测量ViewPager

Question

我有一个自定义的ViewGroup，它有一个子ViewPager。ViewPager由PagerAdapter馈送，该LinearLayout提供到在高度和宽度上都具有WRAP_CONTENT的ViewPager的LayoutParams。

视图可以正确显示，但是在ViewPager上调用child.measure()方法时，它不会返回LinearLayout的实际尺寸，但似乎会填满所有剩余空间。

你知道为什么会发生这种情况，以及如何修正吗？

Answer 1

我对被接受的答案不是很满意(也不是对评论中的预膨胀-所有视图解决方案)，所以我组合了一个ViewPager，它的高度取自第一个可用的孩子。它通过执行第二次测量来实现这一点，允许您窃取第一个孩子的身高。

更好的解决方案是在android.support.v4.view包中创建一个新类，该类实现更好版本的onMeasure (可以访问包可见的方法，如populate())

不过，就目前而言，下面的解决方案很适合我。

public class HeightWrappingViewPager extends ViewPager {

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

    public HeightWrappingViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        boolean wrapHeight = MeasureSpec.getMode(heightMeasureSpec) 
                == MeasureSpec.AT_MOST;

        if(wrapHeight) {
            /**
             * The first super.onMeasure call made the pager take up all the 
             * available height. Since we really wanted to wrap it, we need 
             * to remeasure it. Luckily, after that call the first child is 
             * now available. So, we take the height from it. 
             */

            int width = getMeasuredWidth(), height = getMeasuredHeight();

            // Use the previously measured width but simplify the calculations
            widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, MeasureSpec.EXACTLY);

            /* If the pager actually has any children, take the first child's 
             * height and call that our own */ 
            if(getChildCount() > 0) {
                View firstChild = getChildAt(0);

                /* The child was previously measured with exactly the full height.
                 * Allow it to wrap this time around. */
                firstChild.measure(widthMeasureSpec, 
                        MeasureSpec.makeMeasureSpec(height, MeasureSpec.AT_MOST));

                height = firstChild.getMeasuredHeight();
            }

            heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);

            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }
}

Answer 2

查看兼容性jar中ViewPager类的内部结构：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
{
    // For simple implementation, or internal size is always 0.
    // We depend on the container to specify the layout size of
    // our view. We can't really know what it is since we will be
    // adding and removing different arbitrary views and do not
    // want the layout to change as this happens.
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec), getDefaultSize(0, heightMeasureSpec));

   ...
}

看起来，ViewPager实现并没有测量子视图，而只是根据父视图传入的内容将ViewPager设置为一个标准视图。当您传递wrap_content时，由于视图分页实际上并不测量其内容，因此它占据了整个可用区域。

我的建议是根据子视图的大小在ViewPager上设置一个静态大小。如果这是不可能的(例如，子视图可能不同)，您将需要选择一个最大大小并处理某些视图中的额外空间，或者扩展ViewPager并提供一个测量子视图的onMeasure。您将遇到的一个问题是，视图分页程序的设计不会因为显示不同的视图而改变宽度，因此您可能会被迫选择一个大小并继续使用它

Answer 3

如果你在你的PageAdapter的instantiateItem中定位(SetTag)：

@Override
public Object instantiateItem(ViewGroup collection, int page) {
    LayoutInflater inflater = (LayoutInflater) context
            .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View view = (View) inflater.inflate(R.layout.page_item , null);
    view.setTag(page);

然后，可以使用OnPageChangeListener检索视图(适配器的页面)，测量它，并调整ViewPager的大小：

private ViewPager pager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    pager = findViewById(R.id.viewpager);
    pager.setOnPageChangeListener(new SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            resizePager(position);
        }
    });

    public void resizePager(int position) {
        View view = pager.findViewWithTag(position);
        if (view == null) 
            return;
        view.measure(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        int width = view.getMeasuredWidth();
        int height = view.getMeasuredHeight();
            //The layout params must match the parent of the ViewPager 
        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(width , height); 
        pager.setLayoutParams(params);
    }
}