在应用的使用过程中，需要在屏幕上显示新信息，同时移除旧信息。立即切换显示的内容看起来有些突兀，或者导致用户很容易错过屏幕上的新内容。利用动画可以减慢更改的速度，并通过概念吸引用户的注意，以使更新更加明显。
在显示或隐藏视图时，有三种常见的动画可供使用。您可以使用圆形揭露动画、淡入淡出动画或卡片翻转动画。
创建淡入淡出动画
淡入淡出动画（也称为“叠化”）逐渐淡出一个 View 或 ViewGroup，同时淡入另一个。此动画适用于您希望在应用中切换内容或视图的情况。此处显示的淡入淡出动画使用 ViewPropertyAnimator，适用于 Android 3.1（API 级别 12）及更高版本。
以下是从进度指示器切换到某些文字内容的淡入淡出示例。
淡入淡出动画
创建视图
首先，您需要创建两个要淡入淡出的视图。以下示例创建了一个进度指示器和一个可滚动的文本视图：
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/content"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView style="?android:textAppearanceMedium"
android:lineSpacingMultiplier="1.2"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/lorem_ipsum"
android:padding="16dp" />
</ScrollView>
<ProgressBar android:id="@+id/loading_spinner"
style="?android:progressBarStyleLarge"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center" />
</FrameLayout>
设置淡入淡出动画
如需设置淡入淡出动画，请执行以下操作：
- 为想要淡入淡出的视图创建成员变量。之后在显示动画期间修改视图时，您需要引用这些变量。
- 对于淡入的视图，请将其可见性设置为
GONE。 这样可以避免视图占用布局空间，并在进行布局计算时省去对视图的计算，从而加快处理速度。 - 将
config_shortAnimTimeconfig_longAnimTime和config_mediumAnimTime。
下面的示例使用前一个代码段中的布局作为 Activity 内容视图：
Kotlin
class CrossfadeActivity : Activity() {
private lateinit var contentView: View
private lateinit var loadingView: View
private var shortAnimationDuration: Int = 0
...
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_crossfade)
contentView = findViewById(R.id.content)
loadingView = findViewById(R.id.loading_spinner)
// Initially hide the content view.
contentView.visibility = View.GONE
// Retrieve and cache the system's default "short" animation time.
shortAnimationDuration = resources.getInteger(android.R.integer.config_shortAnimTime)
}
...
}
Java
public class CrossfadeActivity extends Activity {
private View contentView;
private View loadingView;
private int shortAnimationDuration;
...
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_crossfade);
contentView = findViewById(R.id.content);
loadingView = findViewById(R.id.loading_spinner);
// Initially hide the content view.
contentView.setVisibility(View.GONE);
// Retrieve and cache the system's default "short" animation time.
shortAnimationDuration = getResources().getInteger(
android.R.integer.config_shortAnimTime);
}
...
}
淡入淡出视图
现在视图已正确设置，可通过执行以下操作来实现淡入淡出：
- 对于淡入的视图，请将 Alpha 值设置为
0，将可见性设置为VISIBLE。（请记住，初始设置为GONE。）这会使视图可见但完全透明。 - 对于淡入的视图，通过动画将其 Alpha 值从
0过渡到1。对于淡出的视图，通过动画将其 Alpha 值从1过渡到0。 - 在
Animator.AnimatorListener中使用onAnimationEnd()，将淡出视图的可见性设置为GONE。即使 Alpha 值为0，将视图的可见性设置为GONE也会阻止视图占用布局空间，并在进行布局计算时省去对视图的计算，从而加快处理速度。
以下方法通过示例说明了如何执行此操作：
Kotlin
class CrossfadeActivity : Activity() {
private lateinit var contentView: View
private lateinit var loadingView: View
private var shortAnimationDuration: Int = 0
...
private fun crossfade() {
contentView.apply {
// Set the content view to 0% opacity but visible, so that it is visible
// (but fully transparent) during the animation.
alpha = 0f
visibility = View.VISIBLE
// Animate the content view to 100% opacity, and clear any animation
// listener set on the view.
animate()
.alpha(1f)
.setDuration(shortAnimationDuration.toLong())
.setListener(null)
}
// Animate the loading view to 0% opacity. After the animation ends,
// set its visibility to GONE as an optimization step (it won't
// participate in layout passes, etc.)
loadingView.animate()
.alpha(0f)
.setDuration(shortAnimationDuration.toLong())
.setListener(object : AnimatorListenerAdapter() {
override fun onAnimationEnd(animation: Animator) {
loadingView.visibility = View.GONE
}
})
}
}
Java
public class CrossfadeActivity extends Activity {
private View contentView;
private View loadingView;
private int shortAnimationDuration;
...
