What is layout_collapseParallaxMultiplier used in CollapsingToolbarLayout?

Question

What is layout_collapseParallaxMultiplier used in CollapsingToolbarLayout ?

I have seen the android developer doc, but I didn't understand.

please explain this parameter & it's effect when it used with layout_collapseMode with example.

Below is a simple example of xml.

<android.support.design.widget.CollapsingToolbarLayout ... >
<ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        android:src="@drawable/random_pattern"
        android:scaleType="fitXY"
        app:layout_collapseMode="parallax"
        app:layout_collapseParallaxMultiplier="0.75"/>
 </android.support.design.widget.CollapsingToolbarLayout>

Answer 1

In very simple words:

When scrolling we see the following - appbar image starts hiding under the content and beyond the top edge of the screen. Parameter layout_collapseParallaxMultiplier determines what part of the image (in percent) will be hidden under the bottom content.

So, for example, setting this parameter to value 1.0 means that top boundary of appbar's image is bound to the top edge of the screen and doesn't move when scrolling. And main content is moving up the top of the image.

When the parameter is not set this corresponds to the value 0.5 and image will be overlapped above and below synchronously.

Answer 2

This was explained on Android Design Support Library:

In addition to pinning a view, you can use app:layout_collapseMode="parallax" (and optionally app:layout_collapseParallaxMultiplier="0.7" to set the parallax multiplier) to implement parallax scrolling (say of a sibling ImageView within the CollapsingToolbarLayout). This use case pairs nicely with the app:contentScrim="?attr/colorPrimary" attribute for CollapsingToolbarLayout, adding a full bleed scrim when the view is collapsed.

This is a behavior example when you use app:layout_collapseMode="parallax".