Using an NSString in a switch statement

问题:

Is it possible to use an NSString in a switch statement?

Or is it better to just use if / else if?

回答1:

switch statement requires integer constants for it cases so NSString cannot be used here, so it seems you have to go for if/else option.

One more point is you must compare NSStrings using isEqualToString: or compare: method, so even if pointer values have been allowed for switch cases you could not use them anyway

回答2:

I use these macros in my app.

#define CASE(str)                       if ([__s__ isEqualToString:(str)]) 
#define SWITCH(s)                       for (NSString *__s__ = (s); ; )
#define DEFAULT   

SWITCH (string) {
    CASE (@"AAA") {
        break;
    }
    CASE (@"BBB") {
        break;
    }
    CASE (@"CCC") {
        break;
    }
    DEFAULT {
        break;
    }
 }

回答3:

In response and in support of @Cœur's answer.. Here is the same thing, but written in Xcode 4.4+ / clang / Whatever "literal syntax" which is even closer to a simple urnary if, else comparison (and that's the point, isnt it.....)

NSDictionary *actionD = @{ @"A" : ^{ NSLog(@"BlockA!"); }, 
                           @"B" : ^{ NSLog(@"BlockB!"); }};

((void(^)()) actionD[@"A"])(); 

BlockA!

or say, you want to perform a selector based on the title of a button...

- (IBAction) multiButtonTarget:button { 

((void (^)())                           // cast
  @{ @"Click?" : ^{ self.click; }, 
     @"Quit!"  : ^{   exit(-1); }}      // define
        [((NSButton*)button).title])    // select
                                    (); // execute
}

Quit! ⟹ exit -1

Brief, like w.string = kIvar == 0 ? @"StringA" : @"StringB";, and far more useful, as you can shove blocks in there without even a thought of some dreadful (and limited, and convoluted) @selector!

EDIT: This is more obviously constructed as such:

[@[ @"YES", @"NO", @"SIRPOOPSALOT"] do:^(id maybe) {
    [maybe isEqual:@"YES"] ? ^{ NSLog(@"You got it!"); }()
  : [maybe isEqual:@"NO" ] ? ^{ NSLog(@"You lose!!!"); }() 
  :                          ^{ NSLog(@"Not sure!"); [self tryAgain]; }();
}]; 

➜ *** You got it! *** ➜ *** You lose!!! *** ➜ *** Not sure! ***

I have to admit, I'm embarrassingly INTO this kind of syntactic silliness. Another option is to forget about what the string is.. just execute it, lol...

[ @{ NSApplicationWillBecomeActiveNotification : @"slideIn",
     NSApplicationDidResignActiveNotification  : @"slideOut" } each:^( id key, id obj ) {
    [w observeObject:NSApp forName:obj calling: NSSelectorFromString ( obj ) ];       
}];

or taking the UI's word for it, literally..

- (IBAction)setSomethingLiterallyWithSegmentedLabel:(id)sender {
   NSInteger selectedSegment = [sender selectedSegment];
   BOOL isSelected = [sender isSelectedForSegment:selectedSegment];
   BOOL *optionPtr = &isSelected;
   SEL fabricated = NSSelectorFromString
       ([NSString stringWithFormat:@"set%@:",[sender labelForSegment:selectedSegment]]);
   [self performSelector:fabricated withValue:optionPtr];
}

回答4:

Switch statement wouldn't work with NSString: it works only with int.

If/Else statement is too much code and often not optimal.

Optimal solution is to use an NSDictionary indexed by the NSString (or other objects) possibilities. Then you directly access the right value/function.

Example 1, when you want to test for @"A" or @"B" and perform methodA or methodB:

NSDictionary *action = @{@"A" : [NSValue valueWithPointer:@selector(methodA)],
                         @"B" : [NSValue valueWithPointer:@selector(methodB)],
                         };
[self performSelector:[action[stringToTest] pointerValue]];

Example 2, when you want to test for @"A" or @"B" and perform blockA or blockB:

NSDictionary *action = @{@"A" : ^{ NSLog (@"Block A"); },
                         @"B" : ^{ NSLog (@"Block B"); },
                         };
((void (^)())action[stringToTest])();

回答5:

inspired by alex gray, I created a category method, that applies chained filters to its object:

.h

#import <Foundation/Foundation.h>
typedef id(^FilterBlock)(id element,NSUInteger idx, BOOL *stop);

@interface NSObject (Functional)
-(id)processByPerformingFilterBlocks:(NSArray *)filterBlocks;
@end

.m

#import "NSObject+Functional.h"

@implementation NSObject (Functional)
-(id)processByPerformingFilterBlocks:(NSArray *)filterBlocks
{
    __block id blockSelf = self;
    [filterBlocks enumerateObjectsUsingBlock:^( id (^block)(id,NSUInteger idx, BOOL*) , NSUInteger idx, BOOL *stop) {
        blockSelf = block(blockSelf, idx, stop);
    }];

    return blockSelf;
}
@end

You can use it as

FilterBlock fb1 = ^id(id element, NSUInteger idx, BOOL *stop){ if ([element isEqualToString:@"YES"]) { NSLog(@"You did it");  *stop = YES;} return element;};
FilterBlock fb2 = ^id(id element, NSUInteger idx, BOOL *stop){ if ([element isEqualToString:@"NO"] ) { NSLog(@"Nope");        *stop = YES;} return element;};

