In which case we need a pointer of a BOOL variable in Objective C language?
I have a code for collapsible UITableView in which there is a function declaration:
- (void)toggle:(BOOL*)isExpanded section:(NSInteger)section;
and its definition is:
- (void)toggle:(BOOL*)isExpanded section:(NSInteger)section
{
*isExpanded = !*isExpanded;
NSArray *paths = [self indexPathsInSection:section];
if (!*isExpanded)
{
[self.tableview deleteRowsAtIndexPaths:paths withRowAnimation:UITableViewRowAnimationFade];
}
else
{
[self.tableview insertRowsAtIndexPaths:paths withRowAnimation:UITableViewRowAnimationFade];
}
*isExpanded = !*isExpanded; What is the meaning of this statement as I have never used this kind of statement in case of BOOL Variable.
Following are other two functions of same code which are called in the sequence of above function:
- (NSArray*)indexPathsInSection:(NSInteger)section
{
NSMutableArray *paths = [NSMutableArray array];
NSInteger row;
for ( row = 0; row < [self numberOfRowsInSection:section]; row++ )
{
[paths addObject:[NSIndexPath indexPathForRow:row inSection:section]];
}
return [NSArray arrayWithArray:paths];
}
- (NSInteger)numberOfRowsInSection:(NSInteger)section
{
return [[sectionDataArray objectAtIndex:section] count];
}
sectionDataArray is the array for number of rows in each section. I may be unclear but if you got my point please explain all this.
Here is the link to that code
Thanks
Pointers to variables (or pointer to pointers of objects) are useful, if you want to change their value in another function. If you pass a plain BOOL to a method, you can use it, but if you change it, this change is only local as it was passed by value. If you pass the pointer/address of the variable instead, you can also change its real value.
This comes in handy if you need more than one return value and don't want to wrap it up in an object. It's also a common pattern in Cocoa where NSError variables are passed as pointers, i.e -(BOOL) doSomethingError:(NSError **)error.
It's unclear why you did have to pass the pointer to a BOOL. Even if you were tracking the expanded status of all sections using an array of BOOLs, I would guess setting it up as an instance variable would have been better.
*isExpanded = !*isExpanded;
This is basically flipping the value of the BOOL. Since you've passed a pointer, you will be able to see the flip in the caller method.
As such, the second part indulges itself in building a list of all index paths in a particular section so that the app can delete the section using deleteRowsAtIndexPaths:withRowAnimation: method of UITableView. Somehow using deleteSections:withRowAnimation: method could've been easier. Same thing goes for insert.. methods.
An array of booleans
Assuming that you need to maintain a dynamic array of BOOLs. NSMutableArray is a proper fit.
Say, you start off with a certain number of sections that have expanded set to NO.
/* In the interface of the view controller */
NSMutableArray * expandedStatuses;
#property (nonatomic, retain) NSMutableArray * expandedStatuses;
/* In viewDidLoad or viewWillAppear:, as needed */
self.expandedStatuses = [NSMutableArray array];
for ( int i = 0; i < numberOfSections; i++ ) {
[self.expandedStatuses addObject:[NSNumber numberWithBool:NO]];
}
Later, in your toggle:section: method do this,
/* Getting the expanded status of the section */
BOOL expanded = [[self.expandedStatuses objectAtIndex:section] boolValue];
/* Flipping the expanded status of the section */
[self.expandedStatuses replaceObjectAtIndex:section withObject:[NSNumber numberWithBool:!expanded]];
If you are interested in deleting a particular section,
[self.expandedStatuses removeObjectAtIndex:sectionIndexToDelete];
And if you are interested in adding,
/* Adding at the end */
[self.expandedStatuses addObject:[NSNumber numberWithBool:NO]];
/* Or adding it in between */
[self.expandedStatuses insertObject:[NSNumber numberWithBool:NO] atIndex:sectionIndexToInsertAt];
So NSMutableArray is pretty flexible to play with. You can consider it as an option to switch to.
They are intended to use isExpanded even after the exit of toggle: method with the modified value.
*isExpanded = !*isExpanded;
You could avoid using BOOL pointer in method by declaring isExpanded as the member variable (iVar) pf your class .
And use belwo
isExpanded = !isExpanded;
Instead of using
*isExpanded = !*isExpanded;
You can try using
isExpanded[0] = !isExpanded[0];
BOOL is not class type it is
typedef signed char BOOL;
You can not have pointer to BOOL variable.
