緣由

之前老項目一直有個問題阅仔，就是彈窗會頂在屏幕最上面，一直以為是特殊機(jī)型適配的問題台谊，一拖再拖沒有解決蓉媳，后來測試實在忍不了，上網(wǎng)上查一查锅铅，發(fā)現(xiàn)這個bug出現(xiàn)好久了酪呻，是安卓7.0之后源碼發(fā)生改變導(dǎo)致的

兼容性現(xiàn)象

popuwindow設(shè)置 showAsDropDown（view）并沒有在某個view的下面
popupWindow設(shè)置了居中或者底部對齊，但是在7.0機(jī)器是跑到頂部盐须。
很明顯這個bug是和我們設(shè)置了Gravity有關(guān)玩荠。
展示popupWindow的函數(shù)有兩個，showAtLocation 和 update贼邓。
重點看了那兩個函數(shù)的API 24 和 API 23 的區(qū)別阶冈。

源碼分析

通過源碼分析發(fā)現(xiàn)，在update函數(shù)里有一個和gravity相關(guān)的地方塑径，很明顯是個bug女坑。

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);
    }
}

有個 computeGravity 函數(shù)，我們再看看

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

噗统舀，我們發(fā)現(xiàn)匆骗，我們之前設(shè)置的gravity被這個函數(shù)執(zhí)行之后覆蓋了劳景。。
我忽然覺得估計是Google的大牛自測的時候?qū)懰雷兞亢谜{(diào)試碉就，最后發(fā)布的時候忘記改了盟广。。
問題定位到了铝噩，符合2個條件：

1:popupWindow 在 show 的時候定義了 Gravity衡蚂，不是 Gravity.START | Gravity.TOP。
2:PopupWindow 調(diào)用了update骏庸。

下面是比較實用的方法毛甲，通過獲取要展示的view的坐標(biāo)，然后計算屏幕減去坐標(biāo)減去view高度具被，減去偏移量就得到 popuwindow展示的位置玻募，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 版本處理
            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);
        }
    }

還有就是通過反射的方式改變源碼
但是我這個方法有缺陷的，反射的辦法會遇到Google Api如果把變量名改了一姿，那就直接無效了七咧。
但是要解決這種系統(tǒng)bug也只能做API適配了。

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);
            }
        }
    }

    /**
     * 反射獲取對象
     * @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;
    }

    /**
     * 反射賦值對象
     * @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();
        }
    }

    /**
     * 反射執(zhí)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;
    }

    /**
     * 利用遞歸找一個類的指定方法叮叹，如果找不到艾栋，去父親里面找直到最上層Object對象為止。
     *
     * @param clazz
     *            目標(biāo)類
     * @param methodName
     *            方法名
     * @param classes
     *            方法參數(shù)類型數(shù)組
     * @return 方法對象
     * @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;
    }
}