Hands-on with Material Components for Android: Bottom Sheets

Part 3 of a series covering practical usage of Material Components for Android

This post will be covering the features and API of Bottom Sheet components. To find out how to handle initial setup of Material Components for Android (including the Gradle dependency and creating an app theme), please see my original post:

Setting up a Material Components theme for Android

Bottom Sheets are surface components that hold supplementary screen content. They are anchored to the bottom of a screen (making them ergonomic on mobile/tablet devices) and, similar to Dialogs, are elevated over main screen content. In most cases, they can be expanded/dismissed via a drag up/down gesture.

From a design perspective, there are two types of Bottom Sheets:

• Standard Bottom Sheet: They operate independently to (and allow for simultaneous interaction with) primary screen content. They can be in an expanded, collapsed or hidden state.
• Modal Bottom Sheet: They block primary screen content and must be interacted with or dismissed. A semi-transparent scrim, which can be tapped to dismiss, is displayed behind them to indicate the underlying UI is temporarily inaccessible.

Standard Bottom Sheet (left), Modal Bottom Sheet (right)

Note: A third type exists: Expanding Bottom Sheet. At the time of writing, the latest release of Material Components for Android is 1.2.0-alpha06 and there is no standard component or class to handle this. A custom implementation could be achieved with something like MotionLayout, but that is outside the scope of this article and won’t be covered.

Basic usage 🏁

Implementing Bottom Sheets is not as simple as using a single component. There exists different classes within Material Components for Android for each type of sheet.

Standard Bottom Sheet

A child View of a CoordinatorLayout can have Standard Bottom Sheet characteristics enabled by using BottomSheetBehavior. In doing so, the bottom anchoring, drag up/down gesture support and animated state transitions and more are handled for us.

This can be done in your screen layout like so:

<androidx.coordinatorlayout.widget.CoordinatorLayout
  ...>

  <FrameLayout
    android:id="@+id/standardBottomSheet"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    style="?attr/bottomSheetStyle"
    app:layout_behavior="com.google.android.material.bottomsheet.BottomSheetBehavior">

    <!-- Bottom Sheet contents -->

  </FrameLayout>

</androidx.coordinatorlayout.widget.CoordinatorLayout>

In the above example, our Bottom Sheet is in fact the FrameLayout. The most important part is that the layout_behavior attribute is set to com.google.android.material.bottomsheet.BottomSheetBehavior. We have also applied some default styling (background shape/tint, elevation, etc.) with the bottomSheetStyle attribute, which will be discussed in more detail below in the “Theming” section.

Modal Bottom Sheet

For Modal Bottom Sheets, the implementation is different. The BottomSheetDialogFragment class (which extends AppCompatDialogFragment) provides the desired modal behavior in addition to existing DialogFragment functionality.

First of all, a subclass of BottomSheetDialogFragment needs to be created and the onViewCreated callback must be overridden to provide a layout for the contents of the sheet:

class ModalBottomSheet : BottomSheetDialogFragment() {

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? = inflater.inflate(R.layout.modal_bottom_sheet, container, false)

    companion object {
        const val TAG = "ModalBottomSheet"
    }
}

Then, inside an AppCompatActivity, the class can be used to show the sheet like so:

val modalBottomSheet = ModalBottomSheet()
modalBottomSheet.show(supportFragmentManager, ModalBottomSheet.TAG)

It is worth noting that, under the hood, BottomSheetDialogFragment also makes use of BottomSheetBehavior in a BottomSheetDialog class to convert a standard AppCompatDialogFragment to one that has Bottom Sheet characteristics. BottomSheetDialog can be used independently if you don’t want to use a Fragment.

Adjusting behavior ⚙️

There exists a variety of attributes that can be used to adjust the behavior of both Standard and Modal Bottom Sheets. This includes things like “peek” (collapsed) height, hidability, content fitting and more.

