ios - 带有关键帧的动画出错

Question

我正在玩一些动画。

我有一个 UIlabel，我想在单击按钮后从左侧“飞”到中心。标签的初始字符串将位于中心。

我得到的是标签在 3 秒内移动到“原点”，然后在该位置“弹出”。

我希望 UILabel 在这 3 秒内顺利地“滑动”/动画到目标点。 alpha 属性似乎也不能使动画流畅。希望我说清楚了。谢谢。

    - (IBAction)showQuestionButtonPressed:(UIButton *)sender
    {
        // Step to the next question
        self.currentQuestionIndex++;

        // Am I pas the last question?
        if (self.currentQuestionIndex == [self.questions count]) {

            // Go back to the first question
            self.currentQuestionIndex = 0;
        }

        // Get the string at the index in the questions array
        NSString *question = self.questions[self.currentQuestionIndex];

        // Display the string in the question label
        self.questionLabel.text = question;

        // Reset the answer label
        self.answerLabel.text = @"???";
        self.questionLabel.alpha = 1.0;
// the animation animated a little bit after this point:
        CGPoint origin = CGPointMake(0 - (self.questionLabel.frame.size.width/2.0), self.questionLabel.frame.origin.y);
        self.questionLabel.center = origin;


        [UIView animateKeyframesWithDuration:3.0 delay:0.0 options:0 animations:^{
            [UIView addKeyframeWithRelativeStartTime:0.0 relativeDuration:3.0 animations:^{
                self.questionLabel.alpha = 1.0;
            }];
            [UIView addKeyframeWithRelativeStartTime:0.0 relativeDuration:3.0 animations:^{
                self.questionLabel.center = CGPointMake(self.view.center.x, self.questionLabel.frame.origin.y) ;
            }];
        } completion:^(BOOL finished) {
            NSLog(@"finished animation");
        }];
    }

Answer 1

relativeDuration 应该是 0 到 1 之间的数字。来自文档:

The length of time over which to animate to the specified value. This value must be in the range 0 to 1 and indicates the amount of time relative to the overall animation length. If you specify a value of 0, any properties you set in the animations block update immediately at the specified start time. If you specify a nonzero value, the properties animate over that amount of time. For example, for an animation that is two seconds in duration, specifying a duration of 0.5 results in an animation duration of one second.

此外，如果您使用自动布局，则不能简单地设置标签的 center 属性。你必须调整你的约束。