This is code from an addition calculator that does operations by entering the two operands first and then the operation; like "5 enter 2 enter +" would result in "7". When the user taps on a number a double will be sent to pushOperand: When a user taps on the addition button the string #"+" will be sent like to performOperation:. My question is what is the point of making those copies in program and runProgram: if they're all shallow copies and their elements all end up pointing to the same elements of NSNumber and NSString objects as _programStack, program, and stack?
#import <Foundation/Foundation.h>
#interface CalculatorBrain : NSObject
#property (nonatomic, readonly) id program;
+(double)runProgram:(id)program;
-(double)performOperation:(NSString *)operation;
#end
#import "CalculatorBrain.h"
#interface CalculatorBrain ()
#property (nonatomic, strong) NSMutableArray *programStack;
#end
#implementation CalculatorBrain
#synthesize programStack = _programStack;
-(NSMutableArray *) programStack {
if (!_programStack)
_programStack = [[NSMutableArray alloc] init];
return _programStack;
}
-(void)pushOperand:(double)operand {
[self.programStack addObject: [NSNumber numberWithDouble: operand]];
}
-(double)performOperation:(NSString *)operation {
[self.programStack addObject: operation];
double result = [CalculatorBrain runProgram: self.program];
return result;
}
-(id)program {
return [self.programStack copy];
}
+(double)runProgram:(id)program {
NSMutableArray *stack;
if ([program isKindOfClass: [NSArray class]])
stack = [program mutableCopy];
return [self popOperandOffProgramStack: stack];
}
+(double)popOperandOffProgramStack:(NSMutableArray *)stack {
double result = 0;
id topOfStack = [stack lastObject];
if (topOfStack)
[stack removeLastObject];
if ([topOfStack isMemberOfClass: [NSNumber class]])
result = [topOfStack doubleValue];
if ([topOfStack isKindOfClass: [NSString class]]) {
NSString *operation = topOfStack;
if ([operation isEqualToString: #"+"]) {
result = [self popOperandOffProgramStack: stack] + [self popOperandOffProgramStack: stack];
}
return result;
}
#end
NSNumber and NSString are immutable so making a shallow copy of a collection of objects that can't changee is safe.
In program it is important to return a copy of programStack rather than the the actual mutable array. This is because programStack is a private internal property declared in a class extension so it is not externally visible. If you returned programStack directly an external user could change it since it is an NSMutableArray. The program method returns an NSArray since copies of mutable objects are immutable, which has the right semantics. You want to give the external user a snapshot of the programStack array, not access to your class internals.
In runProgram the situation is different. The external user passes CalculatorBrain an NSArray to process and the class' internal logic requires that the stack have elements popped off the array as it is processed. Thus you need to make a mutableCopy so that it can be mutated for processing.
Related
I have some problems about the NSMutableSet in Objective-C.
I learnt that the NSSet will compare the two objects' hash code to decide whether they are identical or not.
The problems is, I implemented a class that is subclass of NSObject myself. There is a property NSString *name in that class. What I want to do is when instances of this custom class has the same variable value of "name" , they should be identical, and such identical class should not be duplicated when adding to an NSMutableSet.
So I override the - (NSUInteger)hash function, and the debug shows it returns the same hash for my two instances obj1, obj2 (obj1.name == obj2.name). But when I added obj1, obj2 to an NSMutableSet, the NSMutableSet still contained both obj1, obj2 in it.
I tried two NSString which has the same value, then added them to NSMutableSet, the set will only be one NSString there.
What could be the solution? Thank you for any help!
The custom Class:
Object.h:
#import <Foundation/Foundation.h>
#interface Object : NSObject
#property (retain) NSString *name;
#end
Object.m
#implementation Object
#synthesize name;
-(BOOL)isEqualTo:(id)obj {
return [self.name isEqualToString:[(Object *)obj name]] ? true : false;
}
- (NSUInteger)hash {
return [[self name] hash];
}
#end
and main:
#import <Foundation/Foundation.h>
#import "Object.h"
int main(int argc, const char * argv[])
{
#autoreleasepool {
Object *obj1 = [[Object alloc]init];
Object *obj2 = [[Object alloc]init];
obj1.name = #"test";
obj2.name = #"test";
NSMutableSet *set = [[NSMutableSet alloc] initWithObjects:obj1, obj2, nil];
NSLog(#"%d", [obj1 isEqualTo:obj2]);
NSLog(#"%ld", [set count]);
}
return 0;
}
Instead of implementing isEqualTo: you have to implement isEqual:
- (BOOL)isEqual:(id)object {
return [object isKindOfClass:[MyObject class]] &&
[self.name isEqual:[(MyObject *)object name]];
}
This will (probably falsely) return NO if both self.name and object.name are nil. If you want to return YES if both properties are nil you should use
- (BOOL)isEqual:(id)object {
if ([object isKindOfClass:[MyObject class]]) {
return (!self.name && ![(MyObject *)object name]) ||
[self.name isEqual:[(MyObject *)object name]];
}
return NO;
}
I'm working on a programmable calculator, and for the life of me I can't understand what I'm doing wrong.
