将RecyclerView的宽高属性设置为“wrap_content”

前言

最近，在使用RecyclerView时遇到一个问题，就是将RecyclerView的高度设置为“wrap_content”时，控件实际测量高度为”match_parent”,为什么会出现这种问题呢？RecyclerView设置如下：

.support.v7.widget.RecyclerView
    android:id="@+id/sv_rv"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@android:color/darker_gray">
.support.v7.widget.RecyclerView>

recyclerView = (RecyclerView)findViewById(R.id.sv_rv);
recyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
recyclerView.setAdapter(new RvAdapter());

而实际显示效果如下：

正文

一般来说，出现这种问题是因为高度测量问题，所以先查看RecyclerView的onMeasure方法，如下：

@Override
    protected void onMeasure(int widthSpec, int heightSpec) {
        if (mAdapterUpdateDuringMeasure) {
            eatRequestLayout();
            processAdapterUpdatesAndSetAnimationFlags();

            if (mState.mRunPredictiveAnimations) {
                // TODO: try to provide a better approach.
                // When RV decides to run predictive animations, we need to measure in pre-layout
                // state so that pre-layout pass results in correct layout.
                // On the other hand, this will prevent the layout manager from resizing properly.
                mState.mInPreLayout = true;
            } else {
                // consume remaining updates to provide a consistent state with the layout pass.
                mAdapterHelper.consumeUpdatesInOnePass();
                mState.mInPreLayout = false;
            }
            mAdapterUpdateDuringMeasure = false;
            resumeRequestLayout(false);
        }

        if (mAdapter != null) {
            mState.mItemCount = mAdapter.getItemCount();
        } else {
            mState.mItemCount = 0;
        }
        if (mLayout == null) {
            defaultOnMeasure(widthSpec, heightSpec);
        } else {
            mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
        }

        mState.mInPreLayout = false; // clear
    }

重要代码:

if (mLayout == null) {
    defaultOnMeasure(widthSpec, heightSpec);
    } else {
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
    }
}

可以发现当RecyclerView设置LayoutManager时，RecyclerView的测量高度由所选择的LayoutManager决定。由于上文所选择的LayoutManager为LinearLayoutManager，故查看其源码的onMeasure函数，然而并没找到，那就查看其父类的onMeasure函数。

public void onMeasure(Recycler recycler, State state, int widthSpec, int heightSpec) {
    mRecyclerView.defaultOnMeasure(widthSpec, heightSpec);
}

然后查看defaultOnMeasure函数，可以发现其并未对子item进行高度测量，所以导致其实测高度异常。故而要纠正此问题，需要重写LayoutManager的onMeasure函数。
如下，是我自己写的一个简单的LayoutManager子类，继承自LinearLayoutManager，仅作抛砖引玉之用:

package how.th.ridelib.RvLayoutManager;

import android.content.Context;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;


public class RecyclerLayoutManager extends LinearLayoutManager {

    public RecyclerLayoutManager(Context context){
        super(context);
    }

    @Override
    public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
        int itemCount = state.getItemCount();
        int measuredWidth = 0;
        int measuredHeight = 0;
        int heightSize = View.MeasureSpec.getSize(heightSpec);
        for(int i = 0; i < itemCount; i ++){
            View view = recycler.getViewForPosition(i);

            if(view != null){
                RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams)view.getLayoutParams();
                int margin = layoutParams.bottomMargin + layoutParams.topMargin;
                measuredWidth = View.MeasureSpec.getSize(widthSpec);
                view.measure(widthSpec, View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
                switch (layoutParams.height){
                    case RecyclerView.LayoutParams.WRAP_CONTENT:
                        measuredHeight += (getDecoratedMeasuredHeight(view) + margin);
                        break;
                    case RecyclerView.LayoutParams.MATCH_PARENT:
                    default:
                        measuredHeight += (layoutParams.height + margin);
                        break;
                }
            }
        }
        if(measuredHeight > heightSize)
            super.onMeasure(recycler, state, widthSpec, heightSpec);
        else
            setMeasuredDimension(measuredWidth, measuredHeight);
    }
}

实际使用：

recyclerView = (RecyclerView)findViewById(R.id.sv_rv);
recyclerView.setLayoutManager(new RecyclerLayoutManager(getApplicationContext()));
recyclerView.setAdapter(new RvAdapter());

实际效果：