动画绘制alpha属性 [英] Animating drawable alpha property

问题描述

我想一个动画的ViewGroup的背景绘制对象的alpha属性。

I want to animate the alpha property of a ViewGroup's background Drawable.

我得到使用view.getBackground（）为背景的绘制参考

I get a reference to the background's drawable using view.getBackground().

然后我用下面的code（从这个线程）：

Then I use the following code (from this thread):

    if (backgroundDrawable.getAlpha() == 0) {
            ObjectAnimator animator = ObjectAnimator.ofPropertyValuesHolder(backgroundDrawable, PropertyValuesHolder.ofInt("alpha", 255));
            animator.setTarget(backgroundDrawable);
            animator.setDuration(2000);
            animator.start();
        } else {
            ObjectAnimator animator = ObjectAnimator.ofPropertyValuesHolder(backgroundDrawable, PropertyValuesHolder.ofInt("alpha", 0));
            animator.setTarget(backgroundDrawable);
            animator.setDuration(2000);
            animator.start();
        }

但动画总是从alpha值0开始（意思是，当我想动画为0，它会立即消失，因为它从动画0比0）。

But the animation always starts from the alpha value 0. (meaning, when I want to animate to 0, it disappears immediately, because it animates from 0 to 0).

有谁知道我怎样才能使这项工作？

Does anyone know how I can make this work?

推荐答案

我相信ü想要的是你的动画设置的初始值和最终值，而不仅仅是最终值，如：

I believe what u want is to set initial and final values for your animations, and not just the final value, like this:

if (backgroundDrawable.getAlpha() == 0) {
        ObjectAnimator animator = ObjectAnimator
            .ofPropertyValuesHolder(backgroundDrawable, 
                      PropertyValuesHolder.ofInt("alpha", 0, 255));
        animator.setTarget(backgroundDrawable);
        animator.setDuration(2000);
        animator.start();
    } else {
        ObjectAnimator animator = ObjectAnimator
             .ofPropertyValuesHolder(backgroundDrawable, 
                       PropertyValuesHolder.ofInt("alpha", 255, 0));
        animator.setTarget(backgroundDrawable);
        animator.setDuration(2000);
        animator.start();
    }

可选地，使用从当前值开始 drawable.getAlpha（）的，但该方法仅可开始于API 19 = /

alternatively, starting from the current value using drawable.getAlpha(), but that method is only available starting on API 19 =/

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