# Top App Bars  `AppBarLayout` is a ViewGroup, most commonly used to wrap a `Toolbar`, that provides many of the Material Design features and interactions for **Top App Bars**, namely responsiveness to scrolling. ## Design & API Documentation * [Material Design guidelines: Top App Bar](https://material.io/go/design-app-bar-top) * [Material Design guidelines: Scrolling techniques](https://material.io/go/design-app-bar-top#behavior) * [Class definition](https://github.com/material-components/material-components-android/tree/master/lib/java/com/google/android/material/appbar/AppBarLayout.java) * [Class overview](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout) As a container for Toolbars, and other views, it works with [CoordinatorLayout](https://developer.android.com/reference/androidx/coordinatorlayout/widget/CoordinatorLayout) in order to respond to scrolling techniques. `AppBarLayout` depends heavily on being used as a direct child of the CoordinatorLayout and reacts to a sibling that supports scrolling (e.g. [NestedScrollView](https://developer.android.com/reference/android/support/v4/widget/NestedScrollView.html), [RecyclerView](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.html)). Flags are added to each child of the `AppBarLayout` to control how they will respond to scrolling. These are interpreted by the `AppBarLayout.LayoutParams`. **Available flags are:** * [enterAlways](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_ENTER_ALWAYS) * [enterAlwaysCollapsed](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_ENTER_ALWAYS_COLLAPSED) * [exitUntilCollapsed](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_EXIT_UNTIL_COLLAPSED) * [scroll](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_SCROLL) * [snap](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_SNAP) * [snapMargins](https://developer.android.com/reference/com/google/android/material/appbar/AppBarLayout.LayoutParams#SCROLL_FLAG_SNAP_MARGINS) Views using the scroll flag should be declared and visually positioned before other views in the `AppBarLayout`. This ensures that they are able to exit at the top of the screen, leaving behind fixed, or pinned, elements. ### Lift On Scroll Top App Bars can also be fixed in place and positioned at the same elevation as content. Upon scroll, they can increase elevation and let content scroll behind them. This design pattern is called "Lift On Scroll" and can be implemented by setting `app:liftOnScroll="true"` on your `AppBarLayout`. By default, `AppBarLayout` assumes that the scrolling view it should use to determine its lifted state is a direct sibling of it. If that is not the case, you may specify the scrolling view it should use to make this determination by using the `liftOnScrollTargetViewId` attribute. Note: the `liftOnScroll` attribute requires that you apply the `@string/appbar_scrolling_view_behavior` `layout_behavior` to your scrolling view (e.g., `NestedScrollView`, `RecyclerView`, etc.). ## Related Concepts The Top App Bar is a way of referencing a specific type of *Toolbar*. It's not a separate Android class. This UI element is often used to provide branding for the app as well as a place to handle common actions like navigation, search, and menus. These are accessible via text or buttons in the Toolbar. A Toolbar that provides some of these features is often referred to as the Top App Bar. They are programmatically identical and use the Toolbar class. The Top App Bar was previously termed *action bar*, and there are methods that utilize this name (e.g. [getSupportActionBar](https://developer.android.com/reference/android/support/v7/app/AppCompatActivity.html#getSupportActionBar())). Other than use of the action bar APIs, references to this prominent Toolbar element should be *Top App Bar*. A CollapsingToolbarLayout is often used as a wrapper around the Toolbar to provide additional UI features in relation to scrolling. * [Top App Bar](https://material.io/go/design-app-bar-top) * [CollapsingToolbarLayout](CollapsingToolbarLayout.md)