这是项目总结第三篇，前两篇分别为：
1. Android 项目总结(1)- 之弧形ViewPager 和弧形HeaderView
2 . Android项目总结（二）时间、数字选择器和省市区三级联动
今天为大家分享一个简单的登录背景动画，图片循环播放动画，具体效果是啥样子的呢？先上一张效果图：
login_alpha
login_alpha2.gif
一、需求
我们开发APP的时候，一般都有一个注册登录的入口页面，这个页面的呈现有很多种方式，如：
- 静态背景图 + 注册登录按钮
- 视频背景 + 注册登录按钮
- 背景动画 + 注册登录按钮
今天分享的就是第三种 ，背景动画，效果图如上所示，接下来就分析一下这个动画：
1 . 有 N 张图片切换（项目中用的4张）
2 . 图片切换过渡：当前图片放大并且淡出，下一张显示的图片淡入。
3 . 图片循环播放，显示到最后一张时又从第一张开始。
二、实现
上面对照效果图分析了动画的几个点，那么接下来就看怎么实现，我们选择用属性动画来实现，具体实现思路如下:
本例中有4张图片：A,B,C,D
有4组动画：
A->B
B->C
C->D
D->A
这样4组就实现了循环切换
然后就是每一组动画的实现，其实很简单，一个Scale 放大效果+ 一个 alpha 效果：
A -> B:
ObjectAnimator animator1 = ObjectAnimator.ofFloat(mBgView1, "alpha", 1.0f, 0f);
ObjectAnimator animator2 = ObjectAnimator.ofFloat(mBgView2, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale1 = ObjectAnimator.ofFloat(mBgView1, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale2 = ObjectAnimator.ofFloat(mBgView1, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet1 = new AnimatorSet();
animatorSet1.setDuration(5000);
animatorSet1.play(animator1).with(animator2).with(animatorScale1).with(animatorScale2);
B->C:
ObjectAnimator animator3 = ObjectAnimator.ofFloat(mBgView2, "alpha", 1.0f, 0f);
ObjectAnimator animator4 = ObjectAnimator.ofFloat(mBgView3, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale3 = ObjectAnimator.ofFloat(mBgView2, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale4 = ObjectAnimator.ofFloat(mBgView2, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet2 = new AnimatorSet();
animatorSet2.setDuration(5000);
animatorSet2.play(animator3).with(animator4).with(animatorScale3).with(animatorScale4);
C->D:
ObjectAnimator animator5 = ObjectAnimator.ofFloat(mBgView3, "alpha", 1.0f, 0f);
ObjectAnimator animator6 = ObjectAnimator.ofFloat(mBgView4, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale5 = ObjectAnimator.ofFloat(mBgView3, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale6 = ObjectAnimator.ofFloat(mBgView3, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet3 = new AnimatorSet();
animatorSet3.setDuration(5000);
animatorSet3.play(animator5).with(animator6).with(animatorScale5).with(animatorScale6);
D->A:
ObjectAnimator animator7 = ObjectAnimator.ofFloat(mBgView4, "alpha", 1.0f, 0f);
ObjectAnimator animator8 = ObjectAnimator.ofFloat(mBgView1, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale7 = ObjectAnimator.ofFloat(mBgView4, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale8 = ObjectAnimator.ofFloat(mBgView4, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet4 = new AnimatorSet();
animatorSet4.setDuration(5000);
animatorSet4.play(animator7).with(animator8).with(animatorScale7).with(animatorScale8);
上面的代码展示了每一组动画，将每组动画中的几个动画放在一个AnimatorSet 中，设置为同时播放。最后我们需要将这4组动画按照顺序链接起来，怎么链接呢？用AnimatorSet的playSequentially方法。如下：
AnimatorSet animatorSet = new AnimatorSet();
animatorSet.playSequentially(animatorSet1, animatorSet2, animatorSet3, animatorSet4);
animatorSet.addListener(new Animator.AnimatorListener() {
@Override
public void onAnimationStart(Animator animation) {
}
@Override
public void onAnimationEnd(Animator animation) {
// 这个是实现循环播放的关键
animation.start();
}
@Override
public void onAnimationCancel(Animator animation) {
}
@Override
public void onAnimationRepeat(Animator animation) {
}
});
animatorSet.start();
其中有一个关键点：在监听动画结束的回调方法中，调用animation.start(); 实现循环播放。
你以为到此这篇文章就结束了吗？ 当然还没有，上面的代码其实效果已经出来了，但是还是有点问题？什么问题呢？就是当播放完第一次，后面循环播放的时候会有一个跳动。 为什么呢？ 看看上面的代码就会发现，当执播放完一轮后，4张图片都放大了 1.3 倍数。
ObjectAnimator animatorScale1 = ObjectAnimator.ofFloat(mBgView1, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale2 = ObjectAnimator.ofFloat(mBgView1, "scaleY", 1.0f, 1.3f);
然后重复播放的时候，又会执行scale 动画，从 1.0 -> 1.3 ，因此实际上会先从 1.3 -> 1.0。再执行缩放动画，这就是跳动的原因，因此在播放完一轮后，我们要将放大的View 先复位到原大小，然后在执行动画。在onAnimationEnd 方法中复位。
最终代码如下：
ObjectAnimator animator1 = ObjectAnimator.ofFloat(mBgView1, "alpha", 1.0f, 0f);
