I want to create a pickerView programmatically and have it use it's own version of a method like pickerView:numberOfRowsInComponent.
I create the instance at runtime like this:
UIPickerView *myPickerView = [[UIPickerView alloc] initWithFrame:CGRectMake(0, 200, 320, 200)];
myPickerView.delegate = self;
myPickerView.dataSource = self;
myPickerView.showsSelectionIndicator = YES;
[self.view addSubview:myPickerView];
The standard method called would be:
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component {
NSUInteger numRows = 5;
return numRows;
}
What I want to do is replace this standard method with another method for this instance only.
-(NSInteger)xxxpickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
{ // special method for this instance only
return 1;
}
I've been able to use method swizzle to do this with other things, but I can't seem to get it to work with UIPickerView.
#implementation UIPickerView (Tracking)
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
SEL originalSelector = #selector(pickerView:numberOfRowsInComponent:);
SEL swizzledSelector = #selector(xxxpickerView:numberOfRowsInComponent:);
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
BOOL didAddMethod =
class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else {
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
I've listed the methods to see if the 2nd method was added to the instance during runtime and it is in the list.
However, the 2nd method doesn't run, the 1st method does run.
Here's a link to the post that got me started on this, and I've confirmed it works, but I seem to be missing something about this.
http://nshipster.com/method-swizzling/
I'm open to other suggestions, the problem I'm trying to solve is that I want to create the instance of a UIPickerView object that won't be dependent on another instance that will be running at the same time. So I want a different method that will work only with the one instance and completely ignore any other instances that might be running and I want to do this programmatically.
At lest one reason for not using a tag/switch, is that I don't know what the conditions will be until runtime.
I don't know why swizzle would work with one object and not another, and I'm open to other way to replace stock methods with others at runtime.
Is there something about the method I'm trying to replace that won't allow it to be replaced?
EDIT: in order to try and make the question clear, the following code in the link works. It swaps one method for another method. What I need to do is the same thing for another object and I can't figure out what it works for 1 object and not another.
This works for another object: http://nshipster.com/method-swizzling/
Here's another link as well: http://blog.newrelic.com/2014/04/16/right-way-to-swizzle/
One simple way to do it is keep a copy of the pointer in a property and then compare pointers in the datasource/delegate methods.
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
{
if ( pickerView == self.myPickerView )
return 1; // response for this particular picker view
else
return 5; // standard response
}
Related
I use a sinlgeton in my application for managing data that is available to the whole application, which accessed via:
static MMProductManager *sharedInstance = nil;
+(MMProductManager*)SharedInstance {
dispatch_once( &resultsToken, ^(void) {
if ( ! sharedInstance ) {
sharedInstance = [[MMProductManager alloc] init];
}
});
return sharedInstance;
}
Everything is working as expected.
In Objective C, there does not seem to be a way to hide any object's init method, and in my case having more than instance of MMProductManager would lead to data being duplicated (in the best case scenario).
What I would like to do is guard against instantiating more than one instance. Other languages seem to have this feature; i.e. marking certain methods/classes as private. I am thinking of implementing something along like:
-(id)init {
// guard against instantiating a more than one instance
if ( sharedInstance )
return sharedInstance;
if ( (self = [super init]) ) {
self->_resultsQueue = dispatch_queue_create( kMMResultQLAbel, NULL );
self->_initialized = FALSE;
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(handleNotification:)
name:UIApplicationDidReceiveMemoryWarningNotification
object:0];
[self initialize];
}
return self;
}
Does this approach seem reasonable?
What would happen in the case of someone allocating this class, then calling the init described above? Would it be reasonable to override +(id)alloc? If so How would I go about doing that?
I know the convention of exposing a SharedInstance method is an implicit message to other developers to go through this method, but I would like a bit more control if possible.
You don't want to override - init (if not for some other reason) - - init is not the method that creates the instance. You want to override + alloc for this:
#implementation SingletonClass
+ (id)alloc
{
static id instance = nil;
if (instance == nil) {
instance = [super alloc];
}
return instance;
}
#end
This way you can actually prevent (almost) completely creating multiple instances of SingletonClass.
