[移动开发]ViewPager设置android:layout_height=“wrap_content“失效的根本原因和解决方案

准备知识：

1，xml布局文件：在LayoutInflater里的parser解析,就和普通的标签文件解析一样，通过反射实例化view并获得attrs.

2，LayoutParams：LayoutInflater.inflate时如果root!=null会去根据xml中解析出来的属性生成的LayoutParams，如果root==null则会出现xml中设置的LayoutParams参数失效。这个时候在addView时如果传入的LayoutParams为空则会generateDefaultLayoutParams生成一个默认的LayoutParams(丢失了xml中width/height等设置的属性值)

3，getMeasureWidth() ：在执行完onMeasure（里面执行了setMeasuredDimension）后得到的measureWidth,

4，getWidth() ：在执行完父容器的onLayout后（执行了自己的layout方法）得到的自己的width

5，计算自己的measureSize是通过onMeasure方法（里面执行了setMeasuredDimension）

6，其中计算子View的measureSpec（包含32位，前2位是mode，后30位是尺寸大小）方法主要是通过下面方法
下面传参分别:spec是当前ViewGroup的measureSpec,padding是ViewGroup的对应方向padding,childDimension一般是ViewGroup的子View在xml中设置的宽、高值。

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

1，原因分析（源码里找原因）

上面已经说了设置尺寸是在onMeasure里，直接看ViewPager里该方法

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
                getDefaultSize(0, heightMeasureSpec));
                ...
	}

里面是直接通过getDefaultSize获取尺寸的，就压根没有用到子View的measureSpec（一般通过getChildMeasureSpec获取），再看看getDefaultSize传参，一个size传0，measureSpec传的是ViewPager自己的measureSpec，这个还记得上面的方法getChildMeasureSpec，这里的ViewPager一般也是被它的父容器（一般measureSpec里的mode是AT_MOST/EXACTLY）这样计算来的，通过getChildMeasureSpec里面的逻辑知道此处ViewPager（xml中的height是wrap_content）从父容器那里获取的measureSpec的mode一般是AT_MOST，heightsize是当前ViewPager父容器的高度。

    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;
    }

所以通过getDefaultSize返回的高度result = specSize（父容器高度）。

2，解决方法

方法一、直接在xml中给ViewPager设置layout_witdh=100dp（具体的某个值）

因为上面说了用到的是ViewPager的measureSpec，根据getChildMeasureSpec里面的逻辑，可以直接让ViewPager的measureSpec尺寸变成具体的值，mode变成EXACTLY。这样大小就固定是设置的具体值了。

方法二、重写ViewPager的onMeasure，在里面修改它的measureSpec

直接上代码

public class MyViewPager extends ViewPager {
    public MyViewPager(@NonNull Context context) {
        super(context);
    }

    public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int childSize = getChildCount();
        int maxHeight = 0;
        for (int i = 0; i < childSize; i++) {
            View child = getChildAt(i);
            ViewGroup.LayoutParams params = child.getLayoutParams();
            child.measure(widthMeasureSpec, getChildMeasureSpec(heightMeasureSpec , getPaddingTop()+getPaddingBottom() , params.height));
            if (child.getMeasuredHeight() > maxHeight) {
                maxHeight = child.getMeasuredHeight();
            }
        }

        //wrap_content一般对应的是mode:MeasureSpec.AT_MOST,size:parentSize所以下面针对MeasureSpec.AT_MOST处理
        // match_parent一般对应的是mode:MeasureSpec.EXACTLY,size:parentSize
        //具体的值一般对应的是mode:MeasureSpec.EXACTLY,size:selfSize(layout_width/heightde值在LayoutParams中的width/height对应关系是具体指具体的参数，match_parent代表-1，wrap_content代表-2)
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        if (maxHeight > 0 && heightMode == MeasureSpec.AT_MOST) {//保持match_parent和exactly具体参数的效果
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(maxHeight ,  heightMode);
        }
        super.onMeasure(widthMeasureSpec , heightMeasureSpec);
    }

注意：如果用上面还是无效的，大概率是因为getChildMeasureSpec的第三个传参有问题，需要确认你自己PagerAdapter的instantiateItem方法里是否如下

            @Override
            public Object instantiateItem(@NonNull ViewGroup container, int position) {
                //子View的LayoutParams是在什么时候设置的（主要里面的width和height），如果是被addView是在父容器的addView里判断如果为空则会generateDefaultLayoutParams，如果ViewGroup里是LayoutParams.WRAP_CONTENT，会导致在xml文件里设置的具体尺寸丢失
//                View view = View.inflate(container.getContext() , R.layout.page , null);/这个执行完，view的LayoutParams为null,因为这里的root传值是null，会导致getChildMeasureSpec的第三个传参值丢失
                //用这个是可以的，因为里面执行了params = root.generateLayoutParams(attrs);根据item xml里设置的具体尺寸去生成LayoutParams
                View view = LayoutInflater.from(container.getContext()).inflate(R.layout.page,container,false);
                TextView textView = view.findViewById(R.id.content);
                textView.setText(String.valueOf(position));
                container.addView(view);
                return view;
            }

好了好了，上面都是通过改变ViewPager的measureSpec实现解决方案的。

建议：源码还是要多看多实践才能变得更加深刻。