Related
I'm testing a custom table view style class:
HorizontalTable
It produces a horizontal table view.
One of the delegate methods equivalent to tableView:numberOfRowsInSection: is:
- (NSInteger)numberOfColumnsForTableView:(HorizontalTableView *)tableView.
If I give this a number (ex: return 10;) it is happy and it give me the number "cells" that I want. But if I feed it a value of someArray.count or an int or NSInteger variable, the table view just comes out blank, delivering no cells.
I think that the method in the custom table view class that receives the NSInteger value is this:
- (NSUInteger)numberOfPages {
NSInteger numPages = 0;
if (_delegate)
numPages = [_delegate numberOfColumnsForTableView:self];
return numPages;
}
Do I need to cast the result of someArray.count to an NSInteger?
Here you get the value from array means your array not nil so just debug and check the if condition that its come in that condition or not and what you get from NSLog
- (NSUInteger)numberOfPages {
NSInteger numPages = 0;
if (_delegate){
numPages = [_delegate numberOfColumnsForTableView:self];
NSLog(#"Total record %d",numPages);//what you get here?
}
return numPages;
}
numPages = [_delegate numberOfColumnsForTableView:self];
//self requires an object of type HorizontalTableView
NSMutableArray is editable, where as NSArray is read-only.
NSMutableArray is a subclass of NSArray and responds to messages such as addObject, removeObject and so forth; i.e. it is mutable, like the name says. Instead, NSArray is immutable, i.e. you can't add/remove objects.
In fact converting the value to be returned to the count of an NSArray, as opposed to the count an NSMutableArray fixes the issue. Why? I am not sure why an NSArray's count value is valid but not the NSMutableArray's. Anyone?
I'm having trouble making the sections in a UITableView. I've looked at the documentation for UILocalizedIndexedCollation as well as this sample code project:
https://developer.apple.com/library/ios/#samplecode/TableViewSuite/Listings/3_SimpleIndexedTableView_Classes_RootViewController_m.html#//apple_ref/doc/uid/DTS40007318-3_SimpleIndexedTableView_Classes_RootViewController_m-DontLinkElementID_18
What I have below is basically a straight copy/paste from the sample project. However, the sample project uses a custom object (TimeZoneWrapper.h) and then places the object in the correct section based on the object's instance variable (TimeZoneWrapper.localeName). However, I'm not using custom objects. I'm using just a bunch of regular NSStrings. So my question is what method on NSString should I pass to the #selector() to compare and place the string in the correct section array?
Currently, I'm calling NSString's copy method as a temporary hack to get things working (which it does), but I'm not sure if this is correct. A little explanation would be much appreciated!
- (void)configureSections {
// Get the current collation and keep a reference to it.
self.collation = [UILocalizedIndexedCollation currentCollation];
NSInteger index, sectionTitlesCount = [[collation sectionTitles] count]; // sectionTitles are A, B, C, etc.
NSMutableArray *newSectionsArray = [[NSMutableArray alloc] initWithCapacity:sectionTitlesCount];
// Set up the sections array: elements are mutable arrays that will contain the locations for that section.
for (index = 0; index < sectionTitlesCount; index++) {
NSMutableArray *array = [[NSMutableArray alloc] init];
[newSectionsArray addObject:array];
}
// Segregate the loctions into the appropriate arrays.
for (NSString *location in locationList) {
// Ask the collation which section number the location belongs in, based on its locale name.
NSInteger sectionNumber = [collation sectionForObject:location collationStringSelector:#selector(/* what do I put here? */)];
// Get the array for the section.
NSMutableArray *sectionLocations = [newSectionsArray objectAtIndex:sectionNumber];
// Add the location to the section.
[sectionLocations addObject:location];
}
// Now that all the data's in place, each section array needs to be sorted.
for (index = 0; index < sectionTitlesCount; index++) {
NSMutableArray *locationsArrayForSection = [newSectionsArray objectAtIndex:index];
// If the table view or its contents were editable, you would make a mutable copy here.
NSArray *sortedLocationsArrayForSection = [collation sortedArrayFromArray:locationsArrayForSection collationStringSelector:#selector(/* what do I put here */)];
// Replace the existing array with the sorted array.
