What is the right timing for setting the opacity of a view's background

Question

I try to set the background alpha of my custom linearLayout.

I have tried:

//ctor
protected MyView(
      Context context, AttributeSet attrs, int defStyle, @LayoutRes int layout) {
    super(context, attrs, defStyle);
    setOrientation(VERTICAL);
    LayoutInflater.from(context).inflate(layout, this);
        getBackground().setAlpha(128);

}

and

vto.addOnGlobalLayoutListener(
    new OnGlobalLayoutListener() {
      @Override
      public void onGlobalLayout() {
        getViewTreeObserver().removeOnGlobalLayoutListener(this);
        getBackground().setAlpha(128);
      }
    });

but the background is still null

I have tried

ViewTreeObserver vto = getViewTreeObserver();
vto.addOnPreDrawListener(new OnPreDrawListener() {
  @Override
  public boolean onPreDraw() {
    vto.removeOnPreDrawListener(this);
    getBackground().setAlpha(128);

    return true;
  }
});

but i get java.lang.IllegalStateException: This ViewTreeObserver is not alive, call getViewTreeObserver() again

What is the right timing to get the background and change its alpha (opacity)?

Answer 1

您是否尝试过在xml本身的android：alpha =“ 0.5”属性中进行线性布局？