转载当 NSDictionary 遇见 nil

相信用 Objective-C 开发 iOS 应用的人对下面的 crash 不会陌生：

*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[1]
*** setObjectForKey: key cannot be nil
*** setObjectForKey: object cannot be nil
Objective-C 里的 NSDictionary 是不支持 nil 作为 key 或者 value 的。但是总会有一些地方会偶然往 NSDictionary 里插入 nil value.

我们希望 NSDictionary 用起来是这样的：

插入 nil 的时候不会 crash
插入 nil 以后它对应的 key 的确存在，且能取到值（NSNull）
被 serialize 成 JSON 的时候，被转成 null
让 NSNull 更接近 nil，可以吃任何方法不 crash

具体见代码

//
//  NSDictionary+NilSafe.m
//  NSDictionary-NilSafe
//
//  Created by Allen Hsu on 6/22/16.
//  Copyright © 2016 Glow Inc. All rights reserved.
//

#import <objc/runtime.h>
#import "NSDictionary+NilSafe.h"

@implementation NSObject (Swizzling)

+ (BOOL)gl_swizzleMethod:(SEL)origSel withMethod:(SEL)altSel {
    Method origMethod = class_getInstanceMethod(self, origSel);
    Method altMethod = class_getInstanceMethod(self, altSel);
    if (!origMethod || !altMethod) {
        return NO;
    }
    class_addMethod(self,
                    origSel,
                    class_getMethodImplementation(self, origSel),
                    method_getTypeEncoding(origMethod));
    class_addMethod(self,
                    altSel,
                    class_getMethodImplementation(self, altSel),
                    method_getTypeEncoding(altMethod));
    method_exchangeImplementations(class_getInstanceMethod(self, origSel),
                                   class_getInstanceMethod(self, altSel));
    return YES;
}

+ (BOOL)gl_swizzleClassMethod:(SEL)origSel withMethod:(SEL)altSel {
    return [object_getClass((id)self) gl_swizzleMethod:origSel withMethod:altSel];
}

@end

@implementation NSDictionary (NilSafe)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [self gl_swizzleMethod:@selector(initWithObjects:forKeys:count:) withMethod:@selector(gl_initWithObjects:forKeys:count:)];
        [self gl_swizzleClassMethod:@selector(dictionaryWithObjects:forKeys:count:) withMethod:@selector(gl_dictionaryWithObjects:forKeys:count:)];
    });
}

+ (instancetype)gl_dictionaryWithObjects:(const id [])objects forKeys:(const id<NSCopying> [])keys count:(NSUInteger)cnt {
    id safeObjects[cnt];
    id safeKeys[cnt];
    NSUInteger j = 0;
    for (NSUInteger i = 0; i < cnt; i++) {
        id key = keys[I];
        id obj = objects[I];
        if (!key) {
            continue;
        }
        if (!obj) {
            obj = [NSNull null];
        }
        safeKeys[j] = key;
        safeObjects[j] = obj;
        j++;
    }
    return [self gl_dictionaryWithObjects:safeObjects forKeys:safeKeys count:j];
}

- (instancetype)gl_initWithObjects:(const id [])objects forKeys:(const id<NSCopying> [])keys count:(NSUInteger)cnt {
    id safeObjects[cnt];
    id safeKeys[cnt];
    NSUInteger j = 0;
    for (NSUInteger i = 0; i < cnt; i++) {
        id key = keys[I];
        id obj = objects[I];
        if (!key) {
            continue;
        }
        if (!obj) {
            obj = [NSNull null];
        }
        safeKeys[j] = key;
        safeObjects[j] = obj;
        j++;
    }
    return [self gl_initWithObjects:safeObjects forKeys:safeKeys count:j];
}

@end

@implementation NSMutableDictionary (NilSafe)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = NSClassFromString(@"__NSDictionaryM");
        [class gl_swizzleMethod:@selector(setObject:forKey:) withMethod:@selector(gl_setObject:forKey:)];
        [class gl_swizzleMethod:@selector(setObject:forKeyedSubscript:) withMethod:@selector(gl_setObject:forKeyedSubscript:)];
    });
}

- (void)gl_setObject:(id)anObject forKey:(id<NSCopying>)aKey {
    if (!aKey) {
        return;
    }
    if (!anObject) {
        anObject = [NSNull null];
    }
    [self gl_setObject:anObject forKey:aKey];
}

- (void)gl_setObject:(id)obj forKeyedSubscript:(id<NSCopying>)key {
    if (!key) {
        return;
    }
    if (!obj) {
        obj = [NSNull null];
    }
    [self gl_setObject:obj forKeyedSubscript:key];
}

@end

@implementation NSNull (NilSafe)


+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [self gl_swizzleMethod:@selector(methodSignatureForSelector:) withMethod:@selector(gl_methodSignatureForSelector:)];
        [self gl_swizzleMethod:@selector(forwardInvocation:) withMethod:@selector(gl_forwardInvocation:)];
    });
}

- (NSMethodSignature *)gl_methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *sig = [self gl_methodSignatureForSelector:aSelector];
    if (sig) {
        return sig;
    }
    return [NSMethodSignature signatureWithObjCTypes:@encode(void)];
}

- (void)gl_forwardInvocation:(NSInvocation *)anInvocation {
    NSUInteger returnLength = [[anInvocation methodSignature] methodReturnLength];
    if (!returnLength) {
        // nothing to do
        return;
    }

    // set return value to all zero bits
    char buffer[returnLength];
    memset(buffer, 0, returnLength);

    [anInvocation setReturnValue:buffer];
}

@end