iridium.jp::Notes

Compile-time keypath safety macro for Objective-C

When using KVO or KVC in Objective-C, we encounter opportunities where we have to write a property name as a string, such as valueForKey:, valueForKeyPath:, willAccessValueForKey:, willChangeValueForKey:, etc.

Specifying a property name as a string means that if the property name is changed, the string part must also be explicitly changed at the same time. If the modification is forgotten, the compiler will not warn us and the code will crash at run-time.

Using NSStringFromSelector() and @selector() together as follows makes the compiler to be able to issue a warning. So, this is a famous solution.

Famous code:

// Use NSStringFromSelector() with @selector()
[self valueForKey:NSStringFromSelector(@selector(param_))];

While this technique is valid, it will cost some little overhead because NSStringFromSelector() operates at run-time. And also, it can't handle keypaths. So, it's time to think about techniques which enable compile-time warnings if possible without using NSStringFromSelector().

Are there any techniques to take advantage of the warning function in @selector(), and at the same time specify a property name as a string? See below codes.

How this works:

// Activate compile-time keypath check with @selector()
// Single level keypath
[self valueForKey:((void)@selector(param_), @"param_")];

// Multi level keypath
[self valueForKeyPath:((void)@selector(items_), (void)@selector(intValue), @"items_.intValue")];

In this example, the return values of @selector()s are cast to void and ignored, while the commas are used to join the syntaxes and express a property string. Also, since the result must represent a single string, the entire syntaxes are enclosed in ().

For multi level keypaths, it seems good to gather @selector()s in front and return a joined string at the end.

To achieve this usefully, we can take the way to implement codes as below by using the technique of pseudo-recursible macro.

One thing to consider, a problem is caused by the warning function in @selector(), we can't avoid the property name contamination because it is indistinguishable from a property name that validly exists in another class. However, this implementation is a realistic solution, I think.

Implementation:

// General use pseudo-recursible macro algorithm (maximum 16 level)
#define VA_ARGS_CONCAT_IMPL(x, y)\
    x##y
#define VA_ARGS_CONCAT(x, y)\
    VA_ARGS_CONCAT_IMPL(x, y)
#define VA_ARGS_COUNT_LOOKUP()\
    16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0
#define VA_ARGS_COUNT_INDEX(a1, a2, a3, a4, a5, a6, a7, a8,\
    a9, a10, a11, a12, a13, a14, a15, a16, n, ...)\
    n
#define VA_ARGS_COUNT_IMPL(...)\
    VA_ARGS_COUNT_INDEX(__VA_ARGS__)
#define VA_ARGS_COUNT(...)\
    VA_ARGS_COUNT_IMPL(__VA_ARGS__, VA_ARGS_COUNT_LOOKUP())
#define VA_ARGS_FOR_1(f, g, a)\
    f(a)
#define VA_ARGS_FOR_2(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_1(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_3(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_2(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_4(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_3(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_5(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_4(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_6(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_5(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_7(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_6(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_8(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_7(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_9(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_8(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_10(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_9(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_11(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_10(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_12(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_11(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_13(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_12(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_14(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_13(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_15(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_14(f, g, __VA_ARGS__))
#define VA_ARGS_FOR_16(f, g, a, ...)\
    g(f(a), VA_ARGS_FOR_15(f, g, __VA_ARGS__))
#define VA_ARGS_FOR(...)\
    VA_ARGS_CONCAT(VA_ARGS_FOR_, VA_ARGS_COUNT(__VA_ARGS__))

// Utility macros
#define VA_ARGS_GLUE_NONE(x, y)\
    x y
#define VA_ARGS_GLUE_SEMICOLON(x, y)\
    x; y
#define VA_ARGS_GLUE_COMMA(x, y)\
    x, y
#define VA_ARGS_GLUE_STRING_DOT(x, y)\
    x @"." y

// KeyPath safety macro algorithm (Core)
#define keyof_impl_selector(param)\
    (void)@selector(param)
#define keyof_impl_string(param)\
    @#param
#define keyof(param)\
    (keyof_impl_selector(param), keyof_impl_string(param))
#define keypathof_impl(prefix, suffix, ...)\
    VA_ARGS_FOR(__VA_ARGS__)(keyof_impl_selector, VA_ARGS_GLUE_COMMA, __VA_ARGS__),\
    prefix VA_ARGS_FOR(__VA_ARGS__)(keyof_impl_string, VA_ARGS_GLUE_STRING_DOT, __VA_ARGS__) suffix
#define keypathof(...)\
    (keypathof_impl(, , __VA_ARGS__))

