循环更改文本字段仅显示一个最终值 [英] Changing text field repeatedly in a loop only displays one final value

问题描述

我希望创建一个循环，该循环每十秒递增一个整数，并执行一百次。但是当我使用以下代码时：

I wish to create a loop that increments an integer every ten seconds, and does this one hundred times. But when I use this code:

- (IBAction)loopTest:(id)sender {

}

- (IBAction)beginLoop:(id)sender {
  for (i=0;i<100 ;i++ ) {
    testingLoops++;
    NSString *feed = [NSString stringWithFormat: @"%d", testingLoops];
    self.feedLabel.stringValue = feed;
    [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:@selector(loopTest:)           userInfo:nil repeats:NO];
}

}

应用程序仅将整数显示为马上100。我有它，这样当我按下按钮时它就会运行 beginLoop 。

the application just displays the integer as 100 straight away. I have it so that it runs the beginLoop when I press a button. What's going on?

推荐答案

您的声明：

[NSTimer scheduledTimerWithTimeInterval:10
                                 target:self
                               selector:@selector(loopTest:)
                               userInfo:nil
                                repeats:NO];

not 不会延迟循环-而是每次循环迭代安排计时器调用 loopTest：，您已定义为什么都不做的方法。

does not delay your loop - rather for every iteration of the loop it schedules a timer to call loopTest:, a method you've defined to do nothing.

使用计时器要延迟循环，您需要安排执行循环其余部分的方法。换句话说，一种非循环方法等效于循环的一次迭代，然后安排执行剩余时间的时间。

To use a timer to delay a loop you need to schedule a method which performs the remainder of the loop. In other words a non-loop method which performs the equivalent of one iteration of your loop and then schedules a time to perform the remainder.

按照您的方法进行操作，但切换到使用由 performSelector：withObject：afterDelay 提供的隐式计时器，因为它在这里更方便，这给了我们：

Following your approach, but switching to use an implicit timer provided by performSelector:withObject:afterDelay as it is more convenient here, this gives us:

- (IBAction)beginLoop:(id)sender
{
   // start "loop"
   // note we only pass the current index and not the limit or delay
   // as there is no performSelector version which directly supports
   // passing three values to the selector
   [self doLoopIndex:@0];
}

- (void) doLoopIndex:(NSNumber *)objIndex
{
   // extract int from NSNumber - we use the later as the argument type so we can use performSelector below
   int index = objIndex.intValue;

   // do the work of one iteration
   NSString *feed = [NSString stringWithFormat: @"%d", index];
   self.feedLabel.stringValue = feed;

   // increment "loop" counter and schedule next iteration if needed
   // note the use of @(index) to create an NSNumber as an object is required
   index++;
   if (index < 100)
      [self performSelector:@selector(doLoopIndex:) withObject:@(index) afterDelay:1];
}

这不是实现目标的唯一方法。使用块和 Grand Central Dispatch（GCD）是另一种方法，在这种情况下，其优点是传递三个值：当前索引，限制和延迟；

This is not the only way to achieve your goal. Using blocks and "Grand Central Dispatch" (GCD) is another and in this case has the advantage that passing three values: current index, limit and delay; is easier.

例如，具有延迟的一般循环可能写为：

For example, a general loop with delay might be written as:

- (void) doLoop:(int)index                      // starting index
          limit:(int)limit                      // limit
          delay:(NSTimeInterval)delayInSeconds  // delay each iteration
           body:(void (^)(int))body             // block for loop body
{
   // invoke the body block
   body(index);

   // increment index and schedule next "iteration" if needed
   index++;
   if (index < 100)
   {
      dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
      dispatch_after(popTime, dispatch_get_main_queue(), ^(void)
      {
         [self doLoop:index limit:limit delay:delayInSeconds body:body];
      });
   }
}

有关dispatch_X方法和方法的详细信息，请参见文档。

See the documentation for the details of the dispatch_X methods and types.

这可能看起来更复杂，但这是因为它更通用。使用上面的代码，您的特定循环变为：

This might look more complicated, but that is because it is more general. Using the above your particular loop becomes just:

- (IBAction)beginLoop:(id)sender
{
   [self doLoop:0 limit:100 delay:1 body:^(int index)
    {
       NSString *feed = [NSString stringWithFormat: @"%d", index];
       self.feedLabel.stringValue = feed;
    }];
}

上述方法中的哪一种，或使用显式计时器作为原始代码，适合您的用例就是您的选择-没有单一的正确答案。

Which of the above approaches, or using an explicit timer as your original code did, is appropriate for your use case is your choice - there is no single "right" answer.

HTH。

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