带有块参数的 NSInvocation

Question

我正在尝试将块参数传递给 NSInvocation，但应用程序崩溃了。该调用发出网络请求并调用成功或失败块。我认为问题是在网络请求完成之前块被释放。我设法让它与一些 Block_copyhackery 一起工作，并且它没有使用 Instruments 报告任何泄漏。

问题： - 即使静态分析仪或仪器没有报告泄漏，是否可能存在泄漏？ - 有没有更好的方法来“保留”区块？

// Create the NSInvocation
NSMethodSignature *methodSignature = [target methodSignatureForSelector:selector];
NSInvocation* invoc = [NSInvocation invocationWithMethodSignature:methodSignature];
[invoc setTarget:target];
[invoc setSelector:selector];

// Create success and error blocks.
void (^successBlock)(id successResponse) = ^(id successResponse) {
    // Some success code here ...
};

void (^errorBlock)(NSError *error) = ^(NSError *error) {
    // Some failure code here ...
};

/*
Without the two Block_copy lines, the block gets dealloced too soon
and the app crashes with EXC_BAD_ACCESS
I tried [successBlock copy] and [failureBlock copy] instead,
but the app still crashes.
It seems like Block_copy is the only way to move the block to the heap in this case.
*/
Block_copy((__bridge void *)successBlock);
Block_copy((__bridge void *)errorBlock);
// Set the success and failure blocks.
[invoc setArgument:&successBlock atIndex:2];
[invoc setArgument:&errorBlock atIndex:3];

[invoc retainArguments]; // does not retain blocks

// Invoke the method.
[invoc invoke];

更新：我将代码更新到下面。块是NSMallocBlocks，但应用仍然崩溃。

// Create success and error blocks.
int i = 0;
void (^successBlock)(id successResponse) = ^(id successResponse) {
    NSLog(@"i = %i", i);
    // Some success code here ...
};

void (^errorBlock)(NSError *error) = ^(NSError *error) {
    NSLog(@"i = %i", i);
    // Some failure code here ...
};

/*** Both blocks are NSMallocBlocks here ***/
// Set the success and failure blocks.
void (^successBlockCopy)(id successResponse) = [successBlock copy];
void (^errorBlockCopy)(NSError *error) = [errorBlock copy];

/*** Both blocks are still NSMallocBlocks here - I think copy is a NoOp ***/

// Set the success and failure blocks.
[invoc setArgument:&successBlockCopy atIndex:2];
[invoc setArgument:&errorBlockCopy atIndex:3];

[invoc retainArguments]; // does not retain blocks

// Invoke the method.
[invoc invoke];

区块在链中的传递如下：

NSInvocation → NSProxy (NSInvocation 使用forwardInvocation:) → method1 → methodN

methodN 最终会根据 HTTP 响应调用成功或失败块。

我需要在每个阶段复制块吗？上面的例子是在谈论第一个NSInvocation。在每个适当的步骤中我是否还需要[invocation retainArguments];？我正在使用 ARC。

Answer 1

Block_copy，实际上是[block copy] return 副本。他们不会神奇地将原件与同一位置的副本交换。所以至少我认为你想要：

successBlock = Block_copy((__bridge void *)successBlock);
errorBlock = Block_copy((__bridge void *)errorBlock); 

（或者，等效地，successBlock = [successBlock copy]; ...）

否则，您将创建副本，对它们不做任何事情，仍然将原件传递给调用。

编辑：所以，我将以下代码放入项目中：

@interface DummyClass: NSObject
@end

typedef void (^ successBlock)(id successResponse);
typedef void (^ failureBlock)(NSError *error);

@implementation DummyClass

- (id)init
{
    self = [super init];

    if(self)
    {
        SEL selector = @selector(someMethodWithSuccess:failure:);
        id target = self;

        // Create the NSInvocation
        NSMethodSignature *methodSignature = [target methodSignatureForSelector:selector];
        NSInvocation* invoc = [NSInvocation invocationWithMethodSignature:methodSignature];
        [invoc setTarget:target];
        [invoc setSelector:selector];

        // Create success and error blocks.
        void (^successBlock)(id successResponse) = ^(id successResponse) {
            // Some success code here ...
            NSLog(@"Off, off, off with %@", successResponse);
        };

        void (^errorBlock)(NSError *error) = ^(NSError *error) {
            // Some failure code here ...
            NSLog(@"Dance, dance, dance till %@", error);
        };

        successBlock = [successBlock copy];
        errorBlock = [errorBlock copy];

        // Set the success and failure blocks.
        [invoc setArgument:&successBlock atIndex:2];
        [invoc setArgument:&errorBlock atIndex:3];

        [invoc retainArguments]; // does not retain blocks

        // Invoke the method.
        double delayInSeconds = 2.0;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
        dispatch_after(popTime, dispatch_get_main_queue(),
        ^{
            [invoc invoke];

        });
    }

    return self;
}

- (void)someMethodWithSuccess:(successBlock)successBlock failure:(failureBlock)failureBlock
{
    NSLog(@"Words:");
    successBlock(@[@"your", @"head"]);
    failureBlock([NSError errorWithDomain:@"you're dead" code:0 userInfo:nil]);
}

@end

并在application:didFinishLaunchingWithOptions:末尾添加以下内容：

DummyClass *unusedInstance = [[DummyClass alloc] init];

结果是启动我的程序两秒钟后，控制台上出现以下内容：

2013-06-02 20:11:56.057 TestProject[3330:c07] Words:
2013-06-02 20:11:56.059 TestProject[3330:c07] Off, off, off with (
    your,
    head
)
2013-06-02 20:11:56.060 TestProject[3330:c07] Dance, dance, dance till Error Domain=you're dead Code=0 "The operation couldn’t be completed. (you're dead error 0.)"