ARC and pointer storing - ios

My situation is a bit more complex than what I've seen here before posting, and I'm not really good with memory management.
I have a custom UITableViewCell (that we will call MyCell here) and I pass its pointer to an UITableViewController (MyController here) when clicking on it. I pass the pointer because I want to call a method of this cell and the reference is only made by copy in Objective-C so it doesn't call the method on the right cell. I have made this:
MyController.h
#interface MyController : UITableViewController {
MyCell * __autoreleasing *_cell;
}
-(instancetype)initWithCell:(MyCell * __autoreleasing *)cell;
#end
MyController.m
- (instancetype)initWithCell:(MyCell **)cell {
if (self = [super init]) {
_cell = cell;
// Checkpoint 1
}
}
Then I want to use this variable later in my code, for example to define the number of sections:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
// Checkpoint 2
return (*_cell).contents.count; // contents being an NSArray property of the custom cell
}
The issue: At the "checkpoints" marked here, I have an NSLog(#"%ld", (unsigned long)(*_cell).contents.count);, however it shows 2 (the right number) in the first checkpoint, but 0 in the second checkpoint, so basically when I click on the cell an empty table view is shown.
I used to pass the cell by copy by storing it in a nonatomic, strong property and everything worked well, but by changing the calls from self.cell to _cell because of the pointer reference, the view is now empty as I said. It is likely a memory management issue, but I have no clue on how to solve it (fairly new to Objective-C, first app).
N.B.: I tried to change the __autoreleasing by a __strong, but this lead to a crash at every access of a property of the _cell. I have also tried to use a nonatomic, assign property to store it instead of using a ivar but it didn't solve my problem.
Thanks!
Edit: Forgot to mention that I call the view controller by using
[self.navigationController pushViewController:[[MyController alloc] initWithCell:(MyCell **)&cell] animated:YES];
in my previous view controller, in the tableView:didSelectRowAtIndexPath: method.

A few points:
The * __autoreleasing * pattern serves a very specific purpose, namely where a called method creates an object and needs to update the caller’s pointer to reference this new object. For example, consider an example from the regular expression documentation:
NSError *error = NULL;
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:#"\\b(a|b)(c|d)\\b"
options:NSRegularExpressionCaseInsensitive
error:&error];
So, error is a NSError pointer, and the caller is
supplying the address to that pointer so that if
regularExpressionWithPattern instantiates a
NSError object, it can update the caller’s reference to point to it.
The only time you need to employ this “pointer to a pointer” pattern
is when the called routine is instantiating an object and wants to
update a pointer of the calling routine. We generally only have to
use that pattern when you want to return pointers to more than one
object (e.g. in the case of regularExpressionWithPattern, it will
return a NSRegularExpression * pointer, but optionally may also
want to update the NSError * pointer, too).
You said:
I pass the pointer because I want to call a method of this cell and the reference is only made by copy in Objective-C so it doesn't call the method on the right cell.
That logic is not quite right. MyCell * is a pointer to that
original object, not a copy of it. You can access it without
resorting to the “pointer to a pointer” pattern. In your case, if
you would just use MyCell *, not MyCell * *.
You should not pass a cell reference to this view controller at all ... one view controller should not be reaching into the view hierarchy of another view controller;
Table views have all sorts of optimizations associated with cell reuse ... one shouldn’t use a cell beyond interaction in its own table view data source and delegate methods; and
One should not use a cell (a “view” object) to store “model” data (what is currently stored in the contents property). Do not conflate the “model” (the data) with the “view” (the UI).
So, I would suggest a simple pointer (not a pointer to a pointer) to a model object:
#interface MyController : UITableViewController
#property (nonatomic, strong) NSArray <ModelObject *> *contents; // or whatever you previously stored in cell `contents`
#end
And then:
MyController *controller = [[MyController alloc] init]; // usually we'd instantiate it using a storyboard reference, but I'm gather you're building your view hierarchy manually
controller.contents = self.contents[indexPath.row]; // note, not `cell.contents`, but rather refer to this current view controller’s model, not the cell (the “view”)
[self.navigationController pushViewController:controller animated:YES];
And for passing data to and from view controllers, see Passing data between view controllers. But, as a general rule, one view controller shouldn’t be accessing the view objects of another.