Here are the relevant parts of the code. (The code is unfinished, so I know there's extra stuff floating around.)
CalculatorViewController.m
#import "CalculatorViewController.h"
#import "CalculatorBrain.h"
#interface CalculatorViewController ()
#property (nonatomic) BOOL userIsEnteringNumber;
#property (nonatomic) BOOL numberIsNegative;
#property (nonatomic,strong) CalculatorBrain *brain;
#property (nonatomic) NSArray *arrayOfDictionaries;
#property (nonatomic) NSDictionary *dictionary;
#end
#implementation CalculatorViewController
#synthesize display = _display;
#synthesize history = _history;
#synthesize userIsEnteringNumber = _userIsEnteringNumber;
#synthesize numberIsNegative;
#synthesize brain = _brain;
#synthesize arrayOfDictionaries;
#synthesize dictionary;
-(CalculatorBrain *)brain
{
if (!_brain) _brain = [[CalculatorBrain alloc] init];
return _brain;
}
/*snip code for some other methods*/
- (IBAction)variablePressed:(UIButton *)sender
{
NSString *var = sender.currentTitle;
NSDictionary *dict = [self.dictionary initWithObjectsAndKeys:[NSNumber numberWithDouble:3],#"x",[NSNumber numberWithDouble:4.1],#"y",[NSNumber numberWithDouble:-6],#"z",[NSNumber numberWithDouble:8.7263],#"foo",nil];
[self.brain convertVariable:var usingDictionary:dict];
self.display.text = [NSString stringWithFormat:#"%#",var];
self.history.text = [self.history.text stringByAppendingString:sender.currentTitle];
[self.brain pushOperand:[dict objectForKey:var] withDictionary:dict];
}
#end
And here's CalculatorBrain.m.
#import "CalculatorBrain.h"
#interface CalculatorBrain ()
#property (nonatomic, strong) NSMutableArray *operandStack;
#end
#implementation CalculatorBrain
#synthesize operandStack = _operandStack;
-(void)pushOperand:(id)operand withDictionary:(NSDictionary *)dictionary
{
NSNumber *operandAsObject;
if (![operand isKindOfClass:[NSString class]])
{
operandAsObject = operand;
}
else
{
operandAsObject = [dictionary objectForKey:operand];
}
[self.operandStack addObject:operandAsObject];
}
-(double)popOperand
{
NSNumber *operandAsObject = [self.operandStack lastObject];
if (operandAsObject) [self.operandStack removeLastObject];
return [operandAsObject doubleValue];
}
-(double)convertVariable:(NSString *)variable usingDictionary:dictionary
{
double convertedNumber = [[dictionary objectForKey:variable] doubleValue];
return convertedNumber;
}
#end
The thing I'm having trouble understanding is in the CalculatorViewController.m method - (IBAction)variablePressed:(UIButton *)sender. This line crashes the program:
NSDictionary *dict = [self.dictionary initWithObjectsAndKeys:[list of objects and keys]];
But if I make it
NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:[list of objects and keys]];
then everything works fine. But if I try to do
NSDictionary *dict = [[self.dictionary alloc] initWithObjectsAndKeys:[list of objects and keys]];
which seems to me the right thing to do, then XCode won't let me, so I'm obviously not understanding something.
Any thoughts?
+alloc allocates memory for an object. -init... methods initialize the object.
[self.dictionary initWithObjectsAndKeys:... calls -dictionary which is either going to return a dictionary set in that property or nil and then attempts to call init... on it. If the dictionary exists then you are attempting to initialize an object more than once which is not valid. If the property has not been set then the getter will return nil and sending an init... message to nil will do nothing. Either way this is not what you want to do.
[[self.dictionary alloc] init... is also invalid, as the compiler warns you. Now you try to obtain an object from -dictionary and then call the class method +alloc on it.
There seems to be some fundamental confusion here about how objects are created and what property accessors do. I'm not sure how to address that besides suggesting looking at object creation and dot syntax.
First I read this article
I think I should use "copy" in my programe.
Problem is using NSMutableDictionary copy it will terminate.
***** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary removeAllObjects]: mutating method sent to immutable object'**
I have no idea about "mutating method sent to immutable object".
I didn't set NSDictionary to NSMutabledictionary pointer.
Here is my code
.h file
#interface Button : NSObject {
#private
NSString* gID;
NSString* gBackColor;
NSString* gIconImage;
int gIndex;
BOOL gEnable;
BOOL gVisible;
NSString* gText;
NSMutableDictionary* gEvents;
BOOL gUseCircle;
}
#property (nonatomic,copy) NSString *ID;
#property (nonatomic,copy) NSString *BackColor;
#property (nonatomic,copy) NSString *IconImage;
#property int Index;
#property BOOL Enable;
#property BOOL Visible;
#property (nonatomic,copy) NSString *Text;
#property (nonatomic,getter=getEvents,retain) NSMutableDictionary *Events;
#property BOOL UseCircle;
#end
.m file
#implementation Button
#synthesize ID = gID;
#synthesize BackColor = gBackColor;
#synthesize IconImage = gIconImage;
#synthesize Index = gIndex;
#synthesize Enable = gEnable;
#synthesize Visible = gVisible;
#synthesize Text = gText;
#synthesize Events = gEvents;
#synthesize UseCircle = gUseCircle;
-(NSMutableDictionary*) getEvents
{
if (!gEvents)
{
gEvents = [[NSMutableDictionary alloc] initWithCapacity:20];
}
return gEvents;
}
- (id) init
{
self = [super init];
if (self != nil)
{
gID = #"";
gBackColor = #"";
gIconImage = #"";
gIndex = 0;
gText = #"";
gUseCircle = NO;
}
return self;
}
- (void) dealloc
{
[gID release];
[gBackColor release];
[gIconImage release];
[gText release];
[gEvents removeAllObjects];
[gEvents release];
gEvents = nil;
[super dealloc];
}
And implement
tBtnXML.Events = [self SplitEvents:tNode];
SplitEvents function:
-(NSMutableDictionary*) SplitEvents:(NSDictionary*)pEvents
{
NSMutableDictionary *tEvents = [[NSMutableDictionary alloc] initWithCapacity:5];
// code blabla
//.
//.
//.
[tEvents setObject:tEvent forKey:[NSNumber numberWithInt:tEventName]];
[tEvent release];
return [tEvents autorelease];
}
But I chage NSMutableDictionary* gEvents property from copy to retain , it execute normal.
Colud anyone tell me what's wrong with my code?
If my code is incorrect with dealloc,please tell me.
Thank you appriciate.
Yes, So I fixed my setter:
-(void) setEvents:(NSMutableDictionary*) pEvents
{
NSMutableDictionary* tNewDict = [pEvents mutableCopy];
[gEvents removeAllObjects];
[gEvents release];
gEvents = tNewDict;
}
This work with no error.
It helps me a lot.
But I can't vote up >"<~
So thank you Bavarious :)
In general, mutable properties should be retain instead of copy. When you declare a property as being copy, the synthesised setter method sends -copy to the object that’s being assigned to the property. In the case of mutable objects (e.g. NSMutableDictionary), sending -copy to them makes an immutable copy, effectively creating an object of immutable type (e.g. NSDictionary) instead.
So in:
tBtnXML.Events = [self SplitEvents:tNode];
the synthesised setter sends -copy to [self SplitEvents:tNode], thus creating an immutable copy of that dictionary (i.e., an NSDictionary instance), and assign it to gEvents. This is the cause of your error: gEvents is declared as NSMutableDictionary but points to an NSDictionary instead.
For the record, mutable classes usually declare a -mutableCopy method that does make a mutable copy. It is not used by declared properties, though. If you do not want to use retain, you need to implement a custom setter that uses -mutableCopy.
How can I call a method with multiple params like below with performSelectorInBackground?
Sample method:
-(void) reloadPage:(NSInteger)pageIndex firstCase:(BOOL)firstCase;
The problem is that performSelectorInBackground:withObject: takes only one object argument. One way to get around this limitation is to pass a dictionary (or array) of arguments to a "wrapper" method that deconstructs the arguments and calls your actual method:
- (void)callingMethod {
NSDictionary * args = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInteger:pageIndex], #"pageIndex",
[NSNumber numberWithBool:firstCase], #"firstCase",
nil];
[self performSelectorInBackground:#selector(reloadPageWrapper:)
withObject:args];
}
- (void)reloadPageWrapper:(NSDictionary *)args {
[self reloadPage:[[args objectForKey:#"pageIndex"] integerValue]
firstCase:[[args objectForKey:#"firstCase"] boolValue]];
}
- (void)reloadPage:(NSInteger)pageIndex firstCase:(BOOL)firstCase {
// Your code here...
