片段存在于活动内部，而活动则独立存在。

片段可以被认为是ui元素复合树中的非根组件，而活动位于复合树(ui树)的顶部。

A rule of thumb on when not to use Fragment is when as a child the fragment has a conflicting attribute, e.g., it may be immersive or may be using a different style all together or has some other architectural / logical difference and doesn't fit in the existing tree homogeneously. A rule of thumb on when to prefer Activity over Fragment is when the task (or set of coherent task) is fully independent and reusable and does some heavy weight lifting and should not be burdened further to conform to another parent-child composite (SRP violation, second responsibility would be to conform to the composite). For e.g., a MediaCaptureActivity that captures audio, video, photos etc and allows for edits, noise removal, annotations on photos etc and so on. This activity/module may have child fragments that do more granular work and conform to a common display theme.

这是我在片段中发现的重要信息:

Historically each screen in an Android app was implemented as a separate Activity. This creates a challenge in passing information between screens because the Android Intent mechanism does not allow passing a reference type (i.e. object) directly between Activities. Instead the object must be serialized or a globally accessible reference made available. By making each screen a separate Fragment, this data passing headache is completely avoided. Fragments always exist within the context of a given Activity and can always access that Activity. By storing the information of interest within the Activity, the Fragment for each screen can simply access the object reference through the Activity.

来源:https://www.pluralsight.com/blog/software-development/android-fragments

1.使用片段的目的是什么?

答: 处理器件形状因子差异。 在应用程序屏幕之间传递信息。 用户界面组织。 高级UI隐喻。

片段是应用程序的用户界面或行为的一部分，可以放在活动中，从而实现更模块化的活动设计。如果我们说片段是一种子活动，那也不会错。

以下是关于片段的要点:

A fragment has its own layout and its own behavior with its own lifecycle callbacks. You can add or remove fragments in an activity while the activity is running. You can combine multiple fragments in a single activity to build a multi-pane UI. A fragment can be used in multiple activities. The fragment life cycle is closely related to the lifecycle of its host activity. When the activity is paused, all the fragments available in the acivity will also be stopped. A fragment can implement a behavior that has no user interface component. Fragments were added to the Android API in Android 3 (Honeycomb) with API version 11.

欲了解更多详情，请访问官方网站碎片。

片段在某些情况下特别有用，比如我们想在所有页面中保留一个导航抽屉。你可以用你想要的任何片段来膨胀框架布局，并且仍然可以访问导航抽屉。

如果您使用了一个活动，那么您将不得不在所有活动中保留抽屉，这将导致冗余代码。这是片段的一个有趣用法。

我是Android的新手，仍然认为碎片是有帮助的。