缘由

之前老项目一直有个问题，就是弹窗会顶在屏幕最上面，一直以为是特殊机型适配的问题，一拖再拖没有解决，后来测试实在忍不了，上网上查一查，发现这个bug出现好久了，是安卓7.0之后源码发生改变导致的

兼容性现象

popuwindow设置 showAsDropDown（view）并没有在某个view的下面
popupWindow设置了居中或者底部对齐，但是在7.0机器是跑到顶部。
很明显这个bug是和我们设置了Gravity有关。
展示popupWindow的函数有两个，showAtLocation 和 update。
重点看了那两个函数的API 24 和 API 23 的区别。

源码分析

通过源码分析发现，在update函数里有一个和gravity相关的地方，很明显是个bug。

public void update(int x, int y, int width, int height, boolean force) {
    if (width >= 0) {
        mLastWidth = width;
        setWidth(width);
    }

    if (height >= 0) {
        mLastHeight = height;
        setHeight(height);
    }

    if (!isShowing() || mContentView == null) {
        return;
    }

    final WindowManager.LayoutParams p =
            (WindowManager.LayoutParams) mDecorView.getLayoutParams();

    boolean update = force;

    final int finalWidth = mWidthMode < 0 ? mWidthMode : mLastWidth;
    if (width != -1 && p.width != finalWidth) {
        p.width = mLastWidth = finalWidth;
        update = true;
    }

    final int finalHeight = mHeightMode < 0 ? mHeightMode : mLastHeight;
    if (height != -1 && p.height != finalHeight) {
        p.height = mLastHeight = finalHeight;
        update = true;
    }

    if (p.x != x) {
        p.x = x;
        update = true;
    }

    if (p.y != y) {
        p.y = y;
        update = true;
    }

    final int newAnim = computeAnimationResource();
    if (newAnim != p.windowAnimations) {
        p.windowAnimations = newAnim;
        update = true;
    }

    final int newFlags = computeFlags(p.flags);
    if (newFlags != p.flags) {
        p.flags = newFlags;
        update = true;
    }

    final int newGravity = computeGravity();
    if (newGravity != p.gravity) {
        p.gravity = newGravity;
        update = true;
    }

    int newAccessibilityIdOfAnchor =
            (mAnchor != null) ? mAnchor.get().getAccessibilityViewId() : -1;
    if (newAccessibilityIdOfAnchor != p.accessibilityIdOfAnchor) {
        p.accessibilityIdOfAnchor = newAccessibilityIdOfAnchor;
        update = true;
    }

    if (update) {
        setLayoutDirectionFromAnchor();
        mWindowManager.updateViewLayout(mDecorView, p);
    }
}

有个 computeGravity 函数，我们再看看

private int computeGravity() {
    int gravity = Gravity.START | Gravity.TOP;
    if (mClipToScreen || mClippingEnabled) {
        gravity |= Gravity.DISPLAY_CLIP_VERTICAL;
    }
    return gravity;
}

噗，我们发现，我们之前设置的gravity被这个函数执行之后覆盖了。。
我忽然觉得估计是Google的大牛自测的时候写死变量好调试，最后发布的时候忘记改了。。
问题定位到了，符合2个条件：

1:popupWindow 在 show 的时候定义了 Gravity，不是 Gravity.START | Gravity.TOP。
2:PopupWindow 调用了update。

下面是比较实用的方法，通过获取要展示的view的坐标，然后计算屏幕减去坐标减去view高度，减去偏移量就得到 popuwindow展示的位置，有效可用

/**
    * android 7.0之后的坑
    */
    public static void showAsDropDownFor_N(PopupWindow pw, View anchor, int xoff, int yoff) {
        if (Build.VERSION.SDK_INT >= 24) {
            int[] location = new int[2];
            anchor.getLocationOnScreen(location);
            // 7.1 版本处理
            if (Build.VERSION.SDK_INT == 25) {
                //【note!】Gets the screen height without the virtual key
                WindowManager wm = (WindowManager) pw.getContentView().getContext().getSystemService(Context.WINDOW_SERVICE);
                int screenHeight = wm.getDefaultDisplay().getHeight();
                /*
                /*
                 * PopupWindow height for match_parent,
                 * will occupy the entire screen, it needs to do special treatment in Android 7.1
                */
                pw.setHeight(screenHeight - location[1] - anchor.getHeight() - yoff);
            }
            pw.showAtLocation(anchor, Gravity.NO_GRAVITY, xoff, location[1] + anchor.getHeight() + yoff);
        } else {
            pw.showAsDropDown(anchor, xoff, yoff);
        }
    }

还有就是通过反射的方式改变源码
但是我这个方法有缺陷的，反射的办法会遇到Google Api如果把变量名改了，那就直接无效了。
但是要解决这种系统bug也只能做API适配了。

package cc.kinva.widget;

import android.content.Context;
import android.os.Build;
import android.text.TextUtils;
import android.util.AttributeSet;
import android.view.View;
import android.view.WindowManager;
import android.widget.PopupWindow;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * Created by pang on 17/9/23.
 */
public class FixedPopupWindow extends PopupWindow {
    public FixedPopupWindow(Context context) {
        super(context);
    }

