View动画：平移、缩放、旋转、透明度;
对应的在xml中的标签为：traslate、scale、rotate、alpha;
四种变换效果对应Animation的四个子类：TranslateAnimation、ScaleAnimation、RotateAnimation、AlphaAnimation.
描述view动画的xml语法如下所示：
<set xmlns:android="http://schemas.android.com/apk/res/android"
android:shareInterpolator="true"
android:fillAfter="true">
<translate android:fromXDelta="0%p"
android:fromYDelta="0%p"
android:toXDelta="80%p"
android:toYDelta="80%p"/>
<scale android:fromXScale="1"
android:fromYScale="1"
android:toXScale="0.2"
android:toYScale="0.2"
/>
<rotate android:fromDegrees="0"
android:toDegrees="360"
/>
<alpha android:fromAlpha="1"
android:toAlpha="0.5"
/>
</set>
其中，<set>标签标示动画的集合，对应于AnimationSet类，其中interpalator属性表示动画所采用的插值器，默认为@android:anim/accelerate_decelerate_interpolator,先加速，后减速的效果。shareInterpolator属性表示集合中的动画是否都共享同一个插值器。
对于xml中各种数值的单位，比如80%指相对于自身的80%，该数值实际上代表了两种属性，一个pivotType和pivotValue,80%在java代码中对应于pivotType = Animation.RELATIVE_TO_SELF,
pivotValue = 0.8f。另外一种是相当于父布局，比如80%p,此时，pivot*Type = Animation.RELATIVE_TO_PARENT。
Java代码如下：
AnimationSet animationSet = new AnimationSet(true);
ScaleAnimation scaleAnimation = new ScaleAnimation(0.1f,1f,0.1f,1f,
Animation.RELATIVE_TO_SELF,0.5f,Animation.RELATIVE_TO_SELF,0.5f);
TranslateAnimation translateAnimation = new TranslateAnimation(Animation.RELATIVE_TO_PARENT,0f,Animation.RELATIVE_TO_PARENT,0.5f,
Animation.RELATIVE_TO_PARENT,0f,Animation.RELATIVE_TO_PARENT,0.5f);
RotateAnimation rotateAnimation = new RotateAnimation(0,360,Animation.RELATIVE_TO_SELF,0.5f,Animation.RELATIVE_TO_SELF,0.5f);
AlphaAnimation alphaAnimation = new AlphaAnimation(0.3f,1f);
animationSet.addAnimation(scaleAnimation);
animationSet.addAnimation(rotateAnimation);
animationSet.addAnimation(alphaAnimation);
animationSet.addAnimation(translateAnimation);
animationSet.setFillAfter(true);
scaleAnimation.setDuration(3000);
translateAnimation.setDuration(3000);
alphaAnimation.setDuration(3000);
rotateAnimation.setDuration(500);
rotateAnimation.setRepeatCount(6);
mShowAnimationTv.startAnimation(animationSet);
上面的代码是将左上角的textView移到右下角，并且伴随着旋转，透明度，缩放的动画（代码实现时要注意set集合add时的顺序）。
实现原理
首先，从动画开始的地方AnimatonUtils.loadAnimaiton()方法开始，源码如下：
public static Animation loadAnimation(Context context, @AnimRes int id)
throws NotFoundException {
XmlResourceParser parser = null;
try {
parser = context.getResources().getAnimation(id);
return createAnimationFromXml(context, parser);
} catch (XmlPullParserException ex) {
NotFoundException rnf = new NotFoundException("Can't load animation resource ID #0x" +
Integer.toHexString(id));
rnf.initCause(ex);
throw rnf;
} catch (IOException ex) {
NotFoundException rnf = new NotFoundException("Can't load animation resource ID #0x" +
Integer.toHexString(id));
rnf.initCause(ex);
throw rnf;
} finally {
if (parser != null) parser.close();
}
}
最后会调用createAnimationFromXml(context, parser),此处使用了XmlResourceParse。
private static Animation createAnimationFromXml(Context c, XmlPullParser parser)
throws XmlPullParserException, IOException {
return createAnimationFromXml(c, parser, null, Xml.asAttributeSet(parser));
}
private static Animation createAnimationFromXml(Context c, XmlPullParser parser,
AnimationSet parent, AttributeSet attrs) throws XmlPullParserException, IOException {
Animation anim = null;
// Make sure we are on a start tag.
int type;
int depth = parser.getDepth();
while (((type=parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > depth)
&& type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
String name = parser.getName();
if (name.equals("set")) {
anim = new AnimationSet(c, attrs);
createAnimationFromXml(c, parser, (AnimationSet)anim, attrs);
} else if (name.equals("alpha")) {
anim = new AlphaAnimation(c, attrs);
} else if (name.equals("scale")) {
anim = new ScaleAnimation(c, attrs);
} else if (name.equals("rotate")) {
anim = new RotateAnimation(c, attrs);
} else if (name.equals("translate")) {
anim = new TranslateAnimation(c, attrs);
} else {
throw new RuntimeException("Unknown animation name: " + parser.getName());
}
if (parent != null) {
parent.addAnimation(anim);
}
}
return anim;
}
最终会调用XmlPullParser,基于事件驱动模型来解析xml文件中的动画，从代码中可以看出来，每一个节点都对应一个相应的View动画类，这样与实际写代码差不多，在Java代码中也是先创建出相应的view动画类，然后按照顺序添加到集合中。