}
This way you're only passing a "single" argument to the backgrounding call, but that method can construct the multiple arguments you need for the real call (which will take place on the same backgrounded thread).
I've just found this question and wasn't happy with any of the answers. In my opinion neither make good use of the tools available, and passing around arbitrary information in arrays and dictionaries generally worries me.
So, I went and wrote a small NSObject category that will invoke an arbitrary selector with a variable number of arguments:
Category Header
#interface NSObject (NxAdditions)
-(void)performSelectorInBackground:(SEL)selector withObjects:(id)object, ... NS_REQUIRES_NIL_TERMINATION;
#end
Category Implementation
#implementation NSObject (NxAdditions)
-(void)performSelectorInBackground:(SEL)selector withObjects:(id)object, ...
{
NSMethodSignature *signature = [self methodSignatureForSelector:selector];
// Setup the invocation
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
invocation.target = self;
invocation.selector = selector;
// Associate the arguments
va_list objects;
va_start(objects, object);
unsigned int objectCounter = 2;
for (id obj = object; obj != nil; obj = va_arg(objects, id))
{
[invocation setArgument:&obj atIndex:objectCounter++];
}
va_end(objects);
// Make sure to invoke on a background queue
NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithInvocation:invocation];
NSOperationQueue *backgroundQueue = [[NSOperationQueue alloc] init];
[backgroundQueue addOperation:operation];
}
#end
Usage
-(void)backgroundMethodWithAString:(NSString *)someString array:(NSArray *)array andDictionary:(NSDictionary *)dict
{
NSLog(#"String: %#", someString);
NSLog(#"Array: %#", array);
NSLog(#"Dict: %#", dict);
}
-(void)someOtherMethod
{
NSString *str = #"Hello world";
NSArray *arr = #[#(1337), #(42)];
NSDictionary *dict = #{#"site" : #"Stack Overflow",
#"url" : [NSURL URLWithString:#"http://stackoverflow.com"]};
[self performSelectorInBackground:#selector(backgroundMethodWithAString:array:andDictionary:)
withObjects:str, arr, dict, nil];
}
Well, I have used this:
[self performSelectorInBackground:#selector(reloadPage:)
withObject:[NSArray arrayWithObjects:pageIndex,firstCase,nil] ];
for this:
- (void) reloadPage: (NSArray *) args {
NSString *pageIndex = [args objectAtIndex:0];
NSString *firstCase = [args objectAtIndex:1];
}
with performSelectorInBackground you can only pass one argument, so make a custom object for this method to hold your data, itll be more concise than an ambiguous dictionary or array. The benefit of this is you can pass the same object around when done containing several return properties.
#import <Foundation/Foundation.h>
#interface ObjectToPassToMethod : NSObject
#property (nonatomic, strong) NSString *inputValue1;
#property (nonatomic, strong) NSArray *inputArray;
#property (nonatomic) NSInteger returnValue1;
#property (nonatomic) NSInteger returnValue2;
#end
and pass that object to your method:
ObjectToPassToMethod *obj = [[ObjectToPassToMethod alloc] init];
obj.inputArray = #[];
obj.inputValue1 = #"value";
[self performSelectorInBackground:#selector(backgroundMethod:) withObject:obj];
-(void)backgroundMethod:(ObjectToPassToMethod*)obj
{
obj.returnValue1 = 3;
obj.returnValue2 = 90;
}
make sure to clean up the object when done to prevent memory leaks
Alright guys, I'm quite confused. So, I have an NSDictionary which is populated by a JSON string which looks like:
{"Success":true,"Devices":[{"UDId":"...","User":"...","Latitude":0.0,"Longitude":0.0}]}
Now, I know how to check if Success is true, but I need to loop through the array of Devices (JSON object) and create an internal array of Devices (internal app object) and I have no idea how to do that. Can someone please explain how to do it?
