@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // For simple implementation, our 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));
    }

在ViewPager的onMeasure()中直接調(diào)用getDefaultSize(), 而該方法在AT_MOST 和EXACTLY模式下，高度為match_parent, 所以在ViewPager中設(shè)置wrap_content會失效梅誓，默認(rèn)為match_parent.

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

如何保證wrap_content 有效桨螺，可以去重寫ViewPager的onMeasure() 方法宾符，主動去測量子View的高度；

 @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //所有子View高度的最大值
        int height = 0;
        //下面遍歷所有child的高度
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            //網(wǎng)絡(luò)上的錯誤做法
            //child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
            ViewGroup.LayoutParams params = child.getLayoutParams();
            int childWidthMeaSpec = getChildMeasureSpec(widthMeasureSpec,
                    getPaddingLeft() + getPaddingRight(), params.width);
            int childHeightMeaSpec = getChildMeasureSpec(heightMeasureSpec,
                    getPaddingTop() + getPaddingBottom(), params.height);
            child.measure(childWidthMeaSpec, childHeightMeaSpec);
            int h = child.getMeasuredHeight();
            if (h > height) height = h; 
        }
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

最后需要注意的是灭翔，在構(gòu)建Fragment請時使用inflate的三個參數(shù)的方法魏烫，即

  // 同樣會導(dǎo)致高度失效
 inflater.inflate(R.layout.fragment_lazy,null)
 //正確使用
 inflater.inflate(R.layout.fragment_lazy,container,false)

因為在inflate()方法中會根據(jù)設(shè)置root和attachToRoot兩個值來構(gòu)建LayoutParams, 使得設(shè)置的高度有效。

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
                   ....
                    ViewGroup.LayoutParams params = null;
                    if (root != null) {
                        if (DEBUG) {
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // Create layout params that match root, if supplied
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }
                .....
}