片段是有趣的东西,但是我认为一旦你了解它们的怪癖,它们就成为了在多个设备上编写良好代码的宝贵工具。
然而,在修复方向更改错误时,我遇到了一个障碍。为了使我的片段正常工作,它需要访问属于其包含 Activity 的 View,这引导我花费很长时间寻找 Activity 和 Fragment 生命周期之间的交互方式。
我在 Activity 的 onCreate()
方法中添加一个片段到其视图中:
// Only add a fragment once, as after it's been added it cannot be replaced (Even though there is a .replace() method. Which is a massive gaping hole in fragments as a technology if you ask me)
if(savedInstanceState == null) {
MainMenuFragment menu= new MainMenuFragment();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.menuFrame, menu);
transaction.commit();
}
导致此Activity->Fragment生命周期:
01-04 15:17:27.226: W/SinglePaneActivity 0: onCreate()
01-04 15:17:27.378: W/MainMenuFragment 0: onAttach() to SinglePaneActivity 0
01-04 15:17:27.378: W/MainMenuFragment 0: onCreate()
01-04 15:17:27.453: W/MainMenuFragment 0: onActivityCreated()
01-04 15:17:27.476: W/MainMenuFragment 0: onStart()
01-04 15:17:27.476: W/SinglePaneActivity 0: onStart()
01-04 15:17:27.476: W/SinglePaneActivity 0: onResume()
01-04 15:17:27.476: W/MainMenuFragment 0: onResume()
然而,屏幕方向变化会突显出通常情况并不是这样的,一个片段的 onCreate()
方法在其父 Activity 的 onCreate()
方法之后并不会被调用。实际上,片段的第一个生命周期调用是发生在 Activity 被创建之前(以 null
作为参数传递)的 onAttach()
方法中:
01-04 15:10:49.589: W/MainMenuFragment 0: onPause()
01-04 15:10:49.589: W/SinglePaneActivity 0: onPause()
01-04 15:10:49.589: W/MainMenuFragment 0: onStop()
01-04 15:10:49.589: W/SinglePaneActivity 0: onStop()
01-04 15:10:49.589: W/MainMenuFragment 0: onDestroyView()
01-04 15:10:49.589: W/MainMenuFragment 0: onDestroy()
01-04 15:10:49.589: W/MainMenuFragment 0: onDetach()
01-04 15:10:49.609: W/SinglePaneActivity 0: onDestroy()
01-04 15:10:49.617: W/MainMenuFragment 1: onAttach() to null
01-04 15:10:49.617: W/MainMenuFragment 1: onCreate()
01-04 15:10:49.617: W/SinglePaneActivity 1: onCreate()
01-04 15:10:49.890: W/MainMenuFragment 1: onActivityCreated()
01-04 15:10:49.917: W/MainMenuFragment 1: onStart()
01-04 15:10:49.917: W/SinglePaneActivity 1: onStart()
01-04 15:10:49.921: W/SinglePaneActivity 1: onResume()
01-04 15:10:49.921: W/MainMenuFragment 1: onResume()
我完全不知道为什么会发生这种情况。有谁能够解释一下为什么在创建其包含的Activity之前就调用了Fragment.onAttach()
吗?
对于那些在UI交互之前不需要访问其包含活动的片段,它们的工作正常。