一、關(guān)于block在內(nèi)存的位置
Block在內(nèi)存中的位置分三種：
1、NSGlobalBlock:類似函數(shù)，位于代碼段（未引用外部變量）：
float (^sum)(float, float) = ^(float a, float b){
return a + b;
};
2、NSStackBlock:位于棧內(nèi)存，函數(shù)返回后block將無效（引用外部變量，但只能在該函數(shù)作用域內(nèi)使用，出作用域該變量內(nèi)存已被釋放，出作用域觸發(fā)block訪問該變量會Crash）：
{
NSArray *testArr = @[@"1", @"2"];
void (^TestBlock)(void) = ^{
NSLog(@"testArr :%@", testArr);
};
NSLog(@"block is %@", ^{
NSLog(@"test Arr :%@", testArr);
});
}
3、NSMallocBlock:位于堆內(nèi)存（對NSStackBlock 使用copy修飾（strong 應該也可以））該類型擁有保存外部變量內(nèi)存的能力。所以調(diào)用外部變量不會crash,下面將講解對外部變量的存取管理。
二、關(guān)于block外部變量的存取管理
#######在這里只討論ARC機制下
- (void)test
{
int a = 0;
__block int b = 0;
self.globstring = @"1";
NSString *__localString = @"1";
__block NSString *_blockString = @"1";
printf("a address: %p\n", &a); //a address: 0x7fff56516b3c
printf("b address: %p\n", &b); //b address: 0x7fff56516b30
printf("local address: %p\n", &__localString); //local address: 0x7fff56516b00
printf("_block address: %p\n", &_blockString); //_block address: 0x7fff56516af8
printf("glob address: %p\n", &_globstring); //glob address: 0x7fed5bc09520
void (^TestBlock)(void) = ^{
printf("a value: %d\n", a); //a value: 0
printf("b value: %d\n", b); //b value: 1
printf("b address: %p\n", &b); //b address: 0x60800002a7f8
printf("local address: %p\n", &__localString); // local address: 0x6080000979a0
NSLog(@"lockstring is : %@\n", __localString); //lockstring is : 1
printf("_blcok address: %p\n", &_blockString); //_blcok address: 0x6080000470d8
NSLog(@"_blcokstring is : %@\n", _blockString); //_blcokstring is : 2
printf("globaddress: %p\n", &_globstring); //globaddress: 0x7fed5bc09520
NSLog(@"globstring is : %@\n", _globstring); //globstring is : 2
};
__weak typeof(self) this = self;
self.copyBlock = ^{
printf("a value: %d\n", a); //a value: 0
printf("b value: %d\n", b); // b value: 1
printf("b address: %p\n", &b); // b address: 0x60800002a7f8
printf("copy block local address: %p\n", &__localString); //copy block local address: 0x608000099660
NSLog(@"copy block string is : %@\n", __localString); //copy block string is : 1
printf("_blcok address: %p\n", &_blockString); //_blcok address: 0x6080000470d8
NSLog(@"_blcokstring is : %@\n", _blockString); //_blcokstring is : 2
printf("glob address: %p\n", &_globstring); //glob address: 0x7fed5bc09520
NSLog(@"string is : %@\n", this.globstring); //string is : 2
};
a = 1;
b = 1;
printf("a address: %p\n", &a); //local address: 0x7fff56516b3c
printf("b address: %p\n", &b); //_block address: 0x60800002a7f8
printf("local address: %p\n", &__localString); //local address: 0x7fff56516b00
printf("_block address: %p\n", &_blockString); //_block address: 0x60800002a7f8
__localString = nil;
_blockString = @"2";
self.globstring = @"2";
TestBlock();
self.copyBlock();
}
通過觀察上面的輸出，得出結(jié)論 :
1、普通的局部變量，BLOCK聲明時會COPY它的值，并且擁有新的地址，也就是說block內(nèi)部a 地址和外部a地址不同（如上）?；ゲ挥绊?。
2、__block局部變量，BLOCK聲明時會在堆中新建一個內(nèi)存地址，并且BLOCK之后的所有b 或者 _blockString （如上）都是這個地址，所以后續(xù)的讀寫都是再這個地址上進行。會不想影響。即使不觸發(fā)block 地址也已經(jīng)不是原來的地址。
3、全局變量 globstring (如上)BLOCK 訪問都是 它本身的地址。直接讀寫操作。
4、BLOCK引用外部對象時會強引用外部變量 ，如果該外部變量也強引用BLOCK就會造成return circle （比如self property block ,block 內(nèi)部訪問self 或者self的屬性），為了解決這個使用weakSelf。
