Coordinator Layout中Toolbar顶部边距异常背景及卡片式Toolbar实现问题

Hey there, let's tackle this issue with your Toolbar's abnormal top background. Looking at your XML and problem description, here's what's going wrong and how to fix it:

What's Causing the Problem? #

The main culprits are likely:

• Conflicting fitsSystemWindows usage: You've set android:fitsSystemWindows="true" on the CoordinatorLayout, AppBarLayout, and CollapsingToolbarLayout all at once. This creates conflicts in how the system handles window insets, leading to unexpected background areas at the top.
• Missing card-style background: Since you want the Toolbar to float like a card, the current setup doesn't have a proper background on the collapsible container to mimic that card effect.

Step-by-Step Solutions #

1. Resolve fitsSystemWindows Conflicts

Remove android:fitsSystemWindows="true" from both AppBarLayout and CollapsingToolbarLayout—only keep it on the root CoordinatorLayout. This ensures insets are handled correctly at the top level:

<android.support.design.widget.AppBarLayout 
    android:id="@+id/app_bar" 
    android:layout_width="match_parent" 
    android:layout_height="@dimen/app_bar_height" 
    android:layout_margin="16dp"> <!-- Removed fitsSystemWindows here -->

    <android.support.design.widget.CollapsingToolbarLayout 
        android:id="@+id/toolbar_layout" 
        android:layout_width="match_parent" 
        android:layout_height="match_parent" 
        android:elevation="4dp" 
        app:contentScrim="?attr/colorPrimary" 
        app:layout_scrollFlags="scroll|exitUntilCollapsed" 
        app:toolbarId="@+id/toolbar"> <!-- Removed fitsSystemWindows here -->

2. Add Card-Style Background

To make the Toolbar look like a floating card, set a solid background (like white) on the CollapsingToolbarLayout—this will cover any unexpected background areas and give the card appearance you want:

<android.support.design.widget.CollapsingToolbarLayout 
    android:id="@+id/toolbar_layout" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:elevation="4dp" 
    android:background="@android:color/white" <!-- Added card background -->
    app:contentScrim="?attr/colorPrimary" 
    app:layout_scrollFlags="scroll|exitUntilCollapsed" 
    app:toolbarId="@+id/toolbar">

3. Optional: Internal CardView for Toolbar

If you want the Toolbar itself to be a card, uncomment your CardView inside the Toolbar and adjust the layout margins to get the desired floating look:

<android.support.v7.widget.Toolbar 
    android:id="@+id/toolbar" 
    android:layout_width="match_parent" 
    android:layout_height="?attr/actionBarSize" 
    app:layout_collapseMode="pin" 
    app:popupTheme="@style/AppTheme.PopupOverlay">

    <android.support.v7.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/colorAccent"
        android:layout_margin="8dp"/>

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

Additional Tips #

• If the issue persists after these changes, share more details about what the "abnormal background" looks like (e.g., color, size relative to the Toolbar) or a clearer screenshot.
• For your future plan of different layouts for expanded/collapsed states, you can use AppBarLayout.OnOffsetChangedListener to detect the collapse state and switch layouts programmatically.

内容的提问来源于stack exchange，提问作者Abhishek Bansal