渲染之前触发ViewPager片段中嵌套片段的动画 [英] Animation of nested fragment within ViewPager fragment is triggered before render

问题描述

我有一个ViewPager片段

class PagerFragment() : Fragment() {
  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)

      val pagerView = view as ViewPager
      val adapter = PagerAdapter(childFragmentManager, items)
      pagerView.adapter = adapter
  }
}

使用以下适配器

class PagerAdapter(val fm: FragmentManager, val items: ArrayList<Item>)
    : FragmentStatePagerAdapter(fm) {

    override fun getItem(position: Int): Fragment {
        val item = items[position]
        return NestedFragment.newInstance(item)
    }

    override fun getCount(): Int {
        return  items.size
    }
}

通过以下方式嵌套片段并触发一些动画onViewCreated:

which nests fragments with a few animations triggered onViewCreated the following way:

class NestedFragment() : Fragment() {
  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)

      val one = AnimationUtils.loadAnimation(context, R.anim.one)
      val two = AnimationUtils.loadAnimation(context, R.anim.two)
      val three = AnimationUtils.loadAnimation(context, R.anim.three)

      view.findViewById<ImageView>(R.id.pic).startAnimation(one)
      view.findViewById<ConstraintLayout>(R.id.content).startAnimation(two)
      view.findViewById<ImageView>(R.id.img).startAnimation(three)
  }
}

但是经常在屏幕上显示嵌套片段时，动画已经开始甚至结束了:(

However often by the time the nested fragment is showed on the screen the animation has already started or even ended :(

是否有一种方法可以确定片段何时确实在设备的屏幕上呈现? 当然，也欢迎任何其他有用的建议来解决该问题.

Is there a way to determine when the fragment is indeed rendered on the screen of the device? Of course any other useful suggestions to overcome the issue are welcome as well.

推荐答案

这是对我有用的.

class NestedFragment() : Fragment() {
  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)

      val one = AnimationUtils.loadAnimation(context, R.anim.one)
      val two = AnimationUtils.loadAnimation(context, R.anim.two)
      val three = AnimationUtils.loadAnimation(context, R.anim.three)
      view.post(Runnable {
        /* 
          If the animation doesn't trigger you may try to hide 
          elements visibility and reveal them before animation starts
        */
        view.findViewById<ImageView>(R.id.pic).startAnimation(one)
        view.findViewById<ConstraintLayout>(R.id.content).startAnimation(two)
        view.findViewById<ImageView>(R.id.img).startAnimation(three)
      })
  }
}

如果动画仍然没有触发，您可以尝试隐藏视图的可见性，并在动画之前显示它们.

If the animation still doesn't trigger you may try to hide views visibility and reveal them right before the animation.

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