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

    public FixedPopupWindow(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public FixedPopupWindow(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    public FixedPopupWindow(View contentView) {
        super(contentView);
    }

    public FixedPopupWindow() {
        super();
    }

    public FixedPopupWindow(int width, int height) {
        super(width, height);
    }

    public FixedPopupWindow(View contentView, int width, int height, boolean focusable) {
        super(contentView, width, height, focusable);
    }

    public FixedPopupWindow(View contentView, int width, int height) {
        super(contentView, width, height);
    }

    @Override
    public void update(int x, int y, int width, int height, boolean force) {
        if (Build.VERSION.SDK_INT < 24) {
            super.update(x, y, width, height, force);
            return;
        }
        if (width >= 0) {
            setParam("mLastWidth", width);
            setWidth(width);
        }

        if (height >= 0) {
            setParam("mLastHeight", height);
            setHeight(height);
        }

        Object obj = getParam("mContentView");
        View mContentView = null;
        if (obj instanceof View) {
            mContentView = (View) obj;
        }
        if (!isShowing() || mContentView == null) {
            return;
        }

        obj = getParam("mDecorView");
        View mDecorView = null;
        if (obj instanceof View) {
            mDecorView = (View) obj;
        }
        final WindowManager.LayoutParams p =
                (WindowManager.LayoutParams) mDecorView.getLayoutParams();

        boolean update = force;

        obj = getParam("mWidthMode");
        int mWidthMode = obj != null ? (Integer) obj : 0;
        obj = getParam("mLastWidth");
        int mLastWidth = obj != null ? (Integer) obj : 0;

        final int finalWidth = mWidthMode < 0 ? mWidthMode : mLastWidth;
        if (width != -1 && p.width != finalWidth) {
            p.width = finalWidth;
            setParam("mLastWidth", finalWidth);
            update = true;
        }

        obj = getParam("mHeightMode");
        int mHeightMode = obj != null ? (Integer) obj : 0;
        obj = getParam("mLastHeight");
        int mLastHeight = obj != null ? (Integer) obj : 0;
        final int finalHeight = mHeightMode < 0 ? mHeightMode : mLastHeight;
        if (height != -1 && p.height != finalHeight) {
            p.height = finalHeight;
            setParam("mLastHeight", finalHeight);
            update = true;
        }

        if (p.x != x) {
            p.x = x;
            update = true;
        }

        if (p.y != y) {
            p.y = y;
            update = true;
        }

        obj = execMethod("computeAnimationResource");
        final int newAnim = obj == null ? 0 : (Integer) obj;
        if (newAnim != p.windowAnimations) {
            p.windowAnimations = newAnim;
            update = true;
        }

        obj = execMethod("computeFlags", new Class[]{int.class}, new Object[]{p.flags});
        final int newFlags = obj == null ? 0 : (Integer) obj;
        if (newFlags != p.flags) {
            p.flags = newFlags;
            update = true;
        }

        if (update) {
            execMethod("setLayoutDirectionFromAnchor");
            obj = getParam("mWindowManager");
            WindowManager mWindowManager = obj instanceof WindowManager ? (WindowManager) obj : null;
            if (mWindowManager != null) {
                mWindowManager.updateViewLayout(mDecorView, p);
            }
        }
    }

    /**
     * 反射获取对象
     * @param paramName
     * @return
     */
    private Object getParam(String paramName) {
        if (TextUtils.isEmpty(paramName)) {
            return null;
        }
        try {
            Field field = PopupWindow.class.getDeclaredField(paramName);
            field.setAccessible(true);
            return field.get(this);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 反射赋值对象
     * @param paramName
     * @param obj
     */
    private void setParam(String paramName, Object obj) {
        if (TextUtils.isEmpty(paramName)) {
            return;
        }
        try {
            Field field = PopupWindow.class.getDeclaredField(paramName);
            field.setAccessible(true);
            field.set(this, obj);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 反射执行方法
     * @param methodName
     * @param args
     * @return
     */
    private Object execMethod(String methodName, Class[] cls, Object[] args) {
        if (TextUtils.isEmpty(methodName)) {
            return null;
        }
        try {
            Method method = getMethod(PopupWindow.class, methodName, cls);
            method.setAccessible(true);
            return method.invoke(this, args);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 利用递归找一个类的指定方法，如果找不到，去父亲里面找直到最上层Object对象为止。
     *
     * @param clazz
     *            目标类
     * @param methodName
     *            方法名
     * @param classes
     *            方法参数类型数组
     * @return 方法对象
     * @throws Exception
     */
    private Method getMethod(Class clazz, String methodName,
                                   final Class[] classes) throws Exception {
        Method method = null;
        try {
            method = clazz.getDeclaredMethod(methodName, classes);
        } catch (NoSuchMethodException e) {
            try {
                method = clazz.getMethod(methodName, classes);
            } catch (NoSuchMethodException ex) {
                if (clazz.getSuperclass() == null) {
                    return method;
                } else {
                    method = getMethod(clazz.getSuperclass(), methodName,
                            classes);
                }
            }
        }
        return method;
    }
}