Related

iOS: How to share data between different views of Tab Bar Controller

My app has two views managed by a Tab Bar Controller. One of the views is Google Map (GMSMapView using their SDK) and the other is a TableView showing a list of the same data. The markers on the map are the same data in the TableView (just alternate presentations of the same data).
I fetch the data from an NSURLSessionDataTask. I'm wondering what is the best way to share that data between the two views. Obviously, I don't want to fetch the data twice for each view. But I'm not sure what is the best practice for making that shared data available/synched between the two views.
A similar question was asked but not answered here.
You can create a model class which holds the map related data in an array/dictionary/custom class objects. You can make this model class as a singleton(can be initialized only once). Both view controllers (i.e the map and table view) can refer to this model to populate date in different views now.
Model Class
-----------
#property (strong, nonatomic) MyCustomDataRepresentationObj *data;
+ (id)sharedModel {
static MyModelClass *sharedModel = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedModel = [[self alloc] init];
});
return sharedModel;
}
-(void)fetchMapDataWithCompletionBlock:(void(^)(id response, NSError *error)onComplete
{
// Check if data is available.
// Note: You can add a refresh data method which will fetch data from remote servers again.
if (!data) {
__weak MyModelClass *weakSelf = self;
// Make HTTP calls here, assume obj is returned value.
// Convert network response to your data structure
MyCustomDataRepresentationObj *objData = [MyCustomDataRepresentationObj alloc] initWith:obj];
// Now hold on to that obj in a property
weakSelf.data = objData;
// Return back the data
onComplete(objData, error);
} else {
onComplete(objData, nil); // Return pre fetched data;
}
}
Now in view controllers you would have to call the model class method which will inturn make the network call(if needed) and returns data in completion block.
View Controller 1
-----------------
-(void)viewDidLoad
{
// This is where the trick is, it returns the same object everytime.
// Hence your data is temporarily saved while your app is running.
// Another trick is that this can be accessed from other places too !
// Like in next view controller.
MyModel *myModelObj = [MyModel sharedModel];
// You can call where ever data is needed.
[myModelObj fetchMapDataWithCompletionBlock:^(id response, NSError *error){
if (!error) {
// No Error ! do whats needed to populate view
}
}];
}
Do the same in other view controller.
View Controller 2
-----------------
-(void)viewDidLoad
{
// Gets the same instance which was used by previous view controller.
// Hence gets the same data.
MyModel *myModelObj = [MyModel sharedModel];
// Call where ever data is needed.
[myModelObj fetchMapDataWithCompletionBlock:^(id response, NSError *error){
if (!error) {
// No Error ! do whats needed to populate view
}
}];
}
Note: I have just jotted down these lines of code here, there might be syntax errors. Its just to get the basic idea.
A UITabBarController act as a Container.
So from your 2 child ViewControllers, you can access the TabBarViewController with the property parentViewController.
So if you want to share the same data with your 2 child ViewControllers, you can fetch and store your data in your UITabBarController. And, from your UIViewControllers, you can access it like this
MyCustomTabBarController *tabBar = (MyCustomTabBarController*)self.parentViewController;
id data = tabBar.myCustomData;
Use Singleton Patterns create a singleton class and initialize singleton instance in your AppDelegate.m this way you can access your singleton class instance from your AppDelegate by using
How about a data fetching object? Make a new class that makes requests for your data bits and stores the results internally.
You then could get the data into your ViewController with a number of different methods:
Direct Reference Associate this object with each ViewController as a property on the ViewControllers before setting the viewControllers property on the Tab Bar Controller.
Your interface to this new class could include the set of fetched results, as well as a method (with a callback when the request finished perhaps) to tell the object to fetch more results.
Notification Center Your object could post notifications when it has more data, and just include a method to start requesting more data.
Delegate + Registration You could create a protocol for objects that want to get told about changes to the data set, make sure all of your necessary ViewControllers conform, and have a delegates NSArray property on your data fetching object. This is far more manual than Notification Center, but it's slightly easier if you need a very robust interface.
Needless to say, there are a lot of ways to handle this, but they all start with designating a class to do the specific task of fetching/storing your data.