private void crossfade() {
// Set the content view to 0% opacity but visible, so that it is visible
// (but fully transparent) during the animation.
contentView.setAlpha(0f);
contentView.setVisibility(View.VISIBLE);
// Animate the content view to 100% opacity, and clear any animation
// listener set on the view.
contentView.animate()
.alpha(1f)
.setDuration(shortAnimationDuration)
.setListener(null);
// Animate the loading view to 0% opacity. After the animation ends,
// set its visibility to GONE as an optimization step (it won't
// participate in layout passes, etc.)
loadingView.animate()
.alpha(0f)
.setDuration(shortAnimationDuration)
.setListener(new AnimatorListenerAdapter() {
@Override
public void onAnimationEnd(Animator animation) {
loadingView.setVisibility(View.GONE);
}
});
}
}
创建卡片翻转动画
卡片翻转通过显示模拟卡片翻转的动画，在内容视图之间添加动画。此处显示的卡片翻转动画使用 FragmentTransaction，它在 Android 3.0（API 级别 11）及更高版本中可用。
卡片翻转的效果如下：
卡片翻转动画
创建 Animator 对象
要创建卡片翻转动画，总共需要四个 Animator。 两个 Animator 用于卡片正面向左侧淡出以及从左侧淡入的动画。您还需要两个 Animator，用于卡片背面从右侧淡入以及向右侧淡出的动画。
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" />
</set>
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>
创建视图
“卡片”的每一面都是一个独立的布局，可以包含您所需的任何内容，例如用于翻转切换的两个文本视图、两张图片或任意视图组合。然后，您将在 Fragment 中使用两个布局供您稍后添加动画。以下布局会创建显示文本的卡片一面：
<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>
卡片 ImageView 的另一面显示：
<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" />
创建 Fragment
为卡片的正面和背面创建 Fragment 类。这些类将返回您之前在每个 Fragment 的 onCreateView() 方法中创建的布局。然后，您可以在要显示该卡片的父 Activity 中创建此 Fragment 的实例。以下示例展示了父 Activity 内使用的嵌套 Fragment 类：
Kotlin
class CardFlipActivity : FragmentActivity() {
...
/**
* A fragment representing the front of the card.
*/
class CardFrontFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View = inflater.inflate(R.layout.fragment_card_front, container, false)
}
/**
* A fragment representing the back of the card.
*/
class CardBackFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View = inflater.inflate(R.layout.fragment_card_back, container, false)
}
}
Java
public class CardFlipActivity extends FragmentActivity {
...
/**
* 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);
}
}
}
为卡片翻转添加动画
现在，您需要显示父 Activity 内的 Fragment。为此，请先为您的 Activity 创建布局。以下示例创建了一个 FrameLayout，您可以在运行时向其中添加 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 代码中，将内容视图设置为您刚刚创建的布局。也可以在创建 Activity 时显示默认 Fragment，以下示例 Activity 向您展示了如何默认显示卡片的正面：
Kotlin
class CardFlipActivity : FragmentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_activity_card_flip)
if (savedInstanceState == null) {
supportFragmentManager.beginTransaction()
.add(R.id.container, CardFrontFragment())
.commit()
}
}
...
}
Java
public class CardFlipActivity extends FragmentActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_activity_card_flip);
if (savedInstanceState == null) {
getSupportFragmentManager()
.beginTransaction()
.add(R.id.container, new CardFrontFragment())
.commit();
}
}
...
}
现在显示的是卡片正面，您可以在适当的时间通过翻转动画显示卡片背面。创建一种方法来显示卡片的另一面，该方法需要完成以下操作：
- 设置您之前为 Fragment 转换创建的自定义动画。
- 使用新 Fragment 替换当前显示的 Fragment，并使用您创建的自定义动画为该事件添加动画。
- 将之前显示的 Fragment 添加到 Fragment 返回堆栈中，以便当用户按下“返回”按钮时，卡片会翻转回来。
Kotlin
class CardFlipActivity : FragmentActivity() {
...
private fun flipCard() {
if (showingBack) {
supportFragmentManager.popBackStack()
return
}
// Flip to the back.
showingBack = 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.
supportFragmentManager.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, 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()
}
}
Java
public class CardFlipActivity extends FragmentActivity {
...
private void flipCard() {
if (showingBack) {
getSupportFragmentManager().popBackStack();
return;
}
// Flip to the back.
showingBack = 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.