[newSectionsArray replaceObjectAtIndex:index withObject:sortedLocationsArrayForSection];
}
self.sectionsArray = newSectionsArray;
}
Thanks in advance!
You should use #selector(self).
Using #selector(copy) will cause memory leaks in your project
I am developing an iphone application which has some data stored in a sqllite database. When my view loads i would like to load the data from the database on a background thread. The problem is the application keeps crashing and i dont know why.
The code:
-(id) init
{
if((self=[super init]))
{
[self performSelectorInBackground:#selector(loadList) withObject:nil];
}
}
-(void) loadList
{
#autoreleasepool
{
Loader * loader = [[Loader alloc] init];
NSMutableArray * array = [loader getItemList];
[array retain];
NSLog(#"Got %d items",[array count]);
[self performSelectorOnMainThread:#selector(createList:) withObject:array waitUntilDone:false];
[loader release];
}
}
-(void) createList: (NSMutableArray*) array
{
items = array;
int i;
Item * it;
for(i = 0; i < [items count]; i++)
{
it = [items objectAtIndex: i];
[it getName]; // crashes
// populate the list
}
}
Loader returns a NSMutableArray with Item objects. The application crashes when i call the item getName (which returns a NSString*). From what i understand it crashes because the item name properties is being released. What am i doing wrong?
Thanks!
It's likely to be a problem with whatever type of object you're using to populate array.
I'm unable to find finger-on-paper proof but I'm confident that performSelectorOnMainThread:withObject:waitUntilDone: retains its object. However if each of the items in array keeps a reference to loader then they need to take responsibility for retaining that object. It looks like you're attempting to keep it alive manually but — as Chuck alludes to — your call to performSelector... will return instantly and not wait for the call you've made to complete.
This particular bug appears to be that you're passing waitUntilDone:NO, so the array is being released immediately and consequently so are its items.
But in general, UIKit is not thread-safe, so this is just a touchy design. I would probably put the loading of this stuff in another class that handles the task for you instead of right in the view.
I'd put a breakpoint on the line:
it = [items objectAtIndex: i];
Then type
po it
in the debugger, and see what's in the name field. As a guess, I'd say one of two things: 1) the field that getName returns isn't initialized with an object (i.e. isn't a real NSString *) or that you're getting a C string from SQLite (which is what it usually returns) and you're trying to treat it as an NSString *. If it's the latter you can use [myCString stringWithUTF8String] to convert the C string into an NSString *
I want to have a NSDictionary that maps from UIViews to something else.
However, since UIViews do not implement the NSCopying protocol, I can't use them directly as dictionary keys.
You can use an NSValue holding the pointer to the UIView and use this as key. NSValues
are copyable. but, if the view is destroyed, the NSValue will hold a
junk pointer.
Here is the actual code (based on the answer by luvieere and further suggestion by Yar):
// create dictionary
NSMutableDictionary* dict = [NSMutableDictionary new];
// set value
UIView* view = [UILabel new];
dict[[NSValue valueWithNonretainedObject:view]] = #"foo";
// get value
NSString* foo = dict[[NSValue valueWithNonretainedObject:view]];
Although this isn't really what they're intended for, you could whip up a functional dictionary-like interface using Associative References:
static char associate_key;
void setValueForUIView(UIView * view, id val){
objc_setAssociatedObject(view, &associate_key, val, OBJC_ASSOCIATION_RETAIN);
}
id valueForUIView(UIView * view){
return objc_getAssociatedObject(view, &associate_key);
}
You could even wrap this up in a class ThingWhatActsLikeADictionaryButWithKeysThatArentCopyable*; in that case you might want to retain the views that you use as keys.
Something like this (untested):
#import "ThingWhatActsLikeADictionaryButWithKeysThatArentCopyable.h"
#import <objc/runtime.h>
static char associate_key;
#implementation ThingWhatActsLikeADictionaryButWithKeysThatArentCopyable
- (void)setObject: (id)obj forKey: (id)key
{
// Remove association and release key if obj is nil but something was
// previously set
if( !obj ){
if( [self objectForKey:key] ){
objc_setAssociatedObject(key, &associate_key, nil, OBJC_ASSOCIATION_RETAIN);
[key release];
}
return;
}
[key retain];
// retain/release for obj is handled by associated objects functions
objc_setAssociatedObject(key, &associate_key, obj, OBJC_ASSOCIATION_RETAIN);
}
- (id)objectForKey: (id)key
{
return objc_getAssociatedObject(key, &associate_key);
}
#end
*The name may need some work.
