按钮点击触发Alpha动画仅生效一次，请求排查异常原因

Hey there! Let's break down why your Alpha animation works perfectly when triggered outside a button click, but only runs once when you tap the button. This is a super common issue with Android's view animations, so let's walk through the most likely causes and fixes:

1. 补间动画的fillAfter属性“卡住”了视图状态 #

If you're using the old-school AlphaAnimation (a view/tween animation), the fillAfter=true setting keeps the view stuck in the animation's final state once it finishes. For example:

• Your anim goes from 1.0f (opaque) to 0.0f (transparent)
• After the first run, the view visually stays transparent
• When you click the button again, the animation tries to animate from 1.0f to 0.0f—but the view's visual state is already 0.0f, so you see nothing happen

Fix 1: Reset the view's alpha before starting the animation #

Add a line to reset the view's alpha back to its initial state every time the button is clicked:

yourButton.setOnClickListener(v -> {
    // 重置视图的Alpha到初始值
    targetView.setAlpha(1.0f);
    
    AlphaAnimation anim = new AlphaAnimation(1.0f, 0.0f);
    anim.setDuration(1000);
    anim.setFillAfter(true);
    targetView.startAnimation(anim);
});

Fix 2: Clear existing animations before starting a new one #

If there's a chance the previous animation hasn't finished, clearing it first ensures the new animation starts fresh:

yourButton.setOnClickListener(v -> {
    targetView.clearAnimation(); // 清除之前的动画
    
    AlphaAnimation anim = new AlphaAnimation(1.0f, 0.0f);
    anim.setDuration(1000);
    anim.setFillAfter(true);
    targetView.startAnimation(anim);
});

2. 换成属性动画（ObjectAnimator）从根本解决问题 #

View animations only change the visual appearance of a view—they don't modify the actual alpha property of the view. Property animations like ObjectAnimator directly update the view's properties, so they don't suffer from this "stuck state" issue.

Here's how to rewrite your animation with ObjectAnimator:

yourButton.setOnClickListener(v -> {
    // 从当前Alpha值动画到0.0f，再可选地回到1.0f
    ObjectAnimator anim = ObjectAnimator.ofFloat(targetView, "alpha", 1.0f, 0.0f, 1.0f);
    anim.setDuration(1000);
    // 如果需要重复动画，可以加这两行
    // anim.setRepeatMode(ValueAnimator.REVERSE);
    // anim.setRepeatCount(ValueAnimator.INFINITE);
    anim.start();
});

This works every time you click the button because it uses the view's actual current alpha value as the starting point for each animation.

为什么外部触发时正常？ #

Chances are, when you trigger the animation outside the button click (like in onResume() or a timer), the view's state is already reset to its initial value (e.g., after a configuration change, or because the app was brought back to the foreground). The button click, however, happens right after the animation finishes—when the view is still stuck in the final animation state due to fillAfter=true.

内容的提问来源于stack exchange，提问作者jobin