Android 动画分三大类
同时,复杂程度,也是从简到繁,由易到难,今天先说帧动画。
帧动画的使用方式网络上很多很多,也很简单,就不多扯淡,主要说下原理。
ImageView iv_anima = (ImageView) findViewById(R.id.iv);
AnimationDrawable ad =(AnimationDrawable) iv_anima.getBackground();
ad.start();
在使用帧动画的时候,会给 ImageView 设置 backgroud,并将backgroud 的drawable 对象 转化为AnimationDrawable 对象,然后用此对象去启动动画。那么问题来了,我们去看看这个牛逼的类
AnimationDrawable :
AnimationDrawable 是创建帧动画的,定义一系列的drawable对象,并且以XML文件的形式放到res/drawable/folder文件夹下,同时将此drawable对象设置到view的backgroud属性.
AnimationDrawable重要属性
AnimationDrawable如何将对象转换为动画的
/**
* Create a drawable from an XML document. For more information on how to
* create resources in XML, see
* "{@docRoot}guide/topics/resources/drawable-resource.html">Drawable Resources</a>.
*/
public static Drawable createFromXml(Resources r, XmlPullParser parser)
throws XmlPullParserException, IOException {
return createFromXml(r, parser, null);
}
由上面的源码得知,createFromXml()方法的使用,将xml文件转化了,转换为什么?接着看源码
public static Drawable createFromXml(Resources r, XmlPullParser parser, Theme theme)
throws XmlPullParserException, IOException {
AttributeSet attrs = Xml.asAttributeSet(parser);
int type;
while ((type=parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty loop
}
if (type != XmlPullParser.START_TAG) {
throw new XmlPullParserException("No start tag found");
}
Drawable drawable = createFromXmlInner(r, parser, attrs, theme);
if (drawable == null) {
throw new RuntimeException("Unknown initial tag: " + parser.getName());
}
return drawable;
}
public static Drawable createFromXmlInner(Resources r, XmlPullParser parser, AttributeSet attrs,
Theme theme) throws XmlPullParserException, IOException {
final Drawable drawable;
final String name = parser.getName();
switch (name) {
case "selector":
drawable = new StateListDrawable();
break;
case "animated-selector":
drawable = new AnimatedStateListDrawable();
break;
case "level-list":
drawable = new LevelListDrawable();
break;
case "layer-list":
drawable = new LayerDrawable();
break;
case "transition":
drawable = new TransitionDrawable();
break;
case "ripple":
drawable = new RippleDrawable();
break;
case "color":
drawable = new ColorDrawable();
break;
case "shape":
drawable = new GradientDrawable();
break;
case "vector":
drawable = new VectorDrawable();
break;
case "animated-vector":
drawable = new AnimatedVectorDrawable();
break;
case "scale":
drawable = new ScaleDrawable();
break;
case "clip":
drawable = new ClipDrawable();
break;
case "rotate":
drawable = new RotateDrawable();
break;
case "animated-rotate":
drawable = new AnimatedRotateDrawable();
break;
case "animation-list":
drawable = new AnimationDrawable();
break;
case "inset":
drawable = new InsetDrawable();
break;
case "bitmap":
drawable = new BitmapDrawable();
break;
case "nine-patch":
drawable = new NinePatchDrawable();
break;
default:
throw new XmlPullParserException(parser.getPositionDescription() +
": invalid drawable tag " + name);
}
drawable.inflate(r, parser, attrs, theme);
return drawable;
}
很明显,直接将xml文件经过XmlPullParser解析转换成了drawable对象,createFromXmlInner()方法中,根据不同的标签名称去生成不同的drawable,最后去调用inflate方法。
到此,我们已经基本理清了从XML文件到Drawable对象的转换流程。对帧动画的工作原理有了一个小了解,这样,也许会让我们对帧动画的使用更加的熟练。那么问题来了,帧动画animation-list节点下的节点是如何形成一个个帧动画效果的?我们继续往下看:
private final static class AnimationState extends DrawableContainerState
private void inflateChildElements(Resources r, XmlPullParser parser, AttributeSet attrs,
Theme theme) throws XmlPullParserException, IOException {
int type;
final int innerDepth = parser.getDepth()+1;
int depth;
while ((type=parser.next()) != XmlPullParser.END_DOCUMENT
&& ((depth = parser.getDepth()) >= innerDepth || type != XmlPullParser.END_TAG)) {
if (type != XmlPullParser.START_TAG) {
continue;
}
if (depth > innerDepth || !parser.getName().equals("item")) {
continue;
}
final TypedArray a = obtainAttributes(r, theme, attrs,
R.styleable.AnimationDrawableItem);
final int duration = a.getInt(R.styleable.AnimationDrawableItem_duration, -1);
if (duration < 0) {
throw new XmlPullParserException(parser.getPositionDescription()
+ ": - tag requires a 'duration' attribute"
);
}
Drawable dr = a.getDrawable(R.styleable.AnimationDrawableItem_drawable);
a.recycle();
if (dr == null) {
while ((type=parser.next()) == XmlPullParser.TEXT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new XmlPullParserException(parser.getPositionDescription()
+ ": - tag requires a 'drawable' attribute or child tag"
+ " defining a drawable");
}
dr = Drawable.createFromXmlInner(r, parser, attrs, theme);
}
mAnimationState.addFrame(dr, duration);
if (dr != null) {
dr.setCallback(this);
}
}
}
上面的代码中我们可以看出, AnimationState 是继承 DrawableContainerState 的,在DrawableContainerState这个类中有一个成员变量Drawable[] mDrawables;用于保存drawable信息。
在 inflateChildElements 这方法里面通过TypeArray获取drawable的信息,然后调用AnimationState的addFrame方法,将一系列动画信息就存储在drawable数组中。
public void start() {
mAnimating = true;
if (!isRunning()) {
// Start from 0th frame.
setFrame(0, false, mAnimationState.getChildCount() > 1
|| !mAnimationState.mOneShot);
}
}
通过setFrame方法设置我们的drawable。接着就可以进行drawable的展示了。
帧动画的大概原理就是这样,个人见解,希望能帮到其他人。