iOS中使用对象的弱引用示例代码

NSObject *obj = [NSObject alloc] init];
NSLog(@"obj retain count: %zd", [obj retainCount]);

- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;

@interface NSObject <NSObject>

+ (NSValue *)valueWithNonretainedObject:(nullable id)anObject;

@interface MZDog : NSObject
@end

#import "MZDog.h"
@implementation MZDog
- (NSString *)description
{
 return [NSString stringWithFORMat:@"MZDog-obj retain count: %zd", [self retainCount]];
}
@end

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 对 dog 使用弱引用，此时其引用计数还是1
NSValue *value = [NSValue valueWithNonretainedObject:dog];
NSLog(@"dog: %@, value: %@", dog, value);
 
// 获取 value 对应的对象
id obj = value.nonretainedObjectValue;
NSLog(@"obj isKindOfClass MZDog: %i", [obj isKindOfClass:[MZDog class]]);
if (obj == dog) {
 NSLog(@"The obj is same dog object.");
}

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 对 dog 使用弱引用，此时其引用计数还是1
NSValue *value = [NSValue valueWithNonretainedObject:dog];
NSLog(@"dog: %@, value: %@", dog, value);
 
// 经过NSValue包装后，可以放到对应的集合对象(如数组，字典等)中，这样这些集合就不会对 dog 进行强引用了
NSArray *array = [NSArray arrayWithObjects:value, nil];
// dog 的引用计数还是1
NSLog(@"dog: %@, array: %@", dog, array);

NSValue *theValue = [NSValue value:&anObject withObjCType:@encode(void *)];

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 对 dog 使用弱引用，此时其引用计数还是1
NSValue *value = [NSValue value:&dog withObjCType:@encode(void *)];
NSLog(@"dog: %@, value: %@", dog, value);
// 经过NSValue包装后，可以放到对应的集合对象(如数组，字典等)中，这样这些集合就不会对 dog 进行强引用了
NSArray *array = [NSArray arrayWithObjects:value, nil];
// dog 的引用计数还是1
NSLog(@"dog: %@, array: %@", dog, array);

@interface NSArray (MZWeak)
- (id)mz_weak_objectAtIndex:(NSUInteger)index;
@end
@interface NSMutableArray (MZWeak)
- (void)mz_weak_addObject:(id)obj;
@end

#import "NSArray+MZWeak.h"
@implementation NSArray (MZWeak)
- (id)mz_weak_objectAtIndex:(NSUInteger)index
{
 NSValue *value = [self objectAtIndex:index];
 
 return value.nonretainedObjectValue;
}
@end
@implementation NSMutableArray (MZWeak)
- (void)mz_weak_addObject:(id)obj
{
 NSValue *value = [NSValue valueWithNonretainedObject:obj];
 if (nil != value) {
 [self addObject:value];
 }
}
@end

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
NSMutableArray *array = [NSMutableArray arrayWithCapacity:1];
// 弱引用
[array mz_weak_addObject:dog];
 
// 此时 dog 的引用计数还是1
NSLog(@"dog: %@", dog);

// 创建 NSPointerArray 对象方式一
NSPointerArray *pointerArray = [NSPointerArray weakObjectsPointerArray];
 
// 创建 NSPointerArray 对象方式二
NSPointerArray *pointerArray1 = [[NSPointerArray alloc] initWithOptions:NSPointerFunctionsWeakMemory];
 
// 创建 NSPointerArray 对象方式三
NSPointerArray *pointerArray2 = [NSPointerArray pointerArrayWithOptions:NSPointerFunctionsWeakMemory];

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 创建 NSPointerArray 对象方式一
// 注意 weakObjectsPointerArray 而不是 stronGobjectsPointerArray
NSPointerArray *pointerArray = [NSPointerArray weakObjectsPointerArray];
[pointerArray addPointer:(__bridge void *)(dog)];
 
// 创建 NSPointerArray 对象方式二
NSPointerArray *pointerArray1 = [[NSPointerArray alloc] initWithOptions:NSPointerFunctionsWeakMemory];
[pointerArray1 addPointer:(__bridge void *)(dog)];
 
// 创建 NSPointerArray 对象方式三
NSPointerArray *pointerArray2 = [NSPointerArray pointerArrayWithOptions:NSPointerFunctionsWeakMemory];
[pointerArray2 addPointer:(__bridge void *)(dog)];
 
// dog 引用计数还是1
NSLog(@"dog: %@", dog);

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 弱应用对象
NSMapTable *map = [NSMapTable weakToWeakObjectsMapTable];
[map setObject:dog forKey:@"first"];
 
// 引用计数还是1，没变
NSLog(@"dog: %@", dog);

// retainCount -> 1
MZDog *dog = [MZDog new];
NSLog(@"dog: %@", dog);
 
// 弱应用对象
NSHashTable *hashTable = [NSHashTable weakObjectsHashTable];
[hashTable addObject:dog];
 
// 引用计数还是1，没变
NSLog(@"dog: %@", dog);

MZDog *dog = nil;
 
NSPointerArray *pointerArray = [NSPointerArray weakObjectsPointerArray];
void *cobj = (__bridge void *)(dog);
NSLog(@"obj: %@", cobj); //NULL
[pointerArray addPointer:cobj];
// 虽然存储的是 NULL，但是 count 仍然是 1
NSLog(@"pointerArray count: %zd", [pointerArray count]);
NSArray *array = [pointerArray allObjects];
NSLog(@"pointerArray allObjects: %@", array);

[pointerArray addPointer:NULL];
[pointerArray compact];

// retainCount -> 1
MZDog *dog = [MZDog new];
NSPointerArray *pointerArray = [NSPointerArray weakObjectsPointerArray];
// 这里会 retain dog 对象，使其引用计数加1，此时retainCount 是 2
[pointerArray addPointer:(__bridge_retained void *)dog];
// 这里会 retain dog 对象，使其引用计数再加1，retainCount 是 3
[pointerArray addPointer:CFBridgingRetain(dog)];
 
// 此时的 retainCount 是 3
NSLog(@"dog: %@", dog);