Applying attributes

For Standard Bottom Sheets, applying these attributes can be done in XML by applying the attributes to the same child View that has the layout_behavior attribute set to BottomSheetBehavior. In our example above in the “Basic usage” section, this is the FrameLayout.

Alternatively, it can be done programmatically:

val standardBottomSheetBehavior = BottomSheetBehavior.from(standardBottomSheet)
// Use this to programmatically apply behavior attributes

For Modal Bottom Sheets there is no XML layout definition for the sheet itself, but we can make use of app-level theme attributes and styles:

<style name="ModalBottomSheet" parent="Widget.MaterialComponents.BottomSheet.Modal">
  <!-- Apply attributes here -->
</style>

<style name="ModalBottomSheetDialog" parent="ThemeOverlay.MaterialComponents.*.BottomSheetDialog">
  <item name="bottomSheetStyle">@style/ModalBottomSheet</item>
</style>

<style name="AppTheme" parent="Theme.MaterialComponents.*">
  <item name="bottomSheetDialogTheme">@style/ModalBottomSheetDialog</item>
</style>

Alternatively, it can be done programmatically:

val modalBottomSheetBehavior = (modalBottomSheet.dialog as BottomSheetDialog).behavior
// Use this to programmatically apply behavior attributes

The attributes

Now that we know how to apply attributes, let’s dive into what attributes are available to us.

• behavior_hideable: Determines whether or not the sheet can be hidden when using a drag down gesture (bearing in mind that it can always be hidden programmatically). The default value is false for Standard Bottom Sheets and true for Modal Bottom Sheets.
• behavior_draggable: Determines whether or not the sheet can be collapsed/expanded when using a drag gesture (bearing in mind that a custom way to expand/collapse the sheet will need to be implemented). The default value is true.
• behavior_skipCollapsed: Determines whether or not the collapsed state should be ignored when hiding the sheet. This has no effect if behavior_hideable is not set to true. The default value is false.
• behavior_fitToContents: Determines whether or not the height of the expanded sheet wraps its contents. Alternatively, it expands in two stages: half the height of the parent container, full height of the parent container. The default value is true.
• behavior_halfExpandedRatio: Determines the height of the sheet (as a ratio of the parent container height) when in half-expanded state. This has no effect if behavior_fitToContents is not set to false and should be greater than the peek height. The default value is 0.5 (the recommended ratio in the Material Guidelines).
• behavior_expandedOffset: Determines the offset of the sheet from the top of the parent container when in expanded state. This has no effect if behavior_fitToContents is not set to false and should be greater than the offset when in half-expanded state. The default value is 0dp (the top of the sheet matches the top of the parent container).
• behavior_peekHeight: The initial “peek” (collapsed state) height of the sheet. The default value is auto, which sets the peek height at the 16:9 ratio keyline of the parent container. A dimension (or pixel value, programmatically) can otherwise be used.

A word on modal dismiss animations

You might notice that, for modal bottom sheets (i.e. BottomSheetDialog or BottomSheetDialogFragment), the animation used when calling dismiss() does not match the normal drag-to-dismiss animation. As of Material Components for Android 1.1.0-alpha10, there exists an opt-in flag to align these animations:

// If using BottomSheetDialog directly
bottomSheetDialog.dismissWithAnimation = true
// If using BottomSheetDialogFragment (do in onActivityCreated)
(requireDialog() as BottomSheetDialog).dismissWithAnimation = true

Setting (and saving) state 💾

The state of a Bottom Sheet can be changed via user interaction, but we can also do so programmatically. By default, these state changes are animated.

Bottom Sheet states

Setting state

The following states can be set on a BottomSheetBehavior:

• STATE_EXPANDED: The sheet is fully expanded.
• STATE_COLLAPSED: The sheet is collapsed (“peeking”).
• STATE_HIDDEN: The sheet is hidden and can only be re-shown programmatically.
• STATE_HALF_EXPANDED: The sheet is half-expanded (only applicable if behavior_fitToContents has been set to false).