// KeyPath safety macro algorithm (Extended)
#define keypath_count_of(...)\
    (keypathof_impl(, @".@count", __VA_ARGS__))
#define avg_keypath_of(...)\
    (keypathof_impl(@"@avg.", , __VA_ARGS__))
#define max_keypath_of(...)\
    (keypathof_impl(@"@max.", , __VA_ARGS__))
#define min_keypath_of(...)\
    (keypathof_impl(@"@min.", , __VA_ARGS__))
#define sum_keypath_of(...)\
    (keypathof_impl(@"@sum.", , __VA_ARGS__))
#define distinctUnionOfObjects_keypath_of(...)\
    (keypathof_impl(@"@distinctUnionOfObjects.", , __VA_ARGS__))
#define unionOfObjects_keypath_of(...)\
    (keypathof_impl(@"@unionOfObjects.", , __VA_ARGS__))
#define distinctUnionOfArrays_keypath_of(...)\
    (keypathof_impl(@"@distinctUnionOfArrays.", , __VA_ARGS__))
#define unionOfArrays_keypath_of(...)\
    (keypathof_impl(@"@unionOfArrays.", , __VA_ARGS__))
#define distinctUnionOfSets_keypath_of(...)\
    (keypathof_impl(@"@distinctUnionOfSets.", , __VA_ARGS__))

Usage 1:

// @"param_"
id value = [self valueForKey:keyof(param_)];

// @"items_.intValue"
id value = [self valueForKeyPath:keypathof(items_, intValue)];

// @"@sum.self"
id sum = [self.items_ valueForKeyPath:sum_keypath_of(self)];

Usage 2:

+ (NSSet<NSString*>*)keyPathsForValuesAffectingDerivedValue
{
    return [NSSet setWithObjects:keypathof(coeff_), keypathof(items_, floatValue), nil];
}

Usage 3:

static void* s_context = &s_context;

- (instancetype)init
{
    if ((self = [super init]) != nil) {
        [self addObserver:self forKeyPath:keypathof(value_) options:0 context:s_context];
        [self addObserver:self forKeyPath:keypath_count_of(items_) options:0 context:s_context];
    }
    return self;
}

- (void)dealloc
{
    [self removeObserver:self forKeyPath:keypath_count_of(items_) context:s_context];
    [self removeObserver:self forKeyPath:keypathof(value_) context:s_context];
    return;
}

- (void)observeValueForKeyPath:(nullable NSString*)keyPath
                      ofObject:(nullable id)object
                        change:(nullable NSDictionary<NSKeyValueChangeKey, id>*)change
                       context:(nullable void*)context
{
    if (context == s_context) {
        if (object == self) {
            if ([keyPath isEqualToString:keypathof(value_)]) {
                // do something...
            }
            else if ([keyPath isEqualToString:keypath_count_of(items_)]) {
                // do something...
            }
        }
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
    return;
}

Limitation:

// NG
id count = [self.items_ valueForKey:keyof(@count)];
// OK
id count = [self.items_ valueForKeyPath:keypath_count_of(self)];

// NG
id avg = [self valueForKeyPath:keypathof(items_, @avg, self)];
// OK
id avg = [self.items_ valueForKeyPath:avg_keypath_of(self)];
// OK (alternative, more complex, run-time)
id avg = [self valueForKeyPath:[NSString stringWithFormat:@"%@.@avg.%@", keyof(items_), keyof(self)]];

To use keyPathsForValuesAffecting<key> with NSObjectController family

When describing the relationship between dependent properties using KVO, it is well known that the compatibility between a class which inherits NSObjectController, and keyPathsForValuesAffectingValueForKey: or keyPathsForValuesAffecting<key> is worse.

Specifically, changes in selectedObjects can't be handled automatically, such as when implementing derived properties that depends on the value of selectedObjects of NSArrayController.

We can implement the goal by explicitly observing property changing with addObserver and removeObserver, but it is unsatisfactory.

Why do only the properties of a class which inherits NSObjectController not work properly? This seems to be due to the specification that NSKeyValueObservance class used internally by NSObjectController always handles the context value as nil.

Example that don't work:

@property (strong) NSArrayController* ac_;
@property (strong, readonly) NSString* secondValue;

- (nullable NSString*)secondValue
{
    NSString* result = nil;

    if (self.ac_.selectedObjects.count >= 2) {
        result = self.ac_.selectedObjects[1];
    }
    return result;
}

+ (NSSet<NSString*>*)keyPathsForValuesAffectingSecondValue
{
    return [NSSet setWithObjects:@"ac_.selectedObjects", nil];
}

Therefore, to fix this problem, make a subclass inherited from NSObjectController and override addObserver and removeObserver like below.

Its point is when the observer is a kind of NSKeyValueObservance class, the context value is always overridden as nil.