Provided you don't need to support before iOS 6, NSMapTable (suggested by neilsbot) works well because it can provide an enumerator over the keys in the collection. That's handy for code common to all of the text fields, like setting the delegate or bi-directionally syncing the text values with an NSUserDefaults instance.
in viewDidLoad
self.userDefFromTextField = [NSMapTable weakToStrongObjectsMapTable];
[self.userDefFromTextField setObject:#"fooUserDefKey" forKey:self.textFieldFoo];
[self.userDefFromTextField setObject:#"barUserDefKey" forKey:self.textFieldBar];
// skipped for clarity: more text fields
NSEnumerator *textFieldEnumerator = [self.userDefFromTextField keyEnumerator];
UITextField *textField;
while (textField = [textFieldEnumerator nextObject]) {
textField.delegate = self;
}
in viewWillAppear:
NSEnumerator *keyEnumerator = [self.userDefFromTextField keyEnumerator];
UITextField *textField;
while (textField = [keyEnumerator nextObject]) {
textField.text = [self.userDefaults stringForKey:[self.textFields objectForKey:textField]];
}
in textField:shouldChangeCharactersInRange:replacementString:
NSString *resultingText = [textField.text stringByReplacingCharactersInRange:range withString:string];
if(resultingText.length == 0) return YES;
NSString *preferenceKey = [self.textFields objectForKey:textField];
if(preferenceKey) [self.userDefaults setString:resultingText forKey:preferenceKey];
return YES;
And now I will go cry, because I implemented all of this before realizing that my iOS 5.1-targeted app can't use it. NSMapTable was introduced in iOS 6.
Rather than store a pointer to the view and risk the garbage issue, just give the UIView a tag and store the tag's value in the dictionary. Much safer.
I'm using a simple solution under ARC provided by Objective-C++.
MyClass.mm:
#import <map>
#implementation MyClass
{
std::map<UIView* __weak, UIColor* __strong> viewMap;
}
- (void) someMethod
{
viewMap[self.someView] = [UIColor redColor];
}
In this example I am getting stronger type checking by making all the values have to be a UIColor* which is all I needed this for. But you could also use id as the value type if you want to allow any object as the value, ex: std::map<UIView* __weak, id __strong> viewMap; Likewise for keys: id __weak, id __strong> viewMap;
You can also vary the __strong and __weak attributes as needed. In my case, the views are already retained by the view controller that I use this in, so I saw no need to take a strong pointer to them.
a simple solution when you just want UIView as key occasionally,I use it to store UILabel and UIColor
NSArray<UIView *> *views = #[viewA,viewB,viewC,viewD];
NSArray *values = #[valueA,valueB,valueC,valueD];
for(int i = 0;i < 4;i++) {
UIView *key = views[i];
id value = values[i]
//do something
}
id value = values[[views indexOfObject:key]]
I asked a similar question, but I couldn't get it working exactly. I'm building an iPhone app, and there is a method that I want called from different files. I figured the easiest way would simply be to make a method in another file, and call the method from the other files.
Here are some problems. I need to return multiple values from the method, after passing it multiple values. For example, I'm passing it: (int, int, int, string, string). And it needs to return all of those values, after they have been changed. Someone showed me this code:
- (NSDictionary *)EndOfTurn:(int)varTurns withFatness:(int)varFatness
{
varTurns--;
if (varTurns <= 0) {
varFatness = varFatness - 5;
}
else {
varFatness += 2;
}
return [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:varFatness], #"FATNESS", [NSNumber numberWithInt:varTurns], #"TURNS", nil];
}
However, this code doesn't work, and I need some more information to really understand it. Let's assuming I'm passing it these values:
int varMoney;
int varNumSheep;
int varNumShepherds;
NSString *test1;
NSString *test2;
So I need to get all of these values back from the method.
How do I declare this in the header file? This should be in an Objective-C file, but could you give me the code for the entire file so I can see where it would go with the #implementation and #end, whatnot. Also, how would I call this method?
What about passing in the values as pointers?