(Unless somebody falls back to calling
id trickyDifferentInstance = class_createInstance(objc_getClass("SingletonClass"), 0));
but that's very unlikely.)
I have general question about designated initializer. I have a some class and from there i want to call a initializer, but before i started to fill my #properties with passing data i want to make data default. For example:
-(id)initWithDefault:(NSDictionary*)defaultTemplate
{
self = [super init];
_fontColor = [defaultTemplate objectForKey:#"color"];
_fontSize = [[defaultTemplate objectForKey:#"size"] intValue];
return self;
}
-(id)initWithTemplate:(NSDictionary*)template
{
self = [self initWithDefault:myDefaultTemplate];
//now i doing something with my template
return self;
}
Is this is a way to prevent null #properties? It this a correct use of designated initializer? Of course you can assume that myDefaultTemplates is not null, and has not null object in keys.
This seems fine with me. I would use the safe way (presented below), but otherwise your code is fine.
-(id)initWithTemplate:(NSDictionary*)template
{
if(self = [self initWithDefault:myDefaultTemplate]) {
//now i doing something with my template
}
return self;
}
Your implementation is perfectly fine, given the fact that _fontColor and _fontSize variables are your local variables of properties.
adig's suggestion is just an enhancement on what you already have implemented. This check takes care of the situation, when your object does not get allocated due to any reason.
I wrote a Refresh function to reload the tableview in iPhone app with the argument of UITableview.
My question is how can I call or invoke this refresh function?
- (void)RefreshTableView:(UITableView *)tableView
{
if(tableView != nil){
[tableView reloadData];
}
}
First of all rename your method like the following:
- (void)refreshTableView:(UITableView *)tableView
Then, to call that method you need to perform the following:
[self refreshTableView:yourTableview];
where self stands for the object that will receive the message. In this case self is an instance of the object that contains that method.
Now, why do you need to pass also an instance of the table view?
if you have a instance variable for that table view and you have synthesized it (#property/#synthesize pattern), you could simple do the following:
- (void)refreshTableView
{
[[self myTable] reloadData];
}
and then invoke that method like the following:
[self refreshTableView];
Edit
As danh suggested, if you have a property (or an instance variable), you can also call directly
[[self myTable] reloadData]; // or [self.myTable reloadData];
without passing through refreshTableView.
If - (void)RefreshTableView:(UITableView *)tableView is in the same file then you can call it
[self RefreshTableView:aTableView]; when you want to call.
from the class itself [self RefreshTableView:yourTableView];
form outside of the class [yourClassInstance RefreshTableView:yourTableView];
P.S:
You should refactor your code and rename RefreshTableView: with refreshTableView:
Could someone help me understand the primitive accessors with this example : i don't understand what is automatically set and the order of those methods :
1.after a person is created, is willSave the first method called? (i guess so, because save: is called after we create a person with insertNewObjectForEntityForName )
2.in RootViewController (the second chunk of code), we then call the getter of eyeColor with : person.eyeColor :
a) in eyeColor, we call : [self eyeColorData] ,
b) but setPrimitiveEyeColorData is in willSave, which is accessible only if primitiveEyeColor exists,
c) but setPrimitiveEyeColor is in eyeColor and only called if [self eyeColorData] exists. So, i'm a bit confused with this code, could someone help me?
here's the code about eyeColor and eyeColorData :
#dynamic eyeColorData;
#dynamic eyeColor;
#interface AWPerson (PrimitiveAccessors)
- (UIColor *)primitiveEyeColor;
- (void)setPrimitiveEyeColor:(UIColor *)value;
- (NSData *)primitiveEyeColorData;
- (void)setPrimitiveEyeColorData:(NSData *)value;
#end
+ (id)personInManagedObjectContext:(NSManagedObjectContext *)moc {
return [NSEntityDescription
insertNewObjectForEntityForName:#"Person"
inManagedObjectContext:moc];
}
+ (id)randomPersonInManagedObjectContext:(NSManagedObjectContext *)moc {
AWPerson *randomPerson = [self personInManagedObjectContext:moc];
//...
