Activity是Android应用开发中的核心组件之一,它负责与用户交互并展示UI。了解Activity的生命周期是每个Android开发者必备的基础知识。在Android系统中,Activity有多个状态,它们之间的转换构成了Activity的生命周期。在这个"Activity生命周期demon"中,我们将深入探讨Activity在不同状态下的行为及其转换。 1. **Activity生命周期的阶段** - **创建(Created)**:当Activity首次被创建时,会调用`onCreate()`方法。在这个阶段,通常会进行初始化工作,如设置布局、实例化对象等。 - **启动(Started)**:`onStart()`方法紧接着`onCreate()`被调用,Activity开始对用户可见,但可能仍被其他Activity遮挡。 - **可见(Resumed)**:当Activity完全可见并处于前台时,`onResume()`会被调用。用户可以与Activity交互,这是Activity的活动状态。 - **暂停(Paused)**:当Activity部分被遮挡,例如弹出一个透明或半透明对话框,会调用`onPause()`。此时,Activity仍然可见,但用户与它的交互减少。 - **停止(Stopped)**:如果Activity完全被遮挡,如新Activity启动,会调用`onStop()`。Activity不再可见,但系统仍保留其状态。 - **销毁(Destroyed)**:当Activity不再需要时,系统会调用`onDestroy()`,可能是用户关闭或系统内存不足时进行清理。 2. **生命周期转换** - `onRestart()`:当已停止的Activity重新变得可见时调用,接着是`onStart()`和`onResume()`。 - `onSaveInstanceState()`:在Activity被销毁之前,系统可能会调用此方法保存当前状态,以便后续恢复。 - `onRestoreInstanceState()`:当Activity重启时,系统会调用此方法,将之前保存的状态数据恢复。 3. **生命周期的注意事项** - 不要在`onCreate()`和`onStart()`里做耗时操作,这可能导致界面延迟显示。 - `onPause()`适合用来保存临时数据,因为它是Activity生命周期中最先被调用的可保证执行的方法。 - `onDestroy()`不一定总会被调用,因此不要依赖它来释放资源。最好在`onPause()`或`onStop()`中进行资源清理。 4. **生命周期与后台服务** 当Activity进入后台,系统可能会杀死它以回收资源。通过结合Service,可以确保即使Activity被销毁,后台任务依然运行。 5. **内存管理与配置变更** - 当设备配置改变(如屏幕旋转)时,系统默认会销毁并重建Activity。开发者可以通过重写`onConfigurationChanged()`来避免这种情况。 - 在`onLowMemory()`中,开发者可以处理内存紧张的情况,比如释放大内存占用的资源。 6. **多Activity间的交互** - 使用`startActivity()`和`startActivityForResult()`启动新的Activity,后者可以接收返回结果。 - 使用`finish()`结束当前Activity,返回上一个Activity。 总结,理解并熟练掌握Activity的生命周期对于优化应用性能、确保用户体验以及正确处理用户交互至关重要。通过这个"Activity生命周期demon",开发者可以深入学习如何在不同的生命周期阶段合理地编写代码,从而实现更高效、更稳定的Android应用。
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- gailanren2016-09-25很好的文档,帮助很大
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