妈个蛋，网上找了一圈，没一个靠谱的

public class ToolbarAlphaScrollBehavior extends CoordinatorLayout.Behavior
    
      {    public ToolbarAlphaScrollBehavior() {    }    public ToolbarAlphaScrollBehavior(Context context, AttributeSet attrs) {        super(context, attrs);    }    @Override    public boolean layoutDependsOn(CoordinatorLayout parent, Toolbar child, View dependency) {        return dependency instanceof AppBarLayout;    }    @Override    public boolean onInterceptTouchEvent(CoordinatorLayout parent, Toolbar child, MotionEvent ev) {        return ev == null || super.onInterceptTouchEvent(parent, child, ev);    }    @Override    public boolean onDependentViewChanged(CoordinatorLayout parent, Toolbar child, View dependency) {        if (dependency instanceof AppBarLayout) {            float ratio = (float) getCurrentScrollValue(child, dependency) / getTotalScrollRange(child, dependency);            float alpha = 1f - Math.min(1f, Math.max(0f, ratio));            int drawableAlpha = (int) (alpha * 255);                        if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {                child.getBackground().setAlpha(drawableAlpha);            } else if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {                ViewGroup toolbarParent = (ViewGroup) child.getParent();                if (toolbarParent.getChildCount() == 2) {                    int count = toolbarParent.getChildCount();                    for (int i = count - 1; i >= 0; i--) {                        toolbarParent.getChildAt(i).getBackground().setAlpha(drawableAlpha);                    }                }                child.getBackground().setAlpha(drawableAlpha);            } else {                child.getBackground().setAlpha(drawableAlpha);            }            if (child.getChildCount() != 0)                child.getChildAt(0).setAlpha(alpha);        }        return false;    }    private int getCurrentScrollValue(Toolbar child, View dependency) {        return dependency.getBottom() - child.getTop();    }    private float getTotalScrollRange(Toolbar child, View dependency) {        return Math.max(dependency.getHeight(), ((AppBarLayout) dependency).getTotalScrollRange()) - child.getTop();    }}
    

初始化设置toolbar透明度 toolbar.getBackground().setAlpha(0);

onDependentViewChanged 主要在这里判断滑动，设置toolbar及其子项透明度变化

效果图：

参考：

http://gold.xitu.io/entry/575d885d2e958a0069e88536