布局动画：LayoutTransition
1、LayoutTransition介绍
Layouttransition位于android.animation包中，Google从API 11时引入这个Layouttransition工具类，专门用来处理ViewGroup布局发生变化时对子控件进行动画处理。
比如一组数据在ViewGroup中显示，这个时候如果添加或者删除其中的数据，同时伴随着每个item的动画，这样能够提高用户的体验，但是开发者实现起来有一点复杂(也可以实现)。
使用Layouttransition类就可以方便的对ViewGroup中的所有View item实现统一的动效，看了源码就会一目了然，是在内部对常见的5个Type动画情况进行封装：内部5个Type、5个Animator、5个ObjectAnimator分别处理不同的ViewGroup item动画：
public class LayoutTransition {
...
/**
* Private bit fields used to set the collection of enabled transition types for
* mTransitionTypes.
*/
private static final int FLAG_APPEARING = 0x01;
private static final int FLAG_DISAPPEARING = 0x02;
private static final int FLAG_CHANGE_APPEARING = 0x04;
private static final int FLAG_CHANGE_DISAPPEARING = 0x08;
private static final int FLAG_CHANGING = 0x10;
/**
* These variables hold the animations that are currently used to run the transition effects.
* These animations are set to defaults, but can be changed to custom animations by
* calls to setAnimator().
*/
private Animator mDisappearingAnim = null;
private Animator mAppearingAnim = null;
private Animator mChangingAppearingAnim = null;
private Animator mChangingDisappearingAnim = null;
private Animator mChangingAnim = null;
/**
* These are the default animations, defined in the constructor, that will be used
* unless the user specifies custom animations.
*/
private static ObjectAnimator defaultChange;
private static ObjectAnimator defaultChangeIn;
private static ObjectAnimator defaultChangeOut;
private static ObjectAnimator defaultFadeIn;
private static ObjectAnimator defaultFadeOut;
/**
* 所有动画使用的默认持续时间300ms.
*/
private static long DEFAULT_DURATION = 300;
...
}2、LayoutTransition API
Layouttransition内部管理5个ObjectAnimator分别处理5种Type的动画，所以大部分方法都需要传入Type，内部用switch语句到具体的ObjectAnimator对象进行操作。
2.1、常量
将LayoutTransition应用于ViewGroup的哪些场景，有如下5种常量分别对应5种情况：
/**
* 由于ViewGroup添加新的元素，其它已有的元素需要应用动画
*/
public static final int CHANGE_APPEARING = 0;
/**
* ViewGroup中移除元素，其它元素需要发生变化应用动画
*/
public static final int CHANGE_DISAPPEARING = 1;
/**
* 元素在容器中出现时所定义的动画
*/
public static final int APPEARING = 2;
/**
* 元素在容器中消失所定义的动画
*/
public static final int DISAPPEARING = 3;
/**
* 由于布局发生变化时所应用的动画(不是由元素的添加或删除所引起)
* 默认关闭，调用enableTransitionType(int)开启
*/
public static final int CHANGING = 4;2.2、方法
①添加、删除Child View方法
addChild(ViewGroup parent, View child, boolean changesLayout)
showChild(ViewGroup parent, View child, int oldVisibility)
removeChild(ViewGroup parent, View child, boolean changesLayout)
hideChild(ViewGroup parent, View child, int newVisibility)②添加/移除LayoutTransition监听器TransitionListener
addTransitionListener(TransitionListener listener)
removeTransitionListener(TransitionListener listener)③启用/判断这5种ViewGroup动画有效
disableTransitionType(int transitionType)
isTransitionTypeEnabled(int transitionType)④setter和getter方法
//此标志控制CHANGE_APPEARING动画或CHANGE_DISAPPEARING动画是否也会导致默认的更改动画也在父层次结构上运行
setAnimateParentHierarchy(boolean animateParentHierarchy)
setAnimator(int transitionType, Animator animator)
setDuration(int transitionType, long duration)
setDuration(long duration)
setInterpolator(int transitionType, TimeInterpolator interpolator)
setStagger(int transitionType, long duration)
//针对每个Type设置每个子item的动画间隔
setStartDelay(int transitionType, long delay)
getAnimator(int transitionType)
getDuration(int transitionType)
getInterpolator(int transitionType)
//获取Type动画子item的动画时间间隔
getStagger(int transitionType)
getStartDelay(int transitionType)3、默认LayoutTransition动画