Here's my Device.m/h:
#import <CoreLocation/CoreLocation.h>
#import <Foundation/Foundation.h>
#interface Device : NSObject {
NSString *udId;
NSString *name;
NSNumber *latitude;
NSNumber *longitude;
}
#property (nonatomic, retain) NSString *udId;
#property (nonatomic, retain) NSString *name;
#property (nonatomic, retain) NSNumber *latitude;
#property (nonatomic, retain) NSNumber *longitude;
#pragma mark -
#pragma mark MKAnnotation Properties
#property (nonatomic, readonly) CLLocationCoordinate2D coordinate;
#end
----
#import "Device.h"
#implementation Device
#synthesize udId, name, latitude, longitude;
- (CLLocationCoordinate2D)coordinate {
CLLocationCoordinate2D internalCoordinate;
internalCoordinate.latitude = [self.latitude doubleValue];
internalCoordinate.longitude = [self.longitude doubleValue];
return internalCoordinate;
}
- (void)dealloc {
[udId release];
udId = nil;
[name release];
name = nil;
[latitude release];
latitude = nil;
[longitude release];
longitude = nil;
[super dealloc];
}
#end
And here's the methods where I should be reading the response and converting it to objects I can use:
- (void)requestFinished:(ASIHTTPRequest *)request {
if (![request error]) {
NSError *jsonError = nil;
NSDictionary *jsonDictionary = [NSDictionary dictionaryWithJSONString:[request responseString] error:&jsonError];
if (!jsonError || ([[jsonDictionary objectForKey:#"Success"] intValue] == 1)) {
// READ "DEVICES" AND CONVERT TO OBJECTS
} else {
// AUTHORIZATION FAILED
}
}
}
I'd really appreciate some help on this. I just can't seem to wrap my head around it...
Thanks in advance!
You are almost there. In your code where you say:
// READ "DEVICES" AND CONVERT TO OBJECTS
do this:
NSArray * devices = [jsonDictionary objectForKey:#"Devices"];
for(NSDictionary * deviceInfo in devices) {
Device * d = [[[Device alloc] init] autorelease];
[d setLatitude:[deviceInfo objectForKey:#"Latitude"]];
[d setLongitude:[deviceInfo objectForKey:#"Longitude"]];
[d setName:[deviceInfo objectForKey:#"User"]];
[d setUdId:[deviceInfo objectForKey:#"UDId"]];
// do some stuff with d
}
What's going on here: I didn't see what JSON library you are using to convert, but presuming it works like TouchJSON or SBJSON, the JSON array is automatically turned into an NSArray instance, while the inner hashes of the NSArray are NSDictionary objects. At the point that you have deserialized that JSON string, everything you're dealing with will be instances of NSString, NSNumber, NSArray and NSDictionary (and depending on the library, NSNull to represent null values).
First you need to define your initializer/constructor for your Device class.
Device.h
- (id)initWithUdid:(NSString *)udid name:(NSString *)name latitude:(NSNumber *)lat longitude:(NSNumber *)lon;
Device.m
- (id)initWithUdid:(NSString *)udid name:(NSString *)name latitude:(NSNumber *)lat longitude:(NSNumber *)lon {
if (self = [super init]) {
self.udid = udid;
self.name = name;
self.latitude = lat;
self.longitude = lon;
}
return self;
}
Then you can initialize a new object like:
Device *dev = [[Device alloc] initWithUdid:#"a udid" name:#"the name" latitude:latNum longitude:lonNum];
So, you should be able to iterate the array and build your Device objects like so:
NSArray *devicesArray = [dict objectForKey:#"Devices"];
for (NSDictionary *d in devicesArray) {
Device *dev = [[Device alloc] initWithUdid:[d objectForKey:#"UDId"]
name:[d objectForKey:#"User"]
latitude:[NSNumber numberWithDouble:[d objectForKey:#"Latitude"]]
longitude:[NSNumber numberWithDouble:[d objectForKey:#"Latitude"]]];
}
You want to access the array of device dictionaries from the top-level dictionary just as you did the Success value. Then iterating over the dictionaries you can use each's -keyEnumerator method to iterate over its keys.
- (void)requestFinished:(ASIHTTPRequest *)request {
if (![request error]) {
NSError *jsonError = nil;
NSDictionary *jsonDictionary = [NSDictionary dictionaryWithJSONString:[request responseString] error:&jsonError];
if (!jsonError || ([[jsonDictionary objectForKey:#"Success"] intValue] == 1)) {
NSArray* deviceArray = [jsonDictionary objectForKey:#"Devices"];
for(NSDictionary* dict in deviceArray)
{
for(NSString* key in [dict keyEnumerator])
{
NSLog(#"%# -> %#", key, [dict objectForKey:key]);
}
}
// READ "DEVICES" AND CONVERT TO OBJECTS
} else {
// AUTHORIZATION FAILED
}
}
}
Sounds like you need to reuse your line:
[jsonDictionary objectForKey:#"Success"]
try having a look at
[jsonDictionary objectForKey:#"Devices"]
You really need to figure out what type it returns.
If you're lucky, it returns an NSDictionary, or alternately something that you can easily turn into an NSDictionary.