Modified ArrayController:

@interface ModArrayController : NSArrayController
@end

@implementation ModArrayController
- (void)addObserver:(NSObject*)observer
         forKeyPath:(NSString*)keyPath
            options:(NSKeyValueObservingOptions)options
            context:(void*)context
{
    [super addObserver:observer
            forKeyPath:keyPath
               options:options
               context:([observer isKindOfClass:NSClassFromString(@"NSKeyValueObservance")]) ? (nil) : (context)];
    return;
}

- (void)removeObserver:(NSObject*)observer
            forKeyPath:(NSString*)keyPath
               context:(void*)context
{
    [super removeObserver:observer
               forKeyPath:keyPath
                  context:([observer isKindOfClass:NSClassFromString(@"NSKeyValueObservance")]) ? (nil) : (context)];
    return;
}

- (void)removeObserver:(NSObject*)observer
            forKeyPath:(NSString*)keyPath
{
    NSMutableDictionary* dictionary;
    
    dictionary = NSThread.currentThread.threadDictionary;
    if ([observer isKindOfClass:NSClassFromString(@"NSKeyValueObservance")] &&
            dictionary[@"NSKeyValueObservanceLockKey"] == nil) {
        dictionary[@"NSKeyValueObservanceLockKey"] = @YES;
        [super removeObserver:observer forKeyPath:keyPath context:nil];
        [dictionary removeObjectForKey:@"NSKeyValueObservanceLockKey"];
    }
    else {
        [super removeObserver:observer forKeyPath:keyPath];
    }
    return;
}
@end

Use above modified class instead of NSArrayController. Just this solves the compatibility problem with keyPathsForValuesAffectingValueForKey: and keyPathsForValuesAffecting<key>.

This technique can be applied not only to NSArrayController, but also to any class which inherits NSObjectController.

Recoverable DC Circuit Breaker

+1.8V ~ 28.0V で使用可能な直流電源用の過電流遮断器。

電源と負荷のあいだに挿入することにより、10mΩ の電流検出抵抗を介して過電流を検出し、出力 MOSFET を遮断する。トランジスタで組まれたラッチ回路により遮断状態は保持され、リセットスイッチを押すか、電源をオフにすることにより導通が復活する。

VR1 の半固定抵抗により制限電流値を最大 2.4A の範囲で設定できるが、基板の寸法を小さく設計したため 2A 程度が実用範囲である。電流検出部は、D1, D2 による簡易的な安定化電源により駆動されるため、2.4V 以下の入力時には、入力電圧の影響を受け制限電流値が低下する。

乾電池などを利用してモーターを駆動する回路での過電流制限や、ブレッドボード上での不注意による部品破損などを防止する目的で設計した。また、電流検出部を除き、ラッチ回路部分をすべて汎用的なディスクリート部品で設計した。

手ハンダで両面実装を楽しめるちょうど良いサイズである。

Schematic

Wiring

Gerber - top

Gerber - bottom

PCB - top x6

PCB - bottom x6

Index	Value	Model	Mfr.Manufacturer	@
R1	47K ohm	(Thick Film Chip, 0805)	(KOA)	1
R2	3.3K ohm	(Thick Film Chip, 0805)	(KOA)	1
R3	10K ohm	(Thick Film Chip, 0805)	(KOA)	1
R4	1K ohm	(Thick Film Chip, 0805)	(KOA)	1
R5, R7, R8, R9, R10, R14, R15	56K ohm	(Thick Film Chip, 0805)	(KOA)	7
R6, R11, R13, R16, R17	20K ohm	(Thick Film Chip, 0805)	(KOA)	5
R12	100K ohm	(Thick Film Chip, 0805)	(KOA)	1
R18	10m ohm	PMR18EZPFU10L0	ROHM	1
VR1	10K ohm	ST-4ETB103	Nidec Copal	1
C1, C2	10uF	GRM21BR6YA106KE43	Murata	2
C3, C6	0.1uF	CGA4J2X7R1H104K125AA	TDK	2
C4	0.33uF	CGA4J2X7R1H334K125AA	TDK	1
C5, C7	0.47uF	CGA4J3X7R1H474K125AB	TDK	2
D1, D9	2mA	S-202T	SEMITEC	2
D2	2.4V	UDZVTE-172.4B	ROHM	1
D3, D4, D5, D6, D7, D8		RB501VM-40TE-17	ROHM	6
D10	Red	GM4ZR83232AE	SHARP	1
D11	18V	UFZVTE-1718B	ROHM	1
Q1, Q2, Q6, Q7, Q8		2SC2712-GR	TOSHIBA	5
Q3, Q4, Q5, Q9		2SA1162-GR	TOSHIBA	4
Q10		TPCA8128,LQ(M	TOSHIBA	1
U1		INA190A2QDCKRQ1	Texas Instruments	1
U2		NJU7109F	JRC	1
SW1		THAU13-AB-R	Zhejiang Jianfu	1
(JP1), (JP2)	OPT

CCXMonitor + CCXLive

Communication Cube X のための openFrameworks アプリケーション。CCXMonitor は、各個体のテレメトリデータを監視する。CCXLive は、各個体のキー押下状態を監視し、擬似キー押下信号を送信することができる。