今天群里不知怎么说起了 block 在栈上还是在堆上的问题。好像之前在哪里看到过，现在 block 的属性已经不用写 copy 关键字，就会自动 copy。于是做了几个实验，想看看什么情况下会自动 copy，什么情况下不会~

实验

代码如下：

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;
}

实验结果：

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

• 作为变量：
  • 一个 block 刚声明的时候是在栈上
  • 赋值给一个普通变量之后就会被 copy 到堆上
  • 赋值给一个 weak 变量不会被 copy
• 作为属性：
  • 用 strong 和 copy 修饰的属性会被 copy
  • 用 weak 和 assign 修饰的属性不会被 copy
• 函数传参：
  • 作为参数传入函数不会被 copy
  • 作为函数的返回值会被 copy

猜测

看着以上结论，感觉可以做出一个猜测：就是 block 被 retain 的时候就会自动被 copy，包括 autoRelease~ 这样就能解释为啥函数的参数不会被 copy，返回值就会被 copy。是不是很有道理呢 =w=