getSupportFragmentManager()
.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();
}
}
创建圆形揭露动画
当您显示或隐藏一组界面元素时，揭露动画可为用户提供视觉连续性。ViewAnimationUtils.createCircularReveal() 方法让您能够为裁剪圆形添加动画以揭露或隐藏视图。此动画在 ViewAnimationUtils 类中提供，适用于 Android 5.0（API 级别 21）及更高版本。
以下示例展示了如何揭露之前不可见的视图：
Kotlin
// previously invisible view
val myView: View = findViewById(R.id.my_view)
// Check if the runtime version is at least Lollipop
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
// get the center for the clipping circle
val cx = myView.width / 2
val cy = myView.height / 2
// get the final radius for the clipping circle
val finalRadius = Math.hypot(cx.toDouble(), cy.toDouble()).toFloat()
// create the animator for this view (the start radius is zero)
val anim = ViewAnimationUtils.createCircularReveal(myView, cx, cy, 0f, finalRadius)
// make the view visible and start the animation
myView.visibility = View.VISIBLE
anim.start()
} else {
// set the view to invisible without a circular reveal animation below Lollipop
myView.visibility = View.INVISIBLE
}
Java
// previously invisible view
View myView = findViewById(R.id.my_view);
// Check if the runtime version is at least Lollipop
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
// get the center for the clipping circle
int cx = myView.getWidth() / 2;
int cy = myView.getHeight() / 2;
// get the final radius for the clipping circle
float finalRadius = (float) Math.hypot(cx, cy);
// create the animator for this view (the start radius is zero)
Animator anim = ViewAnimationUtils.createCircularReveal(myView, cx, cy, 0f, finalRadius);
// make the view visible and start the animation
myView.setVisibility(View.VISIBLE);
anim.start();
} else {
// set the view to invisible without a circular reveal animation below Lollipop
myView.setVisibility(View.INVISIBLE);
}
ViewAnimationUtils.createCircularReveal() 动画采用五个参数。第一个参数是要在屏幕上隐藏或显示的视图。接下来的两个参数是裁剪圆形圆心的 x 和 y 坐标。通常，这将成为视图的中心，但您也可以使用用户轻触的点作为中心，以便动画从用户选择的位置开始。第四个参数是剪裁圆形的起始半径。
在上述示例中，初始半径设置为 0，因此圆形会隐藏要显示的视图。最后一个参数是圆形的最终半径。在显示视图时，请确保最终半径大于视图本身，以便在动画播放完毕之前完全显示视图。
隐藏之前可见的视图：
Kotlin
// previously visible view
val myView: View = findViewById(R.id.my_view)
// Check if the runtime version is at least Lollipop
if (Build.VERSION.SDK_INT == Build.VERSION_CODES.LOLLIPOP) {
// get the center for the clipping circle
val cx = myView.width / 2
val cy = myView.height / 2
// get the initial radius for the clipping circle
val initialRadius = Math.hypot(cx.toDouble(), cy.toDouble()).toFloat()
// create the animation (the final radius is zero)
val anim = ViewAnimationUtils.createCircularReveal(myView, cx, cy, initialRadius, 0f)
// make the view invisible when the animation is done
anim.addListener(object : AnimatorListenerAdapter() {
override fun onAnimationEnd(animation: Animator) {
super.onAnimationEnd(animation)
myView.visibility = View.INVISIBLE
}
})
// start the animation
anim.start()
} else {
// set the view to visible without a circular reveal animation below Lollipop
myView.visibility = View.VISIBLE
}
Java
// previously visible view
final View myView = findViewById(R.id.my_view);
// Check if the runtime version is at least Lollipop
if (Build.VERSION.SDK_INT == Build.VERSION_CODES.LOLLIPOP) {
// get the center for the clipping circle
int cx = myView.getWidth() / 2;
int cy = myView.getHeight() / 2;
// get the initial radius for the clipping circle
float initialRadius = (float) Math.hypot(cx, cy);
// create the animation (the final radius is zero)
Animator anim = ViewAnimationUtils.createCircularReveal(myView, cx, cy, initialRadius, 0f);
// make the view invisible when the animation is done
anim.addListener(new AnimatorListenerAdapter() {
@Override
public void onAnimationEnd(Animator animation) {
super.onAnimationEnd(animation);
myView.setVisibility(View.INVISIBLE);
}
});
// start the animation
anim.start();
} else {
// set the view to visible without a circular reveal animation below Lollipop
myView.setVisibility(View.VISIBLE);
}
在本例中，裁剪圆形的初始半径设置为与视图一样大，因此视图在动画开始播放前可见。最终半径设置为 0，因此视图在动画播放完毕后将被隐藏。一定要在动画中添加监听器，这样可在动画播放完毕时将视图的可见性设置为 INVISIBLE。