【Android Developers Training】 71. 显示翻牌动画

注：本文翻译自Google官方的Android Developers Training文档，译者技术一般，由于喜爱安卓而产生了翻译的念头，纯属个人兴趣爱好。

原文链接：http://developer.android.com/training/animation/cardflip.html

---

这一节课将向您展示如何用自定义的fragment动画来实现翻牌动画（Card Flip）。翻牌动画是在视图切换的时候以翻牌形式为过渡动画的效果，其如下所示：

Card Flip动画

如果你希望略过这部分内容直接看代码样例，可以直接下载样例代码，然后选择淡入淡出动画的例子。下面的文件是实现代码：

• src/CardFlipActivity.java
• animator/card_flip_right_in.xml
• animator/card_flip_right_out.xml
• animator/card_flip_left_in.xml
• animator/card_flip_left_out.xml
• layout/fragment_card_back.xml
• layout/fragment_card_front.xml

---

一）. 创建动画执行器

要创建翻牌动画，在“卡片”移出或者移入时，你需要两个动画执行器（Animator）。

card_flip_left_in.xml

<set xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Before rotating, immediately set the alpha to 0. -->
    <objectAnimator
        android:valueFrom="1.0"
        android:valueTo="0.0"
        android:propertyName="alpha"
        android:duration="0" />

    <!-- Rotate. -->
    <objectAnimator
        android:valueFrom="-180"
        android:valueTo="0"
        android:propertyName="rotationY"
        android:interpolator="@android:interpolator/accelerate_decelerate"
        android:duration="@integer/card_flip_time_full" />

    <!-- Half-way through the rotation (see startOffset), set the alpha to 1. -->
    <objectAnimator
        android:valueFrom="0.0"
        android:valueTo="1.0"
        android:propertyName="alpha"
        android:startOffset="@integer/card_flip_time_half"
        android:duration="1" />
</set>

card_flip_left_out.xml

<set xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Rotate. -->
    <objectAnimator
        android:valueFrom="0"
        android:valueTo="180"
        android:propertyName="rotationY"
        android:interpolator="@android:interpolator/accelerate_decelerate"
        android:duration="@integer/card_flip_time_full" />

    <!-- Half-way through the rotation (see startOffset), set the alpha to 0. -->
    <objectAnimator
        android:valueFrom="1.0"
        android:valueTo="0.0"
        android:propertyName="alpha"
        android:startOffset="@integer/card_flip_time_half"
        android:duration="1" />
</set>

card_flip_right_in.xml

<set xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Before rotating, immediately set the alpha to 0. -->
    <objectAnimator
        android:valueFrom="1.0"
        android:valueTo="0.0"
        android:propertyName="alpha"
        android:duration="0" />

    <!-- Rotate. -->
    <objectAnimator
        android:valueFrom="180"
        android:valueTo="0"
        android:propertyName="rotationY"
        android:interpolator="@android:interpolator/accelerate_decelerate"
        android:duration="@integer/card_flip_time_full" />

    <!-- Half-way through the rotation (see startOffset), set the alpha to 1. -->
    <objectAnimator
        android:valueFrom="0.0"
        android:valueTo="1.0"
        android:propertyName="alpha"
        android:startOffset="@integer/card_flip_time_half"
        android:duration="1" />

card_flip_right_out.xml

<set xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Rotate. -->
    <objectAnimator
        android:valueFrom="0"
        android:valueTo="-180"
        android:propertyName="rotationY"
        android:interpolator="@android:interpolator/accelerate_decelerate"
        android:duration="@integer/card_flip_time_full" />

    <!-- Half-way through the rotation (see startOffset), set the alpha to 0. -->
    <objectAnimator
        android:valueFrom="1.0"
        android:valueTo="0.0"
        android:propertyName="alpha"
        android:startOffset="@integer/card_flip_time_half"
        android:duration="1" />
</set>

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二）. 创建视图

“卡片”的每一面是任何你希望的相互独立的内容，比如两屏的文字，两幅图片，或者任何视图的组合。接下来你就需要后面需要增加动画的fragment的两个布局。下面卡片其中一面的布局：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#a6c"
    android:padding="16dp"
    android:gravity="bottom">

    <TextView android:id="@android:id/text1"
        style="?android:textAppearanceLarge"
        android:textStyle="bold"
        android:textColor="#fff"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/card_back_title" />

    <TextView style="?android:textAppearanceSmall"
        android:textAllCaps="true"
        android:textColor="#80ffffff"
        android:textStyle="bold"
        android:lineSpacingMultiplier="1.2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/card_back_description" />

</LinearLayout>

卡牌的另一面显示一幅图片：

<ImageView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:src="@drawable/image1"
    android:scaleType="centerCrop"
    android:contentDescription="@string/description_image_1" />

---

三）. 创建Fragment

为卡片的正反面创建fragment。这些类返回你之前在每个fragment中的onCreateView()方法中创建的布局。你可以在这些fragment的父activity（你期望显示卡牌的地方）创建这些fragment的实例。下面的代码展示了一个包含有fragment内部类的activity类：

public class CardFlipActivity extends Activity {
    ...
    /**
     * A fragment representing the front of the card.
     */
    public class CardFrontFragment extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_card_front, container, false);
        }
    }

    /**
     * A fragment representing the back of the card.
     */
    public class CardBackFragment extends Fragment {
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_card_back, container, false);
        }
    }
}

---

四）. 卡片翻转动画

现在，你需要展示一个父activity中的fragment。要这么做，首先为你的activity创建布局。下面的例子创建了一个FrameLayout，你可以在运行时添加fragment。

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

在activity的代码中，将视图设置为你刚才创建的布局。也可以展示一个创建activity时默认的fragment，下面的代码展示了如何显示默认的卡牌的正面：

public class CardFlipActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_activity_card_flip);

        if (savedInstanceState == null) {
            getFragmentManager()
                    .beginTransaction()
                    .add(R.id.container, new CardFrontFragment())
                    .commit();
        }
    }
    ...
}

现在你已经有了卡牌的正面，你可以在某个时间通过卡牌反面的动画显示卡牌的背面。创建一个方法来显示卡牌的另一面，它做如下几件事情：

设置你之前为fragment切换所创建的自定义动画。

用新的fragment替换当前显示的fragment，并用你创建的自定义动画来执行这次动画切换。

将之前显示的fragment添加到回退栈中，所以当用户按下返回键时，卡片会翻转回来。

private void flipCard() {
    if (mShowingBack) {
        getFragmentManager().popBackStack();
        return;
    }

    // Flip to the back.

    mShowingBack = true;

    // Create and commit a new fragment transaction that adds the fragment for the back of
    // the card, uses custom animations, and is part of the fragment manager's back stack.

    getFragmentManager()
            .beginTransaction()

            // Replace the default fragment animations with animator resources representing
            // rotations when switching to the back of the card, as well as animator
            // resources representing rotations when flipping back to the front (e.g. when
            // the system Back button is pressed).
            .setCustomAnimations(
                    R.animator.card_flip_right_in, R.animator.card_flip_right_out,
                    R.animator.card_flip_left_in, R.animator.card_flip_left_out)

            // Replace any fragments currently in the container view with a fragment
            // representing the next page (indicated by the just-incremented currentPage
            // variable).
            .replace(R.id.container, new CardBackFragment())

            // Add this transaction to the back stack, allowing users to press Back
            // to get to the front of the card.
            .addToBackStack(null)

            // Commit the transaction.
            .commit();
}