post_code_invoice"]];
[c setClientType:[jsonData valueForKey:#"company_type"]];
[c setCity:[jsonData valueForKey:#"city_invoice"]];
it always stops on setCity no matter what data I pass there, even that call doesn't work
[c setCity:#"aaa"];
Did anyone had similar problem?
The setValueForKey method is calling back into the same function because it has the same name.
From the KVC documentation
Default Search Pattern for setValue:forKey:
When the default implementation of setValue:forKey: is invoked for a property the following search pattern is used:
The receiver’s class is searched for an accessor method whose name matches the pattern set:.
If no accessor is found, and the receiver’s class method accessInstanceVariablesDirectly returns YES, the receiver is searched for an instance variable whose name matches the pattern _, _is, , or is, in that order.
If a matching accessor or instance variable is located, it is used to set the value. If necessary, the value is extracted from the object as described in “Representing Non-Object Values.”
If no appropriate accessor or instance variable is found, setValue:forUndefinedKey: is invoked for the receiver.
So when you call setValue:forKey: with the key city, the implementation calls setCity: and your implementation calls setValue:forKey:, and round and round you go.
Why are you even doing it this way rather than setting the value directly?
Or better still use properties and you don't even need to write setters or getters.
The other two methods work because the key names are different. (they have underscores
)
I am trying to work out the correct way to approach some methodology.
Workflow
When a game is created, I would like to first search to see if a game already exists with this user. If there is a game I will not create one and show a message to the user.
At present I have two methods:
+(void)createNewGameAgainst:(PFUser *)user2 withCompletion:(void (^)(BOOL success))completionHandler
+(BOOL)checkIfGameAlreadyExistsAgainst:(PFUser *)opponentUser
The createNewGame... method is called first. Then within this I make a call to [self checkIfGameAlreadyExistsAgainst:user2];.
How do I check the result of the second method, from within the first? So how do I determine what the BOOL value is of the call to the method checkIfGameAlreadyExistsAgainst?
Is this the correct way to approach this or is there a better/cleaner way possibly?
The return value of a function can be used like a variable:
BOOL gameExists = [self checkIfGameAlreadyExistsAgainst:user2]; // assign result to a new variable
if(gameExists == YES) // compare result to YES
{
}
You can skip creating a new variable and just compare the result
if ([self checkIfGameAlreadyExistsAgainst:user2] == YES) // compare result directly
{
}
And when the type is BOOL, you can omit the comparison and just do this:
if ([self checkIfGameAlreadyExistsAgainst:user2])
{
}