ObjectAnimator animator2 = ObjectAnimator.ofFloat(mBgView2, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale1 = ObjectAnimator.ofFloat(mBgView1, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale2 = ObjectAnimator.ofFloat(mBgView1, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet1 = new AnimatorSet();
animatorSet1.setDuration(5000);
animatorSet1.play(animator1).with(animator2).with(animatorScale1).with(animatorScale2);
ObjectAnimator animator3 = ObjectAnimator.ofFloat(mBgView2, "alpha", 1.0f, 0f);
ObjectAnimator animator4 = ObjectAnimator.ofFloat(mBgView3, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale3 = ObjectAnimator.ofFloat(mBgView2, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale4 = ObjectAnimator.ofFloat(mBgView2, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet2 = new AnimatorSet();
animatorSet2.setDuration(5000);
animatorSet2.play(animator3).with(animator4).with(animatorScale3).with(animatorScale4);
ObjectAnimator animator5 = ObjectAnimator.ofFloat(mBgView3, "alpha", 1.0f, 0f);
ObjectAnimator animator6 = ObjectAnimator.ofFloat(mBgView4, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale5 = ObjectAnimator.ofFloat(mBgView3, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale6 = ObjectAnimator.ofFloat(mBgView3, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet3 = new AnimatorSet();
animatorSet3.setDuration(5000);
animatorSet3.addListener(new Animator.AnimatorListener() {
@Override
public void onAnimationStart(Animator animation) {
}
@Override
public void onAnimationEnd(Animator animation) {
// 放大的View复位
mBgView1.setScaleX(1.0f);
mBgView1.setScaleY(1.0f);
}
@Override
public void onAnimationCancel(Animator animation) {
}
@Override
public void onAnimationRepeat(Animator animation) {
}
});
animatorSet3.play(animator5).with(animator6).with(animatorScale5).with(animatorScale6);
ObjectAnimator animator7 = ObjectAnimator.ofFloat(mBgView4, "alpha", 1.0f, 0f);
ObjectAnimator animator8 = ObjectAnimator.ofFloat(mBgView1, "alpha", 0f, 1.0f);
ObjectAnimator animatorScale7 = ObjectAnimator.ofFloat(mBgView4, "scaleX", 1.0f, 1.3f);
ObjectAnimator animatorScale8 = ObjectAnimator.ofFloat(mBgView4, "scaleY", 1.0f, 1.3f);
AnimatorSet animatorSet4 = new AnimatorSet();
animatorSet4.setDuration(5000);
animatorSet4.play(animator7).with(animator8).with(animatorScale7).with(animatorScale8);
AnimatorSet animatorSet = new AnimatorSet();
animatorSet.playSequentially(animatorSet1, animatorSet2, animatorSet3, animatorSet4);
animatorSet.addListener(new Animator.AnimatorListener() {
@Override
public void onAnimationStart(Animator animation) {
}
@Override
public void onAnimationEnd(Animator animation) {
// 将放大的View 复位
mBgView2.setScaleX(1.0f);
mBgView2.setScaleY(1.0f);
mBgView3.setScaleX(1.0f);
mBgView3.setScaleY(1.0f);
mBgView4.setScaleX(1.0f);
mBgView4.setScaleY(1.0f);
// 循环播放
animation.start();
}
@Override
public void onAnimationCancel(Animator animation) {
}
@Override
public void onAnimationRepeat(Animator animation) {
}
});
animatorSet.start();
xml代码：
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/login_bg_image4"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="centerCrop"
android:src="@drawable/login_bg4" />
<ImageView
android:id="@+id/login_bg_image3"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="centerCrop"
android:src="@drawable/login_bg3" />
<ImageView
android:id="@+id/login_bg_image2"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="centerCrop"
android:src="@drawable/login_bg2" />
<ImageView
android:id="@+id/login_bg_image1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="centerCrop"
android:src="@drawable/login_bg1" />
</FrameLayout>
注意ImageView的顺序，第一张图应该在最上面。
注意： 四个View复位的地方不一样，第一个是在第二组动画执行完毕后复位的，为什么没有和其他几个一起放到最后呢？ 因为 D -> A 的时候就需要显示A,这个时候这一轮是没有播放完的，因此D->A 的时候会跳动。所以我们把他放到前面复位。
三、总结
很简单的一个循环过渡动画，本文是用属性动画实现的。当然肯定还有其他实现方式，如：放一个gif图或者帧动画也是可以的，但是这样可能就需要切很多张图，增加了我们apk 的体积。其他方法大家可以去探索一下，欢迎交流。
