今天群里不知怎么說起了 block 在棧上還是在堆上的問題。好像之前在哪里看到過，現在 block 的屬性已經不用寫 copy 關鍵字，就會自動 copy。于是做了幾個實驗，想看看什么情況下會自動 copy，什么情況下不會~

實驗

代碼如下：

TestClass.h

typedef void(^SimpleBlock)();

@interface TestClass : NSObject

@property (nonatomic, copy) SimpleBlock copyProperty;

@property (nonatomic, strong) SimpleBlock strongProperty;

@property (nonatomic, weak) SimpleBlock weakProperty;

@property (nonatomic, assign) SimpleBlock assignProperty;

@end

main

#import "TestClass.h"

SimpleBlock someFunction(SimpleBlock block) {
    NSLog(@"block as param : %@", block);
    return block;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        __block int a = 1;
        
        NSLog(@"orginal block : %@", ^{
            a = 2;
        });
        
        // as a variable
        SimpleBlock block = ^{
            a = 2;
        };
        NSLog(@"block as variable : %@", block);
        __weak SimpleBlock weakBlock = ^{
            a = 2;
        };
        NSLog(@"block as a weak variable : %@", weakBlock);
        
        // as properties
        TestClass* testClass = [TestClass new];
        testClass.weakProperty = ^{
            a = 2;
        };
        testClass.assignProperty = ^{
            a = 2;
        };
        testClass.copyProperty = ^{
            a = 2;
        };
        testClass.strongProperty = ^{
            a = 2;
        };
        
        NSLog(@"copy property : %@", testClass.copyProperty);
        NSLog(@"strong property : %@", testClass.strongProperty);
        NSLog(@"weak property : %@", testClass.weakProperty);
        NSLog(@"assign property : %@", testClass.assignProperty);
        
        NSLog(@"block as return value : %@", someFunction(^{
            a = 2;
        }));
    }
    return 0;
}

實驗結果：

2017-02-06 17:43:36.207212 test2[27378:1079138] orginal block : <__NSStackBlock__: 0x7fff5fbff728>
2017-02-06 17:43:36.207436 test2[27378:1079138] block as variable : <__NSMallocBlock__: 0x100402f70>
2017-02-06 17:43:36.207457 test2[27378:1079138] block as a weak variable : <__NSStackBlock__: 0x7fff5fbff6b8>
2017-02-06 17:43:36.207492 test2[27378:1079138] copy property : <__NSMallocBlock__: 0x100403140>
2017-02-06 17:43:36.207517 test2[27378:1079138] strong property : <__NSMallocBlock__: 0x100403170>
2017-02-06 17:43:36.207563 test2[27378:1079138] weak property : <__NSStackBlock__: 0x7fff5fbff668>
2017-02-06 17:43:36.207581 test2[27378:1079138] assign property : <__NSStackBlock__: 0x7fff5fbff640>
2017-02-06 17:43:36.207611 test2[27378:1079138] block as param : <__NSStackBlock__: 0x7fff5fbff618>
2017-02-06 17:43:36.207769 test2[27378:1079138] block as return value : <__NSMallocBlock__: 0x100600000>

分析

• 作為變量：
  • 一個 block 剛聲明的時候是在棧上
  • 賦值給一個普通變量之后就會被 copy 到堆上
  • 賦值給一個 weak 變量不會被 copy
• 作為屬性：
  • 用 strong 和 copy 修飾的屬性會被 copy
  • 用 weak 和 assign 修飾的屬性不會被 copy
• 函數傳參：
  • 作為參數傳入函數不會被 copy
  • 作為函數的返回值會被 copy

猜測

看著以上結論，感覺可以做出一個猜測：就是 block 被 retain 的時候就會自動被 copy，包括 autoRelease~ 這樣就能解釋為啥函數的參數不會被 copy，返回值就會被 copy。是不是很有道理呢 =w=