Question

我有以下代码：

- (Transporter *) getTransporter:(NSNumber *)transporterId
{
    for(Transporter *transporter in transporters)
    {
         if([transporter.transporterId isEqualToNumber:transporterId])
         {
             return transporter;
         }
    }
    __block Transporter *transporter;

    [self requestTransporter:transporterId completion:^(NSObject *entity) {
        transporter = (Transporter* )entity;
     }];

    return transporter;
}

我的问题是：__block Tranporter是如何工作的？ iOS等待变量得到值吗？或者我有一个零值，后来是一个对象？

块方法是否适用于具有返回值的方法？

Answer 1

__block关键字允许您修改（读写访问）变量，此处为transporter。如果你的方法requestTransporter同步，他将被执行并将在它的调用结束之前执行他的完成块。您还可以使用以下模式。

- (void) getTransporter:(NSNumber *)transporterId withCompletionBlock:(void(^)(Transporter*))completionBlock
{
    if(!completionBlock)
      return; //Avoid crashs.

    for(Transporter *transporter in transporters)
    {
         if([transporter.transporterId isEqualToNumber:transporterId])
         {
             completionBlock(transporter);
             return;
         }
    }

    [self requestTransporter:transporterId completion:^(NSObject *entity) {
        completionBlock((Transporter* )entity);
     }];
}

并像这样使用它：

// transporterId is an NSNumber containing your transporter id.
[self getTransporter:transporterId withCompletionBlock:^(Transporter *transporter) {
    // Execute your code using transporter object here.
}];

Answer 2

只要块是异步的，就会立即返回传输器，没有分配的实体。

使用完成处理程序，如下所示：

- (void) requestTransporter:(NSNumber *)transporterId withCompletionBlock:(void(^)(Transporter))completionBlock
{
  for(Transporter *transporter in transporters)
   {
     if([transporter.transporterId isEqualToNumber:transporterId])
     {
         completionBlock(transporter);
     }
   }
__block Transporter *transporter;

[self requestTransporter:transporterId completion:^(NSObject *entity) {
    transporter = (Transporter* )entity;
    completionBlock(transporter);
 }];

}

如何返回__block变量？

2 个答案: