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Xamarin.Android CardView

  • Artikkeli

The Cardview widget is a UI component that presents text and image content in views that resemble cards. This guide explains how to use and customize CardView in Xamarin.Android applications while maintaining backward compatibility with earlier versions of Android.

Overview

The Cardview widget, introduced in Android 5.0 (Lollipop), is a UI component that presents text and image content in views that resemble cards. CardView is implemented as a FrameLayout widget with rounded corners and a shadow. Typically, a CardView is used to present a single row item in a ListView or GridView view group. For example, the following screen shot is an example of a travel reservation app that implements CardView-based travel destination cards in a scrollable ListView:

Example app using a CardView for each travel destination

This guide explains how to add the CardView package to your Xamarin.Android project, how to add CardView to your layout, and how to customize the appearance of CardView in your app. In addition, this guide provides a detailed list of CardView attributes that you can change, including attributes to help you use CardView on versions of Android earlier than Android 5.0 Lollipop.

Requirements

The following is required to use new Android 5.0 and later features (including CardView) in Xamarin-based apps:

  • Xamarin.Android – Xamarin.Android 4.20 or later must be installed and configured with either Visual Studio or Visual Studio for Mac.

  • Android SDK – Android 5.0 (API 21) or later must be installed via the Android SDK Manager.

  • Java JDK 1.8 – JDK 1.7 can be used if you are specifically targeting API level 23 and earlier. JDK 1.8 is available from Oracle.

Your app must also include the Xamarin.Android.Support.v7.CardView package. To add the Xamarin.Android.Support.v7.CardView package in Visual Studio for Mac:

  1. Open your project, right-click Packages and select Add Packages.

  2. In the Add Packages dialog, search for CardView.

  3. Select Xamarin Support Library v7 CardView, then click Add Package.

To add the Xamarin.Android.Support.v7.CardView package in Visual Studio:

  1. Open your project, right-click the References node (in the Solution Explorer pane) and select Manage NuGet Packages....

  2. When the Manage NuGet Packages dialog is displayed, enter CardView in the search box.

  3. When Xamarin Support Library v7 CardView appears, click Install.

To learn how to configure an Android 5.0 app project, see Setting Up an Android 5.0 Project. For more information about installing NuGet packages, see Walkthrough: Including a NuGet in your project.

Introducing CardView

The default CardView resembles a white card with minimally rounded corners and a slight shadow. The following example Main.axml layout displays a single CardView widget that contains a TextView:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center_horizontal"
    android:padding="5dp">
    <android.support.v7.widget.CardView
        android:layout_width="fill_parent"
        android:layout_height="245dp"
        android:layout_gravity="center_horizontal">
        <TextView
            android:text="Basic CardView"
            android:layout_marginTop="0dp"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:layout_centerVertical="true"
            android:layout_alignParentRight="true"
            android:layout_alignParentEnd="true" />
    </android.support.v7.widget.CardView>
</LinearLayout>

If you use this XML to replace the existing contents of Main.axml, be sure to comment out any code in MainActivity.cs that refers to resources in the previous XML.

This layout example creates a default CardView with a single line of text as shown in the following screen shot:

Screenshot of CardView with white background and line of text

In this example, the app style is set to the light Material Theme (Theme.Material.Light) so that the CardView shadows and edges are easier to see. For more information about theming Android 5.0 apps, see Material Theme. In the next section, we'll learn how to customize CardView for an application.

Customizing CardView

You can modify the basic CardView attributes to customize the appearance of the CardView in your app. For example, the elevation of the CardView can be increased to cast a larger shadow (which makes the card seem to float higher above the background). Also, the corner radius can be increased to make the corners of the card more rounded.

In the next layout example, a customized CardView is used to create a simulation of a print photograph (a "snapshot"). An ImageView is added to the CardView for displaying the image, and a TextView is positioned below the ImageView for displaying the title of the image. In this example layout, the CardView has the following customizations:

  • The cardElevation is increased to 4dp to cast a larger shadow.
  • The cardCornerRadius is increased to 5dp to make the corners appear more rounded.

Because CardView is provided by the Android v7 support library, its attributes are not available from the android: namespace. Therefore, you must define your own XML namespace and use that namespace as the CardView attribute prefix. In the layout example below, we will use this line to define a namespace called cardview:

    xmlns:cardview="http://schemas.android.com/apk/res-auto"

You can call this namespace card_view or even myapp if you choose (it's accessible only within the scope of this file). Whatever you choose to call this namespace, you must use it to prefix the CardView attribute that you want to modify. In this layout example, the cardview namespace is the prefix for cardElevation and cardCornerRadius:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:cardview="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center_horizontal"
    android:padding="5dp">
    <android.support.v7.widget.CardView
        android:layout_width="fill_parent"
        android:layout_height="245dp"
        android:layout_gravity="center_horizontal"
        cardview:cardElevation="4dp"
        cardview:cardCornerRadius="5dp">
        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="240dp"
            android:orientation="vertical"
            android:padding="8dp">
            <ImageView
                android:layout_width="fill_parent"
                android:layout_height="190dp"
                android:id="@+id/imageView"
                android:scaleType="centerCrop" />
            <TextView
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:textAppearance="?android:attr/textAppearanceMedium"
                android:textColor="#333333"
                android:text="Photo Title"
                android:id="@+id/textView"
                android:layout_gravity="center_horizontal"
                android:layout_marginLeft="5dp" />
        </LinearLayout>
    </android.support.v7.widget.CardView>
</LinearLayout>

