android appbar_scrolling_view_behavior

Android AppBar Scrolling View Behavior

Introduction

In modern Android app design, it is common to have a scrollable content area with an AppBar that stays fixed at the top of the screen. This behavior provides a seamless user experience by allowing the content to scroll underneath the AppBar. To achieve this behavior, Android provides a built-in behavior called appbar_scrolling_view_behavior. In this article, we will explore the usage and implementation of this behavior.

Understanding AppBar Scrolling View Behavior

The appbar_scrolling_view_behavior is a specialized behavior that can be applied to a scrollable view such as RecyclerView or NestedScrollView. It allows the view to scroll in coordination with the AppBar, resulting in a smooth and intuitive scrolling experience.

Implementation

To use the appbar_scrolling_view_behavior, follow these steps:

Step 1: Add the Design library to your project dependencies in the build.gradle file:

implementation 'com.android.support:design:28.0.0'

Step 2: In your layout XML file, define the AppBar and the scrollable view. For example, let's consider a layout with a CoordinatorLayout, an AppBarLayout, and a RecyclerView:

<android.support.design.widget.CoordinatorLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <!-- Add your AppBar content here -->

    </android.support.design.widget.AppBarLayout>

    <android.support.v7.widget.RecyclerView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="android.support.design.widget.AppBarLayout$ScrollingViewBehavior" />

</android.support.design.widget.CoordinatorLayout>

Note the app:layout_behavior attribute added to the RecyclerView which references the appbar_scrolling_view_behavior.

Step 3: Customize the behavior (optional). The appbar_scrolling_view_behavior provides some additional customization options. For example, you can control the scroll flags for the AppBar by using the app:layout_scrollFlags attribute:

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <android.support.v7.widget.Toolbar
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        app:layout_scrollFlags="scroll|enterAlways">

        <!-- Add your Toolbar content here -->

    </android.support.v7.widget.Toolbar>

</android.support.design.widget.AppBarLayout>

In the above example, we've set the scroll and enterAlways flags for the Toolbar. This means that the Toolbar will scroll out of the screen when the content is scrolled down and will appear again when the content is scrolled up.

State Diagram

Below is a state diagram illustrating the different states of the AppBar and the scrollable view:

stateDiagram
    [*] --> Expanded
    Expanded --> ScrolledUp
    Expanded --> ScrolledDown
    ScrolledUp --> Expanded
    ScrolledDown --> Expanded

The initial state is Expanded, where the AppBar is fully visible and the scrollable view is at the top. When the user starts scrolling down, the AppBar transitions to the ScrolledUp state, and when the user scrolls up, the AppBar transitions back to the Expanded state.

Sequence Diagram

Let's take a look at a sequence diagram that illustrates the interaction between the AppBar and the scrollable view:

sequenceDiagram
    participant AppBar
    participant ScrollableView

    AppBar->>ScrollableView: Detect scroll
    ScrollableView->>AppBar: Notify scroll offset
    AppBar->>AppBar: Update visibility and position

When the scrollable view detects a scroll, it sends a notification to the AppBar with the scroll offset. The AppBar then updates its visibility and position based on the scroll offset.

Conclusion

The appbar_scrolling_view_behavior in Android provides an easy way to create a scrolling behavior for AppBar and scrollable views. By utilizing this behavior, you can create a smooth and user-friendly scrolling experience in your app. Remember to customize the behavior according to your specific requirements.