android - 为什么 AppCompatResources.getDrawable(Context context, int resId) 标记为@Nullable

Question

AppCompatResources.getDrawable 如果资源 ID 无效或链接到其他可绘制对象，则可能返回 null。 getDrawable(context, resid) 返回 null 是否还有其他原因？

我有时会遇到这样的代码，这里的null检查是否合理？:

initToolbar() {
    ActionBar actionBar = getSupportActionBar();
    Drawable drawable = AppCompatResources.getDrawable(getContext(), R.drawable.ic_close_24dp);
    //is there any reason to check for null    
    if (drawable != null) {
        drawable.setColorFilter(ColorUtil.getColorPrimary(requireContext()), PorterDuff.Mode.MULTIPLY);
        actionBar.setHomeAsUpIndicator(drawable);
    } else {
        actionBar.setHomeAsUpIndicator(ic_close_primary_24dp);
    }
}

之所以会出现这样的代码，是因为studio会对这样的代码发出警告:

mToolbar.setNavigationIcon(R.drawable.ic_24_close);
mToolbar.getNavigationIcon().setColorFilter(filter); //warning here, but should I react to it?

Answer 1

这是因为AppCompatDrawableManager.getDrawable()可以返回null:

    public Drawable getDrawable(@NonNull Context context, @DrawableRes int resId,
            boolean failIfNotKnown) {
        checkVectorDrawableSetup(context);
        Drawable drawable = loadDrawableFromDelegates(context, resId);
        if (drawable == null) {
            drawable = createDrawableIfNeeded(context, resId);
        }
        if (drawable == null) {
            drawable = ContextCompat.getDrawable(context, resId);
        }
        if (drawable != null) {
            // Tint it if needed
            drawable = tintDrawable(context, resId, failIfNotKnown, drawable);
        }
        if (drawable != null) {
            // See if we need to 'fix' the drawable
            DrawableUtils.fixDrawable(drawable);
        }
        return drawable;
    }