NSArray *filter = @[ fb1, fb2 ];
NSArray *inputArray = @[@"NO",@"YES"];

[inputArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    [obj processByPerformingFilterBlocks:filter];
}];

but you can also do more complicated stuff, like aplied chianed calculations:

FilterBlock b1 = ^id(id element,NSUInteger idx, BOOL *stop) {return [NSNumber numberWithInteger:[(NSNumber *)element integerValue] *2 ];};
FilterBlock b2 = ^id(NSNumber* element,NSUInteger idx, BOOL *stop) {
    *stop = YES;
    return [NSNumber numberWithInteger:[element integerValue]*[element integerValue]];
};
FilterBlock b3 = ^id(NSNumber* element, NSUInteger idx,BOOL *stop) {return [NSNumber numberWithInteger:[element integerValue]*[element integerValue]];};

NSArray *filterBlocks = @[b1,b2, b3, b3, b3];
NSNumber *numberTwo = [NSNumber numberWithInteger:2];
NSNumber *numberTwoResult = [numberTwo processByPerformingFilterBlocks:filterBlocks];    
NSLog(@"%@ %@", numberTwo, numberTwoResult);

回答6:

I know I'm a bit late to the party, but here is my submission for an objective-c switch statement. It's a bit complex, so bear with the ugly macros.

Features:

• looks like a switch statement
• Has thread atomicity built-in
• Works with all objective-c objects, not just NSString (using the -isEqual: selector)
• Could be extended to work with C-types as well, barring structs (as they have no == operator)
• Only one case label can be evaluated per switch statement (break isn't required)
• No chances of infinite loop if no case is ever selected (break isn't required)
• Only reserves one variable name, ____dontuse_switch_var (all others are in the static scope and can be overwritten in the local scope)
• Doesn't result in any odd retain issues (uses __weak references)

Drawbacks:

• Very difficult to understand source
• All case statements are evaluated before the one is selected (so put the most-frequent ones at the top)
• Thread atomicity comes at a cost of performance - it doesn't require any locks, but it does use NSThread quite extensively.
• Without using brackets { or }, Xcode doesn't like to format the statements properly (this is due to the implicit goto label in there.
• Not header only (requires one .m file to work, for NSValue weak refs)

Example:

#include "OBJC_SWITCH.h"

int main()
{
    NSArray *items = @[ @"A", @"B", @"C", @"D", @"E", @"F" ];

    for (int i = 0; i < items.count; i++)
    {
        $switch(items[i]) {
            $case(@"A"):
            {
                NSLog(@"It was A!");
                break;
            }
            $case(@"B"): // no brackets, no break, still works
                NSLog(@"It was B!");

            $case(@"C"): // continue works as well, there's no difference
            {
                NSLog(@"It was C!");
                continue;
            }

            $default: // brackets, but no break.
            {
                NSLog(@"Neither A, B, or C.");
            } 
        }
    }
}

Without further ado, here is the (ugly) code:

OBJC_SWITCH.h:

#import "NSValue+WeakRef.h"

// mapping of threads to the values being switched on
static NSMutableDictionary *____dontuse_switch_variable_dictionary;

// boolean flag to indicate whether or not to stop switching
static NSMutableDictionary *____dontuse_switch_bool_dictionary;

// simple function to return the current thread's switch value
static inline id current_thread_switch_value()
{
    // simple initializer block
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (!____dontuse_switch_variable_dictionary)
            ____dontuse_switch_variable_dictionary = [NSMutableDictionary dictionary];
    });

    return [[____dontuse_switch_variable_dictionary objectForKey:[NSValue valueWithWeakObject:[NSThread currentThread]]] weakObjectValue];
}

// simple function to set the current thread's switch value
static inline void set_current_thread_switch_value(id val)
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (!____dontuse_switch_variable_dictionary)
            ____dontuse_switch_variable_dictionary = [NSMutableDictionary dictionary];
    });

    [____dontuse_switch_variable_dictionary setObject:[NSValue valueWithWeakObject:val] forKey:[NSValue valueWithWeakObject:[NSThread currentThread]]];
}

// check if the current thread has switched yet
static inline BOOL current_thread_has_switched()
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (!____dontuse_switch_bool_dictionary)
            ____dontuse_switch_bool_dictionary = [NSMutableDictionary dictionary];
    });

    return [[____dontuse_switch_bool_dictionary objectForKey:[NSValue valueWithWeakObject:[NSThread currentThread]]] boolValue];
}

// set the current thread's switch state
static inline void set_current_thread_has_switched(BOOL b)
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (!____dontuse_switch_bool_dictionary)
            ____dontuse_switch_bool_dictionary = [NSMutableDictionary dictionary];
    });

