安卓recyclerView中item过多时，固定高度

查了资料可以有三种实现方式：

第一种：直接在代码中判断，当recycler元素大于某个个数时，就将recycler的高度固定；

private fun setRecyclerMaxHeigh(view: RecyclerView, maxHeight: Float) {
    val lp: ViewGroup.LayoutParams = view.getLayoutParams()
    if (view.childCount> 4) {
        Log.d("myLog", view.childCount.toString())
        lp.height = DensityUtil.dip2px(maxHeight)
    } else {
        Log.d("myLog", view.childCount.toString())
        lp.height = DensityUtil.dip2px(0f)
    }
    view.layoutParams = lp
}

第二种：重写RecyclerView加一个最大高度的属性MaxHeightRecyclerView:

public class MaxHeightRecyclerView extends RecyclerView {
    private int mMaxHeight;
    public MaxHeightRecyclerView(Context context) {
        super(context);
    }
    public MaxHeightRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initialize(context, attrs);
    }
    public MaxHeightRecyclerView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initialize(context, attrs);
    }
    private void initialize(Context context, AttributeSet attrs) {
        TypedArray arr = context.obtainStyledAttributes(attrs, R.styleable.MaxHeightRecyclerView);
        mMaxHeight = arr.getLayoutDimension(R.styleable.MaxHeightRecyclerView_max_height, mMaxHeight);
        arr.recycle();
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if (mMaxHeight > 0) {
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(mMaxHeight, MeasureSpec.AT_MOST);
        }
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

第三种：如果你的父布局是constraintLayout，可以用此布局来约束RecyclerView标记进行以下更改：

<android.support.v7.widget.RecyclerView
    android:id="@+id/recycler_view"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintHeight_default="wrap"
    app:layout_constraintHeight_max="280dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/line/>

关键属性是这三个：

android:layout_height="0dp"
app:layout_constraintHeight_default="wrap"
app:layout_constraintHeight_max="280dp"