LayoutTransition默认提供了一种ViewGroup动画实现。
3.1、XML属性：android:animateLayoutChanges
只需要在布局中的ViewGroup节点下加上android:animateLayoutChanges="true"即可实现ViewGroup组件添加删除View显示默认的动画效果。
<GridLayout
android:id="@+id/list"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:animateLayoutChanges="false"
android:background="@android:color/white"
android:columnCount="3" />3.2、代码添加动画
创建LayoutTransition实例，并调用ViewGroup的setLayoutTransition(LayoutTransition transition)方法设置给ViewGroup。
LayoutTransition layoutTransition = new LayoutTransition();
mViewGroup.setLayoutTransition(layoutTransition);默认情况下ViewGroup中的mTransition为null，所以添加和删除时是不会有任何动画效果的。setLayoutTransition(LayoutTransition transition)方法定义如下：
/ **
* 为此ViewGroup设置LayoutTransition对象。
* 如果LayoutTransition对象不为null，则将根据该LayoutTransition对象中定义的动画
* 对由于将孩子添加到ViewGroup或从ViewGroup中删除而引起的布局更改进行动画处理。
* 默认情况下，过渡对象为null（因此不会对布局更改进行动画处理）。
* 替换非null过渡将取消先前的过渡（如果当前正在运行）。
*
* @param transition LayoutTransition对象将对布局中的更改进行动画处理。值为null表示不会在布局更改上进行任何过渡。
* @attr 对应ViewGroup的XML布局属性animateLayoutChanges
* /
public void setLayoutTransition(LayoutTransition transition) {
if (mTransition != null) {
LayoutTransition previousTransition = mTransition;
previousTransition.cancel();
previousTransition.removeTransitionListener(mLayoutTransitionListener);
}
mTransition = transition;
if (mTransition != null) {
mTransition.addTransitionListener(mLayoutTransitionListener);
}
}
3.3、示例
演示代码如下，给ViewGroup添加子View：
mAddBtn.setOnClickListener((v) -> {
Button button = new Button(this/*, null, R.style.ButtonStyle*/);
button.setBackgroundResource(R.drawable.button_bg);
button.setText("按钮 " + (count++));
mViewGroup.addView(button, 0);
});
从ViewGroup中删除子View，注意还要将count计数器同时减一
mRemoveBtn.setOnClickListener((v) -> {
if (mViewGroup.getChildCount() > 0) {
mViewGroup.removeViewAt(0);
--count;
}
});
4、自定义LayoutTransition动画
上面是默认的动画效果，当然开发者也可以自己实现具体的动效让应用与众不同。主要借助PropertyValuesHolder构造ObjectAnimator。
4.1、自定义动画说明
LayoutTransition坑点很多，和ObjectAnimator直接用又有所不一样，看源码设置动画的setAnimator(int transitionType, Animator animator)方法注释说明：
/**
* Sets the animation used during one of the transition types that may run. Any
* Animator object can be used, but to be most useful in the context of layout
* transitions, the animation should either be a ObjectAnimator or a AnimatorSet
* of animations including PropertyAnimators. Also, these ObjectAnimator objects
* should be able to get and set values on their target objects automatically. For
* example, a ObjectAnimator that animates the property "left" is able to set and get the
* <code>left</code> property from the View objects being animated by the layout
* transition. The transition works by setting target objects and properties
* dynamically, according to the pre- and post-layoout values of those objects, so
* having animations that can handle those properties appropriately will work best
* for custom animation. The dynamic setting of values is only the case for the
* CHANGE animations; the APPEARING and DISAPPEARING animations are simply run with
* the values they have.
*
* <p>It is also worth noting that any and all animations (and their underlying
* PropertyValuesHolder objects) will have their start and end values set according
* to the pre- and post-layout values. So, for example, a custom animation on "alpha"
* as the CHANGE_APPEARING animation will inherit the real value of alpha on the target
* object (presumably 1) as its starting and ending value when the animation begins.
