How do you specify attributes for a LayoutManager assigned via RecyclerView's 'layoutManager' attribute?

Issue

According to Google's RecyclerView documentation, you can set a specific LayoutManager from within a layout file by specifying its class name in the RecyclerView's 'layoutManager' attribute. It also specifically mentions LayoutManager has a constructor which accepts an AttributeSet.

My question is since you're specifying the LayoutManager via an attribute on the RecyclerView's element, and not as its own element, where/how do you set the attributes targeted for the LayoutManager itself?

My guess is you add them directly to the RecyclerView element as well. This would make sense as inside the RecyclerView's constructor, when it instantiates the LayoutManager specified in the 'layoutManager' attribute, it could simply pass through the same AttributeSet that was passed in to it. However, this is only a guess.

Here's an example of what I'm thinking is the correct way:

<MyRecyclerView
    app:layoutManager=".MyLayoutManager"
    app:attrOnlyUsedByRecyclerView="I'm used by MyRecyclerView"
    app:attrOnlyUsedByLayoutManager="I'm used by MyLayoutManager" />

Note how all three attributes are technically set on the MyRecyclerView element, but the thinking is the third attribute is ignored by, and passed through from MyRecyclerView's constructor into MyLayoutManager's constructor.

I'm trying to build a demo app to test that theory out now, but in the meantime, can anyone clarify for sure, or at least point me in the right direction if this is incorrect?

Solution

Based on a little testing it does seems that you can just apply the relevant attributes directly to the RecyclerView element and they will be passed through to the LayoutManager.

For example, the relevant constructor for LinearLayoutManager is:

/**
 * Constructor used when layout manager is set in XML by RecyclerView attribute
 * "layoutManager". Defaults to vertical orientation.
 *
 * @attr ref android.support.v7.recyclerview.R.styleable#RecyclerView_android_orientation
 * @attr ref android.support.v7.recyclerview.R.styleable#RecyclerView_reverseLayout
 * @attr ref android.support.v7.recyclerview.R.styleable#RecyclerView_stackFromEnd
 */
public LinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr,
                           int defStyleRes) {
    Properties properties = getProperties(context, attrs, defStyleAttr, defStyleRes);
    setOrientation(properties.orientation);
    setReverseLayout(properties.reverseLayout);
    setStackFromEnd(properties.stackFromEnd);
    setAutoMeasureEnabled(true);
}

...and here's how you specify the 'stackFromEnd' attribute for the LayoutManager. (Note how it's set it on the RecyclerView element even though it's destined for the LayoutManager.)

<android.support.v7.widget.RecyclerView
    android:id="@+id/recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal"
    app:layoutManager="android.support.v7.widget.LinearLayoutManager"
    app:stackFromEnd="true" />

Answered By - Brian Yencho