    [____dontuse_switch_bool_dictionary setObject:[NSNumber numberWithBool:b] forKey:[NSValue valueWithWeakObject:[NSThread currentThread]]];
}

// concatenate two tokens
#define $_concat(A, B) A ## B
#define $concat(A, B) $_concat(A, B)

/* start of switch statement */
#define $switch(value) { \
/* set this thread's switch value */ \
set_current_thread_switch_value(value); \
/* make sure we reset the switched value for the thread */ \
set_current_thread_has_switched(0); \
/* if statement to ensure that there is a scope after the `switch` */ \
} if (1)

/* a case 'label' */
#define $case(value) \
/* make sure we haven't switched yet */ \
if(!current_thread_has_switched() && \
/* check to see if the values are equal */ \
[current_thread_switch_value() isEqual:value]) \
/* basically, we are creating a 'for' loop that only iterates once, so that we support the 'break' statement, without any harm */ \
/* this also sets the 'switched' value for this thread */ \
for (int ____dontuse_switch_var = 0; set_current_thread_has_switched(1), ____dontuse_switch_var != 1; ____dontuse_switch_var++) \
/* this creates the case label (which does nothing) so that it 'looks' like a switch statement. */ \
/* technically, you could to a goto with this, but I don't think anyone would purposely do that */ \
$concat(__objc_switch_label, __COUNTER__)

/* the default 'label' */
#define $default \
/* this only evaluates if we haven't switched yet (obviously) */ \
if (!current_thread_has_switched()) \
/* exact same loop as for the 'case' statement, which sets that we have switched, and only executes once. */ \
for (int ____dontuse_switch_var = 0; set_current_thread_has_switched(1), ____dontuse_switch_var != 1; ____dontuse_switch_var++) \
/* once again, create a case label to make it look like a switch statement */ \
$concat(__objc_switch_label, __COUNTER__)

I'm not going to provide documentation for this, as it's quite self-explanatory. If it's really that hard to understand, leave a comment & I'll document the code.

NSValue+WeakRef.h:

#import <Foundation/Foundation.h>

@interface NSValue(WeakRef)

+(id) valueWithWeakObject:(__weak id) val;
-(id) initWithWeakObject:(__weak id) val;

-(__weak id) weakObjectValue;

@end

NSValue+WeakRef.m:

#import "NSValue+WeakRef.h"

@interface ConcreteWeakValue : NSValue
{
    __weak id _weakValue;
}

@end

@implementation NSValue(WeakRef)

+(id) valueWithWeakObject:(id) val
{
    return [ConcreteWeakValue valueWithWeakObject:val];
}

-(id) initWithWeakObject:(id)val
{
    return [NSValue valueWithWeakObject:val];
}

-(id) weakObjectValue
{
    [self doesNotRecognizeSelector:_cmd];

    return nil;
}

@end

@implementation ConcreteWeakValue

+(id) valueWithWeakObject:(__weak id)val
{
    return [[self alloc] initWithWeakObject:val];
}

-(id) initWithWeakObject:(__weak id)val
{
    if ((self = [super init]))
    {
        _weakValue = val;
    }

    return self;
}

-(const char *) objCType
{
    return @encode(__weak id);
}

-(__weak id) weakObjectValue
{
    return _weakValue;
}

-(void) getValue:(void *)value
{
    * ((__weak id *) value) = _weakValue;
}

-(BOOL) isEqual:(id)object
{
    if (![object isKindOfClass:[self class]])
        return NO;

    return [object weakObjectValue] == [self weakObjectValue];
}

@end

回答7:

As everyone else has noted, it's probably easiest to just use if/else, but you can create something that looks a lot like a switch statement. I created a project on GitHub that does exactly that: WSLObjectSwitch. It's a pretty naive implementation, it doesn't optimise using hashes, etc., but it does work.

回答8:

This is typically where I use something like an enum. If I have to manage that many values, I just create an enum with the same name as the string I would have passed otherwise and pass it in there, for example:

enum {
    EGLFieldSelectionToolbarItem = 0,
    EGLTextSelectionToolbarItem,
};
+(NSImage *)iconForIdentifier:(int)alias WithSize:(NSSize)size; //Alias should be an enum value with the same name
NSImage *icon = [[NSImage alloc]initWithSize:size];
  NSBezierPath *bezierPath = [NSBezierPath bezierPath];
[icon lockFocus];
switch (alias) {
    case EGLFieldSelectionToolbarItem:
        …//Drawing code
        break;
    case EGLTextSelectionToolbarItem:
        …//More drawing code
    default:
        break;
}
[bezierPath stroke];
[icon unlockFocus];
return icon;
}

回答9:

you can easily switch between buttons for different action using their tags.

Example :

- (IBAction)addPost:(id)sender {
switch ([sender tag]) {
    case 1:
        break;
    case 2:
        break;
    case 3:
        break;
    case 4:
        break;
    case 5:
        break;
    default:
        break;
}

}