Trying to store values received in a ViewController and store them in a single instance of an NSMutableArray in another View Controller in iOS

I have an application where A View Controller (A)is called twice in close succession. Now each time it is called, an NSString object is created, and I need this value to be stored in an NSMutableArray that is a public property of ANOTHER View Controller (B).
In A, I create an instance of the second View Controller (B), and using that instance, add the NSString objects into the NSMutableArray which I've created as a public property. Later, when I am inside View Controller B and print the contents of the NSMutableArray property, the array is empty. Why? Here is the code that is inside View Controller A:
-(void)viewDidLoad {
ViewControllerA *aVC = [[ViewControllerA alloc] init];
if (aVC.stringArray == nil) {
aVC.stringArray = [[NSMutableArray alloc] init];
}
[aVC.stringArray addObject:#"hello"];
[aVC.stringArray addObject:#"world"];
for (NSString *wow in aVC.stringArray) {
NSLog(#"The output is: %#", wow);
}
}
Inside my View Controller B class, I have the following code:
- (IBAction)buttonAction:(UIButton *)sender {
NSLog(#"Button selected");
for (NSString *test in self.stringArray) {
NSLog(#"Here are the contents of the array %#", test);
}
}
Now the buttonAction method gets called, as I do see the line Button selected in the system output, but nothing else is printed. Why? One thing I want to ensure is that View Controller A is called twice, which means I would like to see in the output, "Hello World", "Hello World" (i.e. printed twice), and not "Hello World" printed just once.
The other thing I wish to point out is that View Controller B may not be called at all, or it may be called at a later point in time. In any case, whenever View Controller B is called, I would like to have the values inside the array available, and waiting for the user to access. How do I do this?
Your approach is not ideal, potentially leading to a memory cycle, with two objects holding strong pointers to each other.
You can instead achieve your goal in two ways;
Delegate Protocol
This method allows you to set delegates and delegate methods to pass data back and forth between view controllers
in viewControllerA.h
#protocol viewControllerADelegate <NSObject>
- (void)addStringToNSMutableArray:(NSString *)text;
#end
#interface viewControllerA : UIViewController
#property (nonatomic, weak) id <viewControllerADelegate> delegate;
in viewControllerB.m
// create viewControllerA class object
[self.viewControllerA.delegate = self];
- (void)addStringToNSMutableArray:(NSString *)text
{
[self.mutableArray addObject:text];
}
in viewControllerA.m
[self.delegate addStringToNSMutableArray:#"some text"];
Utility Classes
Alternatively you can use a utility class with publicly accessible methods (and temporary data storage). This allows both viewController classes to access a shared data store, also if you use class methods, you don't even need to instantiate the utility class.
in XYZUtilities.h
#import <Foundation/Foundation.h>
#interface XYZUtilities : NSObject
+ (void)addStringToNSMutableArray;
#property (strong, nonatomic) NSMutableArray *array;
#end
in XYZUtilities.m
+ (void)addStringToNSMutableArray
{
NSString *result = #"some text";
[self.array addObject:result];
}
+ (NSArray)getArrayContents
{
return self.array;
}
in viewControllerA.m
NSString *stringFromObject = [XYZUtilities addStringToNSMutableArray];
in viewControllerB.m
self.mutableArray = [[NSMutableArray alloc] initWithArray:[XYZUtilities getArrayContents]];
I'm not sure what kind of a design pattern you are trying to follow but from the looks of it IMHO that's not a very safe one. However, there are many, many ways this could be accomplished.
One thing though, you said that View Controller B may never get allocated and if it is alloc-ed, it will be down the road. So you can't set a value/property on an object that's never been created.
Since you already aren't really following traditional patterns, you could make a static NSMutableArray variable that is declared in the .m of your View Controller B Class and then expose it via class methods.
So it would look like this:
viewControllerB.h
+(void)addStringToPublicArray:(NSString *)string;
viewContrllerB.m
static NSMutableArray *publicStrings = nil;
+(void)addStringToPublicArray:(NSString *)string{
if (publicStrings == nil){
publicStrings = [[NSMutableArray alloc]init];
}
if (string != nil){
[publicStrings addObject:string];
}
}
Then it would be truly public. All instances of view controller B will have access to it. This, of course is not a traditional or recommended way of doing it—I'm sure that you will have many replies pointing that out ;).
Another idea would be to use a singleton class and store the values in there. Then, when or if view controller B is ever created, you can access them from there.

