Hero

HeroTransitions/Hero: Elegant transition library for iOS & tvOS

Hero是一个自定义转场动画的框架。自定义跳转自然是用了UIViewControllerInteractiveTransitioning，Hero通过hero id对VC进行解藕，同时也依赖hero id进行UI变换，默认实现是，在路由时，进行截图，然后拉伸

a在Hero里看见了Chameleon,怀念，不过确实没人维护了

因为是对截图做变换，Hero的Transition主要是view的尺寸、位置变化，而内容的维护需要自己控制，比如我在A VC的hero id为"ironMan"的view内增加了一个label，在转场时，可以看到label被拉伸，并且，如果在B VC中没有添加这个label，那就会在视觉上丢失；同理在B VC，我把hero id为"ironMan"的view设置了另一个背景色，就会看到在路由结束之后，View突然从粉色变成灰色，没有渐变

Simulator Screen Recording - iPhone SE (3rd generation) - 2022-08-11 at 13.32.28.gif

从代码中，看到有个Extension HeroContext，印证了之前的判断：

extension HeroContext {
  /**
   - Returns: a snapshot view for animation
   */
  public func snapshotView(for view: UIView) -> UIView {
    // ...
  }
}

当然，把"ironMan" 设置为不使用快照之后（redView.hero.modifiers = [.useNoSnapshot]）再尝试，就没有那种明显的拉伸感了。

Simulator Screen Recording - iPhone SE (3rd generation) - 2022-08-11 at 13.31.53.gif

Hero有两个关键入口函数：start和animate

Start函数比较长，简单来说包括：

• 提取一张全屏快照，用来防闪烁
• 提取fromViews和toViews，标准是非hidden的view（会考虑容器和子view）并提取modifiers （HeroContext.process(views:,idMap:)）写入<UIView, HeroTargetState>字典targetStates
• processors处理fromViews和toViews，这里有IgnoreSubviewModifiersPreprocessor、ConditionalPreprocessor、DefaultAnimationPreprocessor、MatchPreprocessor、SourcePreprocessor、CascadePreprocessor六个处理器，其中MatchPreprocessor以fromViews和toViews有id映射(MatchPreprocessor.process(fromViews:toViews:))为标准写入<UIView, HeroTargetState>字典targetStates，SourcePreprocessor 提取刚才获得的字典的view的参数（position、opacity、shadow等）
• 提取animatingFromViews和animatingToViews，标准是字典 targetStates[view]的HeroTargetState有值
• 调用animate()开始动画

再看看HeroTransition的animate函数：

extension HeroTransition {
  open func animate() {
    guard state == .starting else { return }
    state = .animating

    if let toView = toView {
      context.unhide(view: toView)
    }

    // auto hide all animated views
    for view in animatingFromViews {
      context.hide(view: view)
    }
    for view in animatingToViews {
      context.hide(view: view)
    }

    var totalDuration: TimeInterval = 0
    var animatorWantsInteractive = false

    if context.insertToViewFirst {
      for v in animatingToViews { _ = context.snapshotView(for: v) }
      for v in animatingFromViews { _ = context.snapshotView(for: v) }
    } else {
      for v in animatingFromViews { _ = context.snapshotView(for: v) }
      for v in animatingToViews { _ = context.snapshotView(for: v) }
    }

    // UIKit appears to set fromView setNeedLayout to be true.
    // We don't want fromView to layout after our animation starts.
    // Therefore we kick off the layout beforehand
    fromView?.layoutIfNeeded()

    for animator in animators {
      let duration = animator.animate(fromViews: animatingFromViews.filter({ animator.canAnimate(view: $0, appearing: false) }),
                                      toViews: animatingToViews.filter({ animator.canAnimate(view: $0, appearing: true) }))
      if duration == .infinity {
        animatorWantsInteractive = true
      } else {
        totalDuration = max(totalDuration, duration)
      }
    }

    self.totalDuration = totalDuration
    if let forceFinishing = forceFinishing {
      complete(finished: forceFinishing)
    } else if let startingProgress = startingProgress {
      update(startingProgress)
    } else if animatorWantsInteractive {
      update(0)
    } else {
      complete(after: totalDuration, finishing: true)
    }

    fullScreenSnapshot?.removeFromSuperview()
  }
}

• hide隐藏了所有animatingFromViews和animatingToViews
• 拍摄快照。
• 调用animator实现动画

这个是简单例子，app store的例子就要复杂很多和好看很多。

Simulator Screen Recording - iPhone SE (3rd generation) - 2022-08-12 at 13.48.45.gif

在实现上多了很多细节，通过modifiers去实现细致的定制效果
首先还是通过hero id去做“复用”，也因此第二个cell不会在animatingFromViews，要知道context.fromViews.count有24个，但animatingFromViews只有两个，原因就在于，MatchPreprocessor在处理的时候，从24个fromViews和15个toViews中，只匹配到了两对id
同时，不在from view里的detail label，因为有modifiers，在前面所述的提取fromViews和toViews步骤中，也被捕获