android view淡入淡出动画优化

这个基本是看了文档之后所写，之前一直没有发现也不知道有没有效果，反正记录一点是一点，打算以后用到的话就可以用上，基本上从文档上照搬，这只是提示自己以后可以用而已。

文档解释：

1. 对于正在淡入的视图，请将alpha值设置为0和能见度VISIBLE。(请记住，它最初被设置为GONE)这使视图可见，但完全透明。
2. 对于正在淡入的视图，将其alpha值从0到1。对于正在消失的视图，将alpha值从1到0.
3. 使用onAnimationEnd()在Animator.AnimatorListener，将逐渐消失的视图的可见性设置为GONE。即使alpha值是0，将视图的可见性设置为GONE防止视图占用布局空间，并从布局计算中省略它，从而加快处理速度。

public class CrossfadeActivity extends Activity {

    private View mContentView;
    private View mLoadingView;
    private int mShortAnimationDuration;

    ...

    private void crossfade() {

        // Set the content view to 0% opacity but visible, so that it is visible
        // (but fully transparent) during the animation.
        mContentView.setAlpha(0f);
        mContentView.setVisibility(View.VISIBLE);

        // Animate the content view to 100% opacity, and clear any animation
        // listener set on the view.
        mContentView.animate()
                .alpha(1f)
                .setDuration(mShortAnimationDuration)
                .setListener(null);

        // Animate the loading view to 0% opacity. After the animation ends,
        // set its visibility to GONE as an optimization step (it won't
        // participate in layout passes, etc.)
        mLoadingView.animate()
                .alpha(0f)
                .setDuration(mShortAnimationDuration)
                .setListener(new AnimatorListenerAdapter() {
                    @Override
                    public void onAnimationEnd(Animator animation) {
                        mLoadingView.setVisibility(View.GONE);
                    }
                });
    }
}

kotlin

class CrossfadeActivity : Activity() {

    private lateinit var mContentView: View
    private lateinit var mLoadingView: View
    private var mShortAnimationDuration: Int = 0

    ...

    private fun crossfade() {
        mContentView.apply {
            // Set the content view to 0% opacity but visible, so that it is visible
            // (but fully transparent) during the animation.
            alpha = 0f
            visibility = View.VISIBLE

            // Animate the content view to 100% opacity, and clear any animation
            // listener set on the view.
            animate()
                    .alpha(1f)
                    .setDuration(mShortAnimationDuration.toLong())
                    .setListener(null)
        }
        // Animate the loading view to 0% opacity. After the animation ends,
        // set its visibility to GONE as an optimization step (it won't
        // participate in layout passes, etc.)
        mLoadingView.animate()
                .alpha(0f)
                .setDuration(mShortAnimationDuration.toLong())
                .setListener(object : AnimatorListenerAdapter() {
                    override fun onAnimationEnd(animation: Animator) {
                        mLoadingView.visibility = View.GONE
                    }
                })
    }
}

设置gone让系统不再绘制这个控件减少占用布局，到达优化效果

附上官方文档：

https://developer.android.google.cn/training/animation/reveal-or-hide-view