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：

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。