This can be done like so:

bottomSheetBehavior.state = BottomSheetBehavior.STATE_COLLAPSED

While these should not be set programmatically, a BottomSheetBehavior can also be in one of the following states:

• STATE_DRAGGING: The sheet is being dragged up/down via a gesture.
• STATE_SETTLING: The sheet is animating up/down as a result of programmatically setting its state.

Retaining attribute state on configuration change

We can choose which aspects of state we wish to preserve when the host Activity experiences a configuration change (i.e. the View representing our Bottom Sheet is destroyed and recreated). In this case, “state” refers to the attributes discussed in the “Adjusting behavior” section above.

The following flags can be set (or combined with bitwise OR operations) on a BottomSheetBehavior:

• SAVE_PEEK_HEIGHT: The behavior_peekHeight attribute will be preserved.
• SAVE_FIT_TO_CONTENTS: The behavior_fitToContents attribute will be preserved.
• SAVE_HIDEABLE: The behavior_hideable attribute will be preserved.
• SAVE_SKIP_COLLAPSED: The behavior_skipCollapsed attribute will be preserved.
• SAVE_ALL: All aforementioned attributes will be preserved.
• SAVE_NONE: No attributes will be preserved. This is the default value.

This can be done like so:

bottomSheetBehavior.saveFlags = BottomSheetBehavior.SAVE_ALL

There is also a corresponding behavior_saveFlags XML attribute which can be used in layouts and styles.

Listening for state change and slide callbacks 👂

We can listen for changes to Bottom Sheet state as well as the current slide offset. This can be used to coordinate other UI changes, such as fading in/out other Views, adjusting system bar color, etc.

Adjusting background color on slide

A BottomSheetCallback can be added to a BottomSheetBehavior like so:

val bottomSheetCallback = object : BottomSheetBehavior.BottomSheetCallback() {
    
    override fun onStateChanged(bottomSheet: View, newState: Int) {
        // Do something for new state
    }

    override fun onSlide(bottomSheet: View, slideOffset: Float) {
        // Do something for slide offset
    }
}
bottomSheetBehavior.addBottomSheetCallback(bottomSheetCallback)

In the onStateChanged callback, the newState parameter will be one of the state constants discussed above in the “Setting (and saving) state” section.

In the onSlide callback, the slideOffset parameter is a Float value in the [-1.0, 1.0] range. Hidden state is -1.0, collapsed state is 0.0 and expanded state is 1.0. The value interpolates linearly and increases as the sheet moves upwards.

Note: Multiple callbacks can be added (and removed via BottomSheetBehavior#removeBottomSheetCallback).

Theming 🎨

Bottom Sheets can be themed in terms of the three Material Theming subsystems: color, typography and shape. For Standard Bottom Sheets, there are no existing style variants but we can create our own. When implementing a global Standard Bottom Sheet style, reference it in your app theme with the bottomSheetStyle attribute. For Modal Bottom Sheets, we have already shown which styles/attributes to use in the “Adjusting behavior” section above. Theming attributes are applicable to both sheet types.

Color

The color of a Bottom Sheet background can be customized with the backgroundTint attribute. This defaults to colorSurface.

Color theming

Typography

There is no primary text as part of Bottom Sheet components. Text included in the contents of a Bottom Sheet will be styled according to the class/component used as well as the fontFamily app theme attribute.

Type theming

Shape

The shape of a Bottom Sheet background can be customized with the shapeAppearance attribute. This defaults to shapeAppearanceLargeComponent.

Shape theming

More resources 📚

• The source code for the Playground app used in this article can be found on GitHub.
• Bottom Sheets Design Documentation
• Bottom Sheet API Documentation
• Modal Bottom Sheet API Documentation

I hope this post has provided some insight into Bottom Sheets and how they can be used in your Android app(s). If you have any questions, thoughts or suggestions then I’d love to hear from you!

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