在 Scroll 上顯示和隱藏 FloatingActionButton

從支援庫版本 22.2.1 開始，可以使用利用 show() 和 hide() 方法的 FloatingActionButton.Behavior 子類來顯示和隱藏 FloatingActionButton的滾動行為。

請注意，這僅適用於 CoordinatorLayout 以及支援巢狀滾動的內部檢視，例如 RecyclerView 和 NestedScrollView 。

這個 ScrollAwareFABBehavior 類來自 Codepath 上的 Android 指南 （cc-wiki，需要歸屬）

public class ScrollAwareFABBehavior extends FloatingActionButton.Behavior {
    public ScrollAwareFABBehavior(Context context, AttributeSet attrs) {
        super();
    }

    @Override
    public boolean onStartNestedScroll(final CoordinatorLayout coordinatorLayout, final FloatingActionButton child,
                                       final View directTargetChild, final View target, final int nestedScrollAxes) {
        // Ensure we react to vertical scrolling
        return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL
                || super.onStartNestedScroll(coordinatorLayout, child, directTargetChild, target, nestedScrollAxes);
    }

    @Override
    public void onNestedScroll(final CoordinatorLayout coordinatorLayout, final FloatingActionButton child,
                               final View target, final int dxConsumed, final int dyConsumed,
                               final int dxUnconsumed, final int dyUnconsumed) {
        super.onNestedScroll(coordinatorLayout, child, target, dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed);
        if (dyConsumed > 0 && child.getVisibility() == View.VISIBLE) {
            // User scrolled down and the FAB is currently visible -> hide the FAB
            child.hide();
        } else if (dyConsumed < 0 && child.getVisibility() != View.VISIBLE) {
            // User scrolled up and the FAB is currently not visible -> show the FAB
            child.show();
        }
    }
}

在 FloatingActionButton 佈局 xml 中，使用完全限定類名稱 ScrollAwareFABBehavior 指定 app:layout_behavior：

app:layout_behavior="com.example.app.ScrollAwareFABBehavior"

例如，使用此佈局：

<android.support.design.widget.CoordinatorLayout
    android:id="@+id/main_layout"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/appBarLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:elevation="6dp">
        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:background="?attr/colorPrimary"
            android:minHeight="?attr/actionBarSize"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
            app:elevation="0dp"
            app:layout_scrollFlags="scroll|enterAlways"
            />

        <android.support.design.widget.TabLayout
            android:id="@+id/tab_layout"
            app:tabMode="fixed"
            android:layout_below="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            app:elevation="0dp"
            app:tabTextColor="#d3d3d3"
            android:minHeight="?attr/actionBarSize"
            />

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

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_below="@+id/tab_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        app:layout_behavior="com.example.app.ScrollAwareFABBehavior"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_dialog_email" />

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

結果如下：

https://i.stack.imgur.com/ijLDS.gif