objective-c - iOS NSObject 在 block 中初始化后为零

Question

我有一个在 block 内创建的 NSObject。按照下面的代码:

__block NSObject *myObject;

[self myMethod:^{
    myObject = [[NSObject alloc] init];
    ....
}];

if(myObject == nil){
    NSLog(@"Why is my object nil?!");
}

在 myMethod 的定义中，我有以下内容:

  backgroundQueue = dispatch_queue_create("backgroundqueue", NULL);

            dispatch_async(backgroundQueue, 
                           ^{

                               dispatch_async(dispatch_get_main_queue(), 
                                              ^{
                                                  if(block){
                                                      block();//Never called.
                                                  }
                                              });

但是该 block 永远不会被调用。

Answer 1

这里的问题是，您似乎从未执行实例化 myObject 的 block 。为了便于说明，请运行这个小程序:

#import <Foundation/Foundation.h>

typedef void(^MyTestBlock)(void);

@interface Foo:NSObject
- (id)initWithBlock:(MyTestBlock)aBlock;
- (void)someMethod;
@end

@implementation Foo {
    MyTestBlock _block;
}

- (id)initWithBlock:(MyTestBlock)aBlock {
    self = [super init];
    if( !self ) { return nil; }
    _block = aBlock;
    return self;
}

- (void)someMethod {
    _block();
}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        __block NSObject *myObject;

        Foo *myFoo = [[Foo alloc] initWithBlock:^{
            myObject = [[NSObject alloc] init];
        }];
        [myFoo someMethod];

        NSLog((myObject)?@"Your object was created":@"Why is my object nil?");
    }
}

这会将 2012-11-26 05:00:58.519 Untitled 2[23467:707] Your object was Created 打印到控制台。关键是 block 不会自行执行。在上面的代码中，虽然我们将 block 设置为类的 ivar，但直到我们在 Foo 上调用 someMethod 时才会执行它。

编辑:

对您问题的编辑指出，该 block 不是在发送到主队列的异步调度 block 的上下文中执行的。如果这是命令行应用程序，则必须在 main 末尾调用 dispatch_main()。请参阅 dispatch_get_main_queue() 的 man 页面。这是一个完整的工作命令行应用程序来说明这一点，以及与竞争条件相关的问题:

#import <Foundation/Foundation.h>

typedef void(^MyTestBlock)(void);

@interface Foo:NSObject
- (id)initWithBlock:(MyTestBlock)aBlock;
- (void)someMethod;
@end

@implementation Foo {
    MyTestBlock _block;
}

- (id)initWithBlock:(MyTestBlock)aBlock {
    self = [super init];
    if( !self ) { return nil; }
    _block = aBlock;
    return self;
}

- (void)someMethod {
    dispatch_queue_t backgroundQueue = dispatch_queue_create("backgroundqueue", NULL);
    dispatch_async(backgroundQueue, ^{
        dispatch_queue_t innerQueue = dispatch_get_main_queue();
        dispatch_async(innerQueue, ^{
            if( _block){
                NSLog(@"Will call block.");
                _block();
            }
        });
    });

}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        __block NSObject *myObject;

        Foo *myFoo = [[Foo alloc] initWithBlock:^{
            myObject = [[NSObject alloc] init];
        }];
        [myFoo someMethod];

        //  this log statement should show that myObject is nil because it will (probably)
        //  be executed before your block.
        NSLog((myObject)?@"Your object was created":@"Why is my object nil?");

        //  wait a little bit to resolve race condition (just for illustrative purposes)
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, 0.4f * NSEC_PER_SEC);
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            NSLog((myObject)?@"Your object was created":@"Why is my object nil?");
        });
    }
    //  this isn't a Cocoa app, so must call dispatch_main() at end of main
    dispatch_main();
}