淡出动画

为了使视图慢慢淡入或淡出视图，请使用 ObjectAnimator。如下面的代码所示，使用 .setDuration(millis) 设置持续时间，其中 millis 参数是动画的持续时间（以毫秒为单位）。在下面的代码中，视图将在 500 毫秒或 1/2 秒内淡入/淡出。要启动 ObjectAnimator 的动画，请致电 .start()。一旦动画完成，就会调用 onAnimationEnd(Animator animation)。这是一个设置视图对 View.GONE 或 View.VISIBLE 的可见性的好地方。

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.ValueAnimator;

void fadeOutAnimation(View viewToFadeOut) {
    ObjectAnimator fadeOut = ObjectAnimator.ofFloat(viewToFadeOut, "alpha", 1f, 0f);

    fadeOut.setDuration(500);
    fadeOut.addListener(new AnimatorListenerAdapter() {
        @Override
        public void onAnimationEnd(Animator animation) {
            // We wanna set the view to GONE, after it's fade out. so it actually disappear from the layout & don't take up space.
            viewToFadeOut.setVisibility(View.GONE);
        }
    });

    fadeOut.start();
}

void fadeInAnimation(View viewToFadeIn) {
    ObjectAnimator fadeIn = ObjectAnimator.ofFloat(viewToFadeIn, "alpha", 0f, 1f);
    fadeIn.setDuration(500);

    fadeIn.addListener(new AnimatorListenerAdapter() {
        @Override
        public void onAnimationStar(Animator animation) {
            // We wanna set the view to VISIBLE, but with alpha 0. So it appear invisible in the layout.
            viewToFadeIn.setVisibility(View.VISIBLE);
            viewToFadeIn.setAlpha(0);
        }
    });

    fadeIn.start();
}