Confusion on memory address for NSString between view Controllers

In my child view controller, I have a property defined as:
#property (nonatomic, copy) NSString *name;
In view controller A, the Parent, I have the following:
NSString *temp = currency.name; //This is because currency is a Core Data Managed Object.
//I wanted to make sure it wasn't a confounding factor.
childViewController.name = temp;
if(childViewController.name == temp)
NSLog(#"I am surprised");
The problem is that if statement finds equivalency and the "I am surprised" is printed. I thought that == should be checking if they're the same object, and that the use of copy in the property declaration should ensure the setter is making a copy. I checked in the debugger and they are both pointing to the same string. (Which I believe is immutable, which may be why this is happening?)
The same thing happens even if I write childViewController.name = [temp copy];, which I find shocking!
Can anyone explain what is going on here?
Edit: I removed a bit here on worrying about a circular reference which I realized wasn't a concern.
This is an optimization.
For immutable objects, it's superfluous to create an actual copy, so - copy is often implemented as a simple retain, i. e.
- (id)copy
{
[self retain];
return self;
}
Try assigning a mutable object (e. g. NSMutableString) to the property, and you will get the "expected" behavior.

Where to initialise an object

I have a UITableView in my main view and an 'add' button in a UINavigationBar that will push to another view that allows the user to add another object to the tableview. I have a protocol in this view that allows the information to be sent back to the main view.
My problem is that whenever I try to add this new object sent from the protocol (which is a NSMutableDictionary) to a NSMutableDictionary property in the main view, it does not add. I have tried adding an NSLog and it says that this object is null. If I initialise this object in the viewDidLoad method, it will run whenever the UINavigationController pops the view, and resets everything in the dictionary. I do not know where to initialise the object to make sure that it keeps everything stored in it.
The protocol works fine, but I cannot do anything with the object it sends.
In AddCellViewController.h:
#import <UIKit/UIKit.h>
#protocol AddCellDelegate <NSObject>
#required
-(void)passCellInfo:(NSMutableDictionary *)cellInfo;
#end
#interface AddCellViewController : UITableViewController <UITextFieldDelegate> {
id <AddCellDelegate> delegate;
}
#property (strong) id <AddCellDelegate> delegate;
#end
in AddCellViewController.m (the method that utilises the protocol):
-(void)sendObject{
[[self delegate] passCellInfo:newCellInfo];
[self.navigationController popToRootViewControllerAnimated:YES];
}
in MainView.m:
-(void)passCellInfo:(NSMutableDictionary *)cellInfo{
[self.cellInformation setValue:cellInfo forKey:[cellInfo objectForKey:#"cell_title"]];
[self.cells addObject:[cellInfo objectForKey:#"cell_title"]];
[self.tableView reloadData];
NSLog(#"%#: cellInfo - cellInformation: %# - cells: %#",cellInfo ,self.cellInformation,self.cells); //logs the object passed from the protocol (this works), the cellInformation object, and cells object (these return null)
}
You can use a NSMutableDictionary property in a static or global class, and access it very easy from the tableview or any other view:
You feed your table from that dictionary
And you add elements from your other view into that dictionary (the protocol is not needed anymore).
Everytime your tableView appears, you should refresh the data of the table from this global class.
Some example of how to use it:
How to implement global static class
In your code you do this. You using same key every time. So it will get replaced.
[self.cells addObject:cellInfo];
I will tell you a simple way instead. Send your mutableDictionary to secondView from init method. Copy in class level mutableDictionary. Do not allocate or initialize. Add new item in that Dictionary. It will reflect in mainView dictionary. Call [tableView reloadData] in mainView viewWillAppear method.

