window background and translucent

Activity、Window、Dialog、View 相关知识点

发现很多同学在线上都会问到这个问题；遂，作此篇！

1. windowBackground为透明时, windowIsTranslucent为false, Activity背景为何还是黑色，为什么不是透明？
2. windowBackground为红色时, windowIsTranslucent为true, Activity背景还是红色，为什么不是透明？

windowBackground 、windowIsTranslucent顾名思义是Window属性，但是具体的实现是在哪里呢？是在RootView中———也就是DecorView类中

1. 我们进入DecorView类中：
2. 找到setWindowBackground方法

   public void setWindowBackground(Drawable drawable) {
       if (getBackground() != drawable) {
           setBackgroundDrawable(drawable);
           if (drawable != null) {
               mResizingBackgroundDrawable = enforceNonTranslucentBackground(drawable,
                       mWindow.isTranslucent() || mWindow.isShowingWallpaper());
           } else {
               mResizingBackgroundDrawable = getResizingBackgroundDrawable(
                       getContext(), 0, mWindow.mBackgroundFallbackResource,
                       mWindow.isTranslucent() || mWindow.isShowingWallpaper());
           }
           if (mResizingBackgroundDrawable != null) {
               mResizingBackgroundDrawable.getPadding(mBackgroundPadding);
           } else {
               mBackgroundPadding.setEmpty();
           }
           drawableChanged();
       }
   }

   我们可以看到enforceNonTranslucentBackground中使用到 Window.isTranslucent()，于是我们向下进一步寻找

3. 找到 enforceNonTranslucentBackground

   /**
   * Enforces a drawable to be non-translucent to act as a background if needed, i.e. if the
   * window is not translucent.
   */
   private static Drawable enforceNonTranslucentBackground(Drawable drawable,
           boolean windowTranslucent) {
       if (!windowTranslucent && drawable instanceof ColorDrawable) {
           ColorDrawable colorDrawable = (ColorDrawable) drawable;
           int color = colorDrawable.getColor();
           if (Color.alpha(color) != 255) {
               ColorDrawable copy = (ColorDrawable) colorDrawable.getConstantState().newDrawable()
                       .mutate();
               copy.setColor(
                       Color.argb(255, Color.red(color), Color.green(color), Color.blue(color)));
               return copy;
           }
       }
       return drawable;
   }

   enforceNonTranslucentBackground中的大致逻辑：当windowTranslucent == false,enforceNonTranslucentBackground会强制将背景更改为不透明背景（PS: 即使windowBackground 是 透明的 ）

未完待续。。。