For example:
- (void) getValuesForInt:(int *)int1 anotherInt:(int *)int2 aBool:(BOOL *)bool1 anotherBool:(BOOL *)bool2 {
if (*int1 == 42 && *int2 == 0) {
*int1 = 0;
*int2 = 42;
}
if (*bool1 == NO) {
*bool2 = YES;
}
}
Then you can invoke it like:
int int1 = 42;
int int2 = 0;
BOOL bool1 = NO;
BOOL bool2 = NO;
[self getValuesForInt:&int1 anotherInt:&int2 aBool:&bool1 anotherBool:&bool2];
NSLog(#"int1: %d int2: %d bool1: %d bool2: %d", int1, int2, bool1, bool2);
//prints "int1: 0 int2: 42 bool1: 0 bool2: 1"
Edit:
This works equally well with objects. You'll often see this used when dealing with NSError objects:
NSError *error = nil;
[anObject doSomething:foo error:&error];
Can be implemented as:
- (void) doSomething:(id)terrible error:(NSError **)error {
if ([terrible isEqual:reallyBad]) {
if (error != nil) { *error = [NSError errorWithDomain:#"domain" code:42 userInfo:nil]; }
}
}
You can use a block closure to pass back multiple values from a method like this. -rrh
[self heyFunctionGiveMeBackTwoValuesFromThisFruitArray:#[#"apple", #"orange", #"banana", #"apple"] findThisFruit:#"apple" closureFunction:^(int fruitCount, NSString* fruitString)
{
NSLog(#"Two values returned, int-fruitCount:%d, NSString-fruiteString:%#", fruitCount, fruitString);
}];
- (void)heyFunctionGiveMeBackTwoValuesFromThisFruitArray:(NSArray*)fruitsArray findThisFruit:(NSString*)findThisFruit closureFunction:(void (^)(int fruitCount, NSString *fruitString))passBackResultsUsingThisClosure
{
NSInteger fruitsFound = 0;
NSString* fruitsMessage = [NSString stringWithFormat:#"No %# Found", findThisFruit];
for (NSString* string in fruitsArray)
{
if ([string compare:findThisFruit] == NSOrderedSame)
{
fruitsFound++;
}
}
if (fruitsFound > 0)
{
fruitsMessage = [NSString stringWithFormat:#"You have %# on your list this many times:%d", findThisFruit, fruitsFound];
}
passBackResultsUsingThisClosure(fruitsFound, fruitsMessage);
}
Results:
Two values returned, int-fruitCount:2, NSString-fruiteString:You have apple on your list this many times:2
If you have that many different things that need to be returned from a method, either encapsulate it into an NSDictionary as others have suggested or consider just defining a class. You can declare the instance variables and properties to encapsulate the data, as needed.
Defining a class to encapsulate such information proves to be quite efficient and maximizes flexibility. If you need to refactor your app such that the collection of data gains new fields, needs to be saved for later, or might need to gain functionality, a class will ease these changes.
Since you can only return a single value from any method in C and C-derived languages, you simply need to return a single value that represents all of your other values. This is what your sample code is doing with an NSDictionary.
The sample code is correct, even if it's a bit contrary to common Objective-C style.
What you declare in the header file is simply the declaration of the method, that is:
#interface MyClass : NSObject
- (NSDictionary *)EndOfTurn:(int)varTurns withFatness:(int)varFatness;
#end
In the source file, then:
#implementation MyClass
// code, as given above
#end
If you only need to return primitive values, then returning a struct may be the optimal solution. You get compile-time error checking (e.g. as opposed to an NSDictionary where you could attempt to read an invalid key), while not requiring all the code/files involved in creating a class.
typedef struct myStruct {
int varMoney;
int varNumSheep;
int varNumShepherds;
} myStruct;
Apple uses structs in many of their methods too (e.g. CGPoint, CGRect).
The reason this won't work with objects is because ARC forbids this.
One slight improvement to the last point in some designs is to use a struct holding enum members. This gives you the compile-time checking already mentioned, something that looks like an object in the return value, and the benefit of clear cases if you need to check the values in the return.
The struct:
typedef struct _SIXRecorderStateChange {
SIXRecorderState oldState;
SIXRecorderState newState;
} SIXRecorderStateChange;
The client code:
SIXRecorderStateChange stateChange = [recorderState stop];
if (stateChange.newState == SIXRecorderStopped) {
...
...