passing data to UIView before its loaded onto the navigation controller

I have a very complex situation (well for me) I am trying to resolve but thus far am having trouble with it.
I will outline the structure of the application now and then explain the problem I am having.
The names I am using are made up due to sensitivity of the data I am using.
secondToLastViewController // is a UITableView on the navigation stack
lastViewController // is just a normal UIView that i want to push onto the navigation stack
RequestClass // this class dose requests to my database and passed the data back to correct classes
getInfoClass // class is used for this specific request stores the information correctly and passes it back to secondToLastViewController
So as the user initiates didSelectRowAtIndexPath inside secondToLastViewController I make a request for the data using the RequestClass
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
//..
[RequestClass Getinfo:storedInfoPram];
}
now the thread shoots off to my RequestClass, which in turn queries the DB for some data which is then received and this data is passed off to my getInfoClass the reason I have done this is because there are dozens and dozens of different calls in RequestClass all doing different things, this particular request brings back alot of data I have to sort into correct object types so have created this class to do that for me.
anyway inside getInfoClass I sort everything into their correct types etc and pass this data back to secondToLastViewController in a method called recivedData, this is also where I think things are going wrong... as I create a new instance of secondToLastViewController the thing is I dont know how to pass the data back to the same secondToLastViewController that is already on the stack and was where the original request came from.
- (void) recivedData {
// do some stuff then pass data back to secondToLastViewController
SecondToLastViewController *sec = [[SecondToLastViewController alloc] init];
[sec sendGetSeriesArrays:pram1 Pram2:pram2 Pram3:pram3 Pram4:pram4 Pram5:pram5];
}
Now going back into SecondToLastViewController the thread lands in this method
- (void)sendGetSeriesArrays:pram1 Pram2:pram2 Pram3:pram3 Pram4:pram4 Pram5:pram5{
// call detailed view onto the stack
lastViewController *last = [[lastViewController alloc] initWithNibName:#"lastViewController" bundle:nil];
[self.navigationController pushViewController:last animated:YES];
}
after the thread reaches this point nothing happens... all the data is there and ready to be sent but the new view is never pushed to the controller stack.. and I think it is due to me declaring another version of secondToLastViewController when I am inside getInfoClass
what I would like to know firstly is how do I pass the recived data in sendGetSeriesArrays to the final view and secondly how do i even load the lastview onto the navigation stack?
Your observation is correct you are creating the secondToLastViewController instance again inside the getInfoClass. Dont do like that you have to use delegate/protocol approach for passing the data back to the secondToLastViewController.
Do like this
Define a protocol in getInfo class
getInfoClass.h
#protocol GetInfoClassProtocol <NSObject>
//delegate method calling after getting data
// I dont know the argument types give it properly
- (void)sendGetSeriesArrays:pram1 Pram2:pram2 Pram3:pram3 Pram4:pram4 Pram5:pram5;
#end
// declare the delegate property
#property (assign, nonatomic)id<GetInfoClassProtocol>delegate;
getInfoClass.m
- (void) recivedData {
// do some stuff then pass data back to secondToLastViewController
if ([self.delegate respondsToSelector:#selector(sendGetSeriesArrays: param2:)])
{
[self.delegate sendGetSeriesArrays:pram1 Pram2:pram2 Pram3:pram3 Pram4:pram4 Pram5:pram5];
}
}
secondToLastViewController.m
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
//..
RequestClass.delegate = self;
[RequestClass Getinfo:storedInfoPram];
}
Your secondToLastViewController should conform to the GetInfoClassProtocol
There are lots of ways you can accomplish this. In your revivedData function, instead of creating a new instance, you could:
1) Maintain a pointer to the navigation controller in getInfoClass, then you can get the last view controller from the view controllers on the navigation stack and use that. This will be the active instance of the view controller. There are ways to recover this from the window object, but those seem fragile and I would not recommend that approach.
2) You can pass a pointer to self from secondToLastViewController to your RequestClass getInfo call, then hold on to that and pass it back. This is probably a pain depending on the amount of code you have already.
3) You can maintain a static instance of the class if you will never have more than one secondToLastViewController. See How do I declare class-level properties in Objective-C?

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