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.

不确定你指的是什么视频，但我怀疑他们说你应该使用片段而不是活动，因为它们不能直接互换。实际上，在开发指南中有一个相当详细的条目，考虑阅读它的详细信息。

In short, fragments live inside activities, and each activity can host many fragments. Like activities, they have a specific lifecycle, unlike activities, they are not top-level application components. Advantages of fragments include code reuse and modularity (e.g., using the same list view in many activities), including the ability to build multi-pane interfaces (mostly useful on tablets). The main disadvantage is (some) added complexity. You can generally achieve the same thing with (custom) views in a non-standard and less robust way.

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。

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

活动是应用程序中带有工具栏的全屏组件，其他的最好是片段。 一个带有工具栏的全屏父活动可以有多个窗格、可滚动页面、对话框等(所有片段)，所有这些都可以从父活动访问并通过父活动进行通信。

例子:

活动A、活动B、活动C

所有的活动都需要有相同的代码重复，以显示基本 工具栏，或者从父活动继承 管理起来很麻烦)。 要从一个活动移动到另一个活动，要么所有活动都需要在内存中(开销)，要么需要销毁一个活动以便打开另一个活动。 活动之间的通信可以通过intent完成。

vs

活动A，片段1，片段2，片段3:

没有代码重复，所有屏幕都有来自该活动的工具栏等。 几种方法从一个片段移动到下一个视图页，多窗格等。 活动拥有最多的数据，因此需要最少的片段间通信。如果仍然需要，可以通过接口轻松完成。 片段不需要全屏显示，在设计上有很大的灵活性。 如果不需要视图，则片段不需要扩展布局。 多个活动可以使用相同的片段。

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。