Block内存管理
2020-04-20 本文已影响0人
言霏
对block自身内存的管理
对于block，有两个内存管理方法：Block_copy, Block_release;Block_copy与copy等效，Block_release与release等效；
不管是对block进行retian,copy,release,block的引用计数都不会增加，始终为1；
-
NSGlobalBlock:使用retain,copy,release都无效，block依旧存在全局区，且没有释放, 使用copy和retian只是返回block的指针； -
NSStackBlock:使用retain,release操作无效；栈区block会在方法返回后将block空间回收； 使用copy将栈区block复制到堆区，可以长久保留block的空间，以供后面的程序使用； -
NSMallocBlock:支持retian,release，虽然block的引用计数始终为1，但内存中还是会对引用进行管理，使用retain引用+1，release引用-1； 对于NSMallocBlock使用copy之后不会产生新的block，只是增加了一次引用，类似于使用retian;
对引用变量的内存管理
如果该block为NSMallocBlock或者被copy成NSMallocBlock类型则其引用的外界变量的引用计数会被+1，当block释放时外界变量的引用计数-1，而block为NSStackBlock或NSGlobalBlock类型时外界变量的引用计数不会变，例子NSMallocBlock：
- (void)testBlock {
NSObject *obj = [[NSObject alloc] init];
NSLog(@"obj.retainCount = %ld", obj.retainCount);
void(^normalBlock)(int, int) = ^(int a, int b) {
NSLog(@"normalBlock = %d %@", a + b, obj);
};
NSLog(@"obj.retainCount = %ld", obj.retainCount);
NSLog(@"normalBlock %@", normalBlock);
void(^normalCopyBlock)(int, int) = [normalBlock copy];
NSLog(@"normalCopyBlock %@", normalCopyBlock);
NSLog(@"obj.retainCount = %ld", obj.retainCount);
}
输出：
2020-04-20 16:03:56.412850+0800 obj.retainCount = 1
2020-04-20 16:03:56.414244+0800 obj.retainCount = 1
2020-04-20 16:03:56.414685+0800 normalBlock <__NSStackBlock__: 0x7ffeefbff538>
2020-04-20 16:03:56.414798+0800 normalCopyBlock <__NSMallocBlock__: 0x100625400>
2020-04-20 16:03:56.414985+0800 obj.retainCount = 2
例子NSGlobalBlock：
NSObject *obj = nil;
- (void)testBlock {
obj = [[NSObject alloc] init];
NSLog(@"obj.retainCount = %ld", obj.retainCount);
void(^normalBlock)(int, int) = ^(int a, int b) {
NSLog(@"normalBlock = %d %@", a + b, obj);
};
NSLog(@"obj.retainCount = %ld", obj.retainCount);
NSLog(@"normalBlock %@", normalBlock);
void(^normalCopyBlock)(int, int) = [normalBlock copy];
NSLog(@"normalCopyBlock %@", normalCopyBlock);
NSLog(@"obj.retainCount = %ld", obj.retainCount);
}
输出：
0-04-20 16:14:01.728789+0800 obj.retainCount = 1
2020-04-20 16:14:01.729195+0800 obj.retainCount = 1
2020-04-20 16:14:01.729499+0800 normalBlock <__NSGlobalBlock__: 0x100002030>
2020-04-20 16:14:01.729582+0800 normalCopyBlock <__NSGlobalBlock__: 0x100002030>
2020-04-20 16:14:01.729636+0800 obj.retainCount = 1
循环引用
-
__block在MRC中使用，NSMallocBlock类型的block不会对__block修饰的的变量引用计数+1，从而消除循环引用，并且可以让该变量在block内被赋值；在ARC中使用__block只是可以让该变量在block内被赋值。 -
__weak在ARC中使用，作用和__block一样，从而消除循环引用；在非ARC中不可以使用__weak;
MRC下例子：
@property (nonatomic, copy, readwrite) void (^copyBlock)(void);
- (void)testBlock {
NSObject *obj = [[NSObject alloc] init];
NSLog(@"obj.retainCount = %ld", obj.retainCount);
self.copyBlock = ^() {
NSLog(@"copyBlock = %@", obj);
};
NSLog(@"obj.retainCount = %ld", obj.retainCount);
NSLog(@"normalBlock %@", self.copyBlock);
}
输出：
2020-04-20 16:23:12.712386+0800 obj.retainCount = 1
2020-04-20 16:23:12.713195+0800 obj.retainCount = 2
2020-04-20 16:23:12.713557+0800 normalBlock <__NSMallocBlock__: 0x100707d40>
MRC下例子：
@property (nonatomic, copy, readwrite) void (^copyBlock)(void);
- (void)testBlock {
__block NSObject *obj = [[NSObject alloc] init];
NSLog(@"obj.retainCount = %ld", obj.retainCount);
self.copyBlock = ^() {
NSLog(@"copyBlock = %@", obj);
};
NSLog(@"obj.retainCount = %ld", obj.retainCount);
NSLog(@"normalBlock %@", self.copyBlock);
}
输出：
2020-04-20 16:28:59.888767+0800 obj.retainCount = 1
2020-04-20 16:28:59.889190+0800 obj.retainCount = 1
2020-04-20 16:28:59.889454+0800 normalBlock <__NSMallocBlock__: 0x1006518e0>