randomPerson.eyeColor = [self randomColor]; //setter eyeColor
return randomPerson;
}
+ (UIColor *)randomColor {
static NSArray *colorsArray = nil;
if( !colorsArray ) {
colorsArray = [[NSArray alloc] initWithObjects:
[UIColor lightGrayColor],
[UIColor blueColor],
[UIColor greenColor], nil];
}
int randomIndex = arc4random() % [colorsArray count];
return [colorsArray objectAtIndex:randomIndex];
}
- (void)willSave {
UIColor *color = [self primitiveEyeColor];
if( color ) {
[self setPrimitiveEyeColorData:
[NSKeyedArchiver archivedDataWithRootObject:color]];
} else {
[self setPrimitiveEyeColorData:nil];
}
[super willSave];
}
- (UIColor *)eyeColor {
[self willAccessValueForKey:#"eyeColor"];
UIColor *tmpValue = [self primitiveEyeColor];
[self didAccessValueForKey:#"eyeColor"];
if( tmpValue ) return tmpValue;
NSData *colorData = [self eyeColorData];
if( !colorData ) return nil;
tmpValue = [NSKeyedUnarchiver unarchiveObjectWithData:colorData];
[self setPrimitiveEyeColor:tmpValue];
return tmpValue;
}
in RootViewController :
- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
AWPerson *person = [[self fetchedResultsController] objectAtIndexPath:indexPath];
[cell setBackgroundColor:person.eyeColor];
}
Thanks
EDIT - Added info on willSave
To answer your first question, willSave is called whenever the object is saved (using the save method). So the first method called will be one of the class methods (used to create the object) or init and then, since you said that the object is saved just after it is created, willSave gets called.
I think the key to understanding this is to realize that eyeColor, primitiveEyeColor, and their setters are all ultimately interacting with the same variable in memory (the iVar named eyeColor). The difference is whether or not the code in the setter/getter (in this case the - (UIColor *)eyeColor { function) is called.
There are just a few different ways to interact with it:
[self primitiveEyeColor]; - This reads the value of the iVar directly.
[self setPrimitiveEyeColor:tmpValue]; - This sets the value of the iVar directly.
[self eyeColor] - This calls the - (UIColor *)eyeColor method in your class (which should ultimately retrieve the iVar or a representation of it).
[self setEyeColor:value] - This calls the - (void)setEyeColor:(UIColor *)newColor method in your class. Note that in this case it doesn't exist so it simply calls the primitive method (and does the KVO magic).
In this particular code, they are using a "non-standard persistent attribute" because NSManagedObject does not support UIColor's. Read about it here.
EDIT 2
To answer your other questions:
a) The color in randomPerson.eyeColor = [self randomColor] is
accessible with [self primitiveEyeColor] (in willSave)?
Yes, once eyeColor is set (either via the setEyeColor method or the setPrimitiveEyeColor method), you can read it from primitiveEyeColor and it will return the same value.
Note that once it is set, eyeColor and primitiveEyeColor return the same value and can be called from anywhere in your class (not just willSave).
b) So if [self primitiveEyeColor] != nil : in eyeColor, the line :
if( tmpValue ) return tmpValue; should therefore always be true...
when can we unarchive eyeColorData if UIColor *tmpValue = [self
primitiveEyeColor] is always returned in -(UIColor *)eyeColor?
This method only looks at eyeColorData (which was stored during the last call to willSave) if eyeColor is nil. This is an optimization because we could skip all of this and just unarchive eyeColorData every time if we wanted to. In this case, once a value is unarchived or set to a new value, it always stores that value and returns it so that we don't have to call unarchive again.
Also, there is really what I believe to be an error here (although it could be by design). Let's say that we perform the following steps:
Set eyeColor to a random color (let's say blue).
save the object.