* Animations which need to use values at the beginning and end that may not match the
* values queried when the transition begins may need to use a different mechanism
* than a standard ObjectAnimator object.</p>
*
* @param transitionType One of {@link #CHANGE_APPEARING}, {@link #CHANGE_DISAPPEARING},
* {@link #CHANGING}, {@link #APPEARING}, or {@link #DISAPPEARING}, which determines the
* animation whose animator is being set.
* @param animator The animation being assigned. A value of <code>null</code> means that no
* animation will be run for the specified transitionType.
*/
public void setAnimator(int transitionType, Animator animator) {
switch (transitionType) {
case CHANGE_APPEARING:
mChangingAppearingAnim = animator;
break;
case CHANGE_DISAPPEARING:
mChangingDisappearingAnim = animator;
break;
case CHANGING:
mChangingAnim = animator;
break;
case APPEARING:
mAppearingAnim = animator;
break;
case DISAPPEARING:
mDisappearingAnim = animator;
break;
}
}总结为一下几点(参考《Android自定义控件开发入门与实战》)：
①LayoutTransition.CHANGE_APPEARING和LayoutTransition.CHANGE_DISAPPEARING必须使用PropertyValuesHolder构造动画才会有效果，其他任何方式构造动画都不会有效果。
②在使用PropertyValuesHolder时，”left”、”top”基础属性就算不需要变化也必须要写
PropertyValuesHolder leftValuesHolder = PropertyValuesHolder.ofInt("left", 0, 0);
PropertyValuesHolder topValuesHolder = PropertyValuesHolder.ofInt("top", 0, 0);③ofInt，ofFloat中的参数值，第一个值和最后一个值必须相同，不然此属性所对应的的动画将被放弃，在此属性值上将不会有效果。
//首尾值一样，中间插入变化值
PropertyValuesHolder rotationDisAppearing = PropertyValuesHolder.ofFloat("rotationY", 90f, 0f, 90f);④使用的ofInt，ofFloat中，如果所有参数值都相同，也将不会有动画效果。所以至少需要三个值，相同的首尾两个值，中间一个变化值。
//无效的动画设置，首尾值一样。
PropertyValuesHolder leftValuesHolder = PropertyValuesHolder.ofInt("left", 0, 0);4.2、构造ObjectAnimator
演示如何设置一个item移除DISAPPEARING时的动画。其它几种类型都是举一反三的。
①如果动画复杂，需要构造多个动画。先构造若干PropertyValuesHolder对象
PropertyValuesHolder rotationDisAppearing = PropertyValuesHolder.ofFloat("rotationY", 0f, 90f);
PropertyValuesHolder alphaDisappearing = PropertyValuesHolder.ofFloat("alpha", 1f, 0f);
PropertyValuesHolder colorDisappearing = PropertyValuesHolder.ofObject("BackgroundColor",
new ArgbEvaluator(),
Color.parseColor("#84bf96"),
Color.parseColor("#ed1941"));②使用PropertyValuesHolder对象构造ObjectAnimator实例，详见《属性动画：PropertyValuesHolder》
ObjectAnimator disappearingAnimator = ObjectAnimator.ofPropertyValuesHolder(mViewGroup,
leftValuesHolder, topValuesHolder, colorDisappearing, rotationDisAppearing, alphaDisappearing);③构造LayoutTransition对象
LayoutTransition layoutTransition = new LayoutTransition();④ObjectAnimator动画指定设置给ayoutTransition对象指定的Type变换类型，这里指定DISAPPEARING时进行动画。
layoutTransition.setAnimator(LayoutTransition.DISAPPEARING, disappearingAnimator);⑤还可以对LayoutTransition进行一些设置
layoutTransition.setDuration(1000);
layoutTransition.setDuration(LayoutTransition.DISAPPEARING,750);
layoutTransition.setInterpolator(LayoutTransition.DISAPPEARING, new AccelerateInterpolator());⑥最后将构造好的LayoutTransition设置给ViewGroup
mViewGroup.setLayoutTransition(layoutTransition);
相关资料：
Docs > Reference > LayoutTransition">Android Developers > Docs > Reference > LayoutTransition
Sample project for Android Animations
LayoutAnimation和LayoutTransition