When this layout example is used to display an image in a photo viewing app, the CardView has the appearance of a photo snapshot, as depicted in the following screenshot:

CardView with an image and caption below the image

Notice that a CardView can display more than one child view in its content area. For example, in the above photo viewing app example, the content area is comprised of a ListView that contains an ImageView and a TextView. Although CardView instances are often arranged vertically, you can also arrange them horizontally (see Creating a Custom View Style for an example screenshot).

CardView Layout Options

CardView layouts can be customized by setting one or more attributes that affect its padding, elevation, corner radius, and background color:

Diagram of CardView attributes

Each attribute can also be changed dynamically by calling a counterpart CardView method (for more information on CardView methods, see the CardView class reference). Note that these attributes (except for background color) accept a dimension value, which is a decimal number followed by the unit. For example, 11.5dp specifies 11.5 density-independent pixels.

Padding

CardView offers five padding attributes to position content within the card. You can set them in your layout XML or you can call analogous methods in your code:

Diagram of CardView padding attributes

The padding attributes are explained as follows:

  • contentPadding – Inner padding between the child views of the CardView and all edges of the card.

  • contentPaddingBottom – Inner padding between the child views of the CardView and the bottom edge of the card.

  • contentPaddingLeft – Inner padding between the child views of the CardView and the left edge of the card.

  • contentPaddingRight – Inner padding between the child views of the CardView and the right edge of the card.

  • contentPaddingTop – Inner padding between the child views of the CardView and the top edge of the card.

Content padding attributes are relative to the boundary of the content area rather than to any given widget located within the content area. For example, if contentPadding were sufficiently increased in the photo viewing app, the CardView would crop both the image and the text shown on the card.

Elevation

CardView offers two elevation attributes to control its elevation and, as a result, the size of its shadow:

Diagram of CardView elevation attributes

The elevation attributes are explained as follows:

  • cardElevation – The elevation of the CardView (represents its Z axis).

  • cardMaxElevation – The maximum value of the CardView's elevation.

Larger values of cardElevation increase the shadow size to make CardView seem to float higher above the background. The cardElevation attribute also determines the drawing order of overlapping views; that is, the CardView will be drawn under another overlapping view with a higher elevation setting and above any overlapping views with a lower elevation setting. The cardMaxElevation setting is useful for when your app changes elevation dynamically – it prevents the shadow from extending past the limit that you define with this setting.

Corner Radius and Background Color

CardView offers attributes that you can use to control its corner radius and its background color. These two properties allow you change the overall style of the CardView:

Diagram of CardView corner radious and background color attributes

These attributes are explained as follows:

  • cardCornerRadius – The corner radius of all corners of the CardView.

  • cardBackgroundColor – The background color of the CardView.

In this diagram, cardCornerRadius is set to a more rounded 10dp and cardBackgroundColor is set to "#FFFFCC" (light yellow).

Compatibility

You can use CardView on versions of Android earlier than Android 5.0 Lollipop. Because CardView is part of the Android v7 support library, you can use CardView with Android 2.1 (API level 7) and higher. However, you must install the Xamarin.Android.Support.v7.CardView package as described in Requirements, above.

CardView exhibits slightly different behavior on devices before Lollipop (API level 21):

  • CardView uses a programmatic shadow implementation that adds additional padding.

  • CardView does not clip child views that intersect with the CardView's rounded corners.

To help in managing these compatibility differences, CardView provides several additional attributes that you can configure in your layout:

  • cardPreventCornerOverlap – Set this attribute to true to add padding when your app is running on earlier Android versions (API level 20 and earlier). This setting prevents CardView content from intersecting with the CardView's rounded corners.

  • cardUseCompatPadding – Set this attribute to true to add padding when your app is running in versions of Android at or greater than API level 21. If you want to use CardView on pre-Lollipop devices and have it look the same on Lollipop (or later), set this attribute to true. When this attribute is enabled, CardView adds additional padding to draw shadows when it runs on pre-Lollipop devices. This helps to overcome the differences in padding that are introduced when pre-Lollipop programmatic shadow implementations are in effect.

For more information about maintaining compatibility with earlier versions of Android, see Maintaining Compatibility.

Summary

This guide introduced the new CardView widget included in Android 5.0 (Lollipop). It demonstrated the default CardView appearance and explained how to customize CardView by changing its elevation, corner roundness, content padding, and background color. It listed the CardView layout attributes (with reference diagrams), and explained how to use CardView on Android devices earlier than Android 5.0 Lollipop. For more information about CardView, see the CardView class reference.