objective-c - 将 block 传递给异步方法

Question

我将一个 block 传递给一个稍后执行该 block 的异步方法。如果我在将 block 传递给 someMethod:success:failure:

之前不复制 block ，我的应用程序会崩溃

有没有办法复制 forwardInvocation: 中的 block ，而不是在将其传递给 someMethod:success:failure: 之前复制它？

流程是someMethod:success:failure: -> forwardInvocation: -> httpGet:success:failure

httpGet:success:failure:根据 HTTP 状态代码执行成功或失败 block 。

// AppDelegate.h

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) id response;
@property (strong, nonatomic) NSError *error;

@end

// AppDelegate.m

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // The app crashes if the blocks are not copied here!
    [[MyController new] someMethod:[^(NSString *response) {
        self.response = response;
        NSLog(@"response = %@", response);
    } copy] failure:[^(NSError *error) {
        self.error = error;
    } copy]];

    return YES;
}

@end

// MyController.h

@protocol MyControllerProtocol <NSObject>

@optional


- (void)someMethod:(void (^)(NSString *response))success
           failure:(void (^)(NSError *error))failure;

@end

@interface MyController : NSObject <MyControllerProtocol>

@end

// MyController.m

#import "MyController.h"
#import "HTTPClient.h"

@implementation MyController

- (void)forwardInvocation:(NSInvocation *)invocation {
    [invocation retainArguments];

    NSUInteger numberOfArguments = [[invocation methodSignature] numberOfArguments];

    typedef void(^SuccessBlock)(id object);
    typedef void(^FailureBlock)(NSError *error);

    __unsafe_unretained SuccessBlock successBlock1;
    __unsafe_unretained SuccessBlock failureBlock1;
    [invocation getArgument:&successBlock1 atIndex:(numberOfArguments - 2)]; // success block is always the second to last argument (penultimate)
    SuccessBlock successBlock = [successBlock1 copy];
    [invocation getArgument:&failureBlock1 atIndex:(numberOfArguments - 1)]; // failure block is always the last argument
    FailureBlock failureBlock = [failureBlock1 copy];

    NSLog(@"successBlock copy = %@", successBlock);
    NSLog(@"failureBlock copy = %@", failureBlock);

    // Simulates a HTTP request and calls the success block later!
    [HTTPClient httpGet:@"somerequest" success:successBlock failure:failureBlock];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    NSMethodSignature *methodSignature = [super methodSignatureForSelector:sel];
    return methodSignature;
}

@end


// HTTPClient.h

@interface HTTPClient : NSObject

+ (void)httpGet:(NSString *)path
        success:(void (^)(id object))success
        failure:(void (^)(NSError *error))failure;

@end

// HTTPClient.m

#import "HTTPClient.h"

@implementation HTTPClient

+ (void)httpGet:(NSString *)path
        success:(void (^)(id object))success
        failure:(void (^)(NSError *error))failure
{
    // 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(),
               ^{
                   success(@"foo");
               });
}

@end

完整的源代码可以在这里找到:https://github.com/priteshshah1983/BlocksWithNSInvocation

你能帮忙吗？

Answer 1

罪魁祸首是 [invocation retainArguments] 行。如果您注释掉该行，它就可以正常工作。 (无论如何，该行从来都不是必需的，因为调用永远不会被异步存储或使用。)

解释:

想想 -retainArguments 做了什么。它对对象指针类型的所有参数调用 retain。然后当调用被解除分配时，它会调用它们的 release。

但参数是堆栈(非复制) block 。 retain 和 release 对它没有影响(因为它不是堆对象)。因此，当它被保留时，什么也没有发生，然后(从崩溃中)看起来调用在某个时候被自动释放(发生的完全正常的事情)，所以调用的最终释放和释放是异步发生的。当调用被解除分配时，它会尝试释放其保留的参数，但此时它会尝试向不再有效的堆栈 block 发送消息，从而导致崩溃。

附言forwardInvocation: 中的 block 的复制也是不必要的