Set eyeColor to nil
Now, if you check the color using [self eyeColor] it will see that primitiveEyeColor is nil and unarchive eyeColorData again, therefore returning the blue color that was stored previously. You should probably be over-riding the set function so that it sets eyeColorData to nil when eyeColor is set to nil. That way, checking the value of eyeColor after setting it to nil will return nil as expected.
Hi I had an implementation previous versions of iOS for a singleton as follows:
.h file
#interface CartSingleton : NSObject
{
}
+(CartSingleton *) getSingleton;
.m file
#implementation CartSingleton
static CartSingleton *sharedSingleton = nil;
+(CartSingleton *) getSingleton
{
if (sharedSingleton !=nil)
{
NSLog(#"Cart has already been created.....");
return sharedSingleton;
}
#synchronized(self)
{
if (sharedSingleton == nil)
{
sharedSingleton = [[self alloc]init];
NSLog(#"Created a new Cart");
}
}
return sharedSingleton;
}
//==============================================================================
+(id)alloc
{
#synchronized([CartSingleton class])
{
NSLog(#"inside alloc");
NSAssert(sharedSingleton == nil, #"Attempted to allocate a second instance of a singleton.");
sharedSingleton = [super alloc];
return sharedSingleton;
}
return nil;
}
//==============================================================================
-(id)init
{
self = [super init];
}
However on the web I see people have implemented the Singleton design pattern using this code:
+ (id)sharedInstance
{
static dispatch_once_t pred = 0;
__strong static id _sharedObject = nil;
dispatch_once(&pred, ^{
_sharedObject = [[self alloc] init]; // or some other init method
});
return _sharedObject;
}
Could someone who is experience please guide me.
Im a newbie and thoroughly confused between the old iOS implementation of the Singleton and the new one and which is the correct one?
Thanks a lot
Strictly speaking, you must use:
+ (MySingleton*) instance {
static dispatch_once_t _singletonPredicate;
static MySingleton *_singleton = nil;
dispatch_once(&_singletonPredicate, ^{
_singleton = [[super allocWithZone:nil] init];
});
return _singleton;
}
+ (id) allocWithZone:(NSZone *)zone {
return [self instance];
}
Now you guarantee that one cannot call alloc/init and create another instance.
Explanation: The instance method is at the class level and is your main access method to get a reference to the singleton. The method simply uses the dispatch_once() built-in queue that will only execute a block once. How does the runtime guarantee that the block is only executed once? Using the predicate you supply (of type dispatch_once_t). This low-level call will guarantee that even if there are multiple threads trying to call it, only one succeeds, the others wait until the first one is done and then returns.
The reason we override allocWithZone is because alloc calls allocWithZone passing nil as the zone (for the default zone). To prevent rogue code from allocating and init-ializing another instance we override allocWithZone so that the instance passed back is the already initialized singleton. This prevents one from creating a second instance.
The dispatch_once snippet is functionally identical to other one. You can read about it at http://developer.apple.com/library/mac/#documentation/Darwin/Reference/Manpages/man3/dispatch_once.3.html.
This is what I use for singletons:
+ (MySingleton*) getOne {
static MySingleton* _one = nil;
#synchronized( self ) {
if( _one == nil ) {
_one = [[ MySingleton alloc ] init ];
}
}
return _one;
}
NOTE: In most cases, you do not even need to use #synchronized (but it is safe this way).
A singleton is a special kind of class where only one instance of the class exists for the current process. (In the case of an iPhone app, the one instance is shared across the entire app.) Some examples in UIKit are [UIApplication sharedApplication] (which returns the sole instance of the application itself), and [NSFileManager defaultManager] (which returns the file manager instance). Singletons can be an easy way to share data and common methods across your entire app.
Rather than create instances of the singleton class using alloc/init, you'll call a class method that will return the singleton object. You can name the class method anything, but common practice is to call it sharedName or defaultName.
Please check a link with best answer
:http://www.idev101.com/code/Objective-C/singletons.html