如何確定UIAttachmentBehavior中的長度？

Question

UIAttachmentView中的length屬性的文檔如下：

創建附件后，可以根據需要使用此屬性來調整附件的長度。 系統會根據您初始化附件的方式自動設置初始長度。

我的問題是關於最后一句話：初始長度是如何計算的？

Answer 1

首次創建附件時，初始長度由錨的選擇決定。 例如，當您調用initWithItem:offsetFromCenter:attachedToAnchor:時，它是offsetFromCenter到attachedToAnchor之間的距離。

例如，考慮如下手勢識別器：

- (void)handlePan:(UIPanGestureRecognizer *)gesture
{
    static UIAttachmentBehavior *attachment;
    CGPoint location = [gesture locationInView:self.animator.referenceView];

    if (gesture.state == UIGestureRecognizerStateBegan) {
        attachment = [[UIAttachmentBehavior alloc] initWithItem:self.viewToAnimate attachedToAnchor:location];
        NSLog(@"before adding behavior to animator: length = %.0f", attachment.length);       // this says zero, even though it's not really
        [self.animator addBehavior:attachment];
        NSLog(@"after adding behavior to animator:  length = %.0f", attachment.length);       // this correctly reflects the length
    } else if (gesture.state == UIGestureRecognizerStateChanged) {
        attachment.anchorPoint = location;
        NSLog(@"during gesture: length = %.0f", attachment.length);       // this correctly reflects the length
    } else if (gesture.state == UIGestureRecognizerStateEnded || gesture.state == UIGestureRecognizerStateCancelled) {
        [self.animator removeBehavior:attachment];
        attachment = nil;
    }
}

報告：

2014-05-10 14:50:03.590 MyApp[16937:60b] before adding behavior to animator: length = 0
2014-05-10 14:50:03.594 MyApp[16937:60b] after adding behavior to animator:  length = 43
2014-05-10 14:50:03.606 MyApp[16937:60b] during gesture: length = 43
2014-05-10 14:50:03.607 MyApp[16937:60b] during gesture: length = 43

看來，如果您在實例化UIAttachmentBehavior之后立即查看length （但在將行為添加到動畫制作者之前），則該length似乎為零。 但是，只要將行為添加到動畫師， length就會正確更新。