前言

Android无需权限显示悬浮窗, 兼谈逆向分析app这篇文章阅读量很大, 这篇文章是通过逆向分析UC浏览器的实现和兼容性处理来得到一个悬浮窗的实现小技巧, 但有很多问题没有弄明白, 比如为什么在API 18及以下TYPE_TOAST的悬浮窗无法接受触摸事件, 为什么使用TYPE_TOAST就不需要权限.

期间@廖祜秋liaohuqiu_秋百万和我有较多探讨, 原文贴的一个demo android-UCToast也是他做的, 他也有写Android 悬浮窗的小结. 这几篇关于悬浮窗的文章, 是我和他共同探索的结果, 非常感谢.

思路

老实说一开始我是想看看整个事件的传播过程, 从EventHub开始, 到View.onTouchEvent, 想看看Android系统内事件分发, 不过由于绝大部分代码在Native层, 我并没有搞清楚.

其实要想知道原因很简单, 只要grep一下TYPE_TOAST, 把每个用到的地方看一看, 自然就知道了, 但是恰好周末我手上没有源码, 只能在grepcode上面一个一个的查, 所以也花了不少时间.

正文

还是从最简单的地方开始, 我们调用了WindowManager.addView, WindowManager是个接口, 我们使用的是他的实现类WindowManagerImpl, 看看它的addView方法:

@Override
public void addView(View view, ViewGroup.LayoutParams    params) {
    mGlobal.addView(view, params, mDisplay, mParentWindow);
}

mGlobal是WindowManagerGlobal的实例, 再看看WindowManagerGlobal.addView:

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        ......
        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
        ......

        synchronized (mLock) {
            ......
            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);
            ......
        }

        // do this last because it fires off messages to start doing things
        try {
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            ......
    }

代码中创建了一个ViewRootImpl, 调用了它的setView, 将我们要添加的view传入. 继续看ViewRootImpl.setView:

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                ......
                mWindowAttributes.copyFrom(attrs);
                if (mWindowAttributes.packageName == null) {
                    mWindowAttributes.packageName = mBasePackageName;
                }
                ......
                try {
                    ......
                    res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                            getHostVisibility(), mDisplay.getDisplayId(),
                            mAttachInfo.mContentInsets, mInputChannel);
                } catch (RemoteException e) {
                    ......
                    throw new RuntimeException("Adding window failed", e);
                } finally {
                    ......
                }
                ......
            }
        }
    }

对我们的分析来说最关键的代码是

res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
        getHostVisibility(), mDisplay.getDisplayId(),
        mAttachInfo.mContentInsets, mInputChannel);

mWindowSession的类型是IWindowSession, mWindow的类型是IWindow.Stub, 这句代码就是利用AIDL进行IPC, 实际被调用的是Session.addToDisplay:

    @Override
    public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
            int viewVisibility, int displayId, Rect outContentInsets,
            InputChannel outInputChannel) {
        return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId,
                outContentInsets, outInputChannel);
    }

mService是WindowManagerService, 继续往下跟:

    public int addWindow(Session session, IWindow client, int seq,
            WindowManager.LayoutParams attrs, int viewVisibility, int displayId,
            Rect outContentInsets, InputChannel outInputChannel) {
        int[] appOp = new int[1];
        int res = mPolicy.checkAddPermission(attrs, appOp);
        if (res != WindowManagerGlobal.ADD_OKAY) {
            return res;
        }
        ......
        final int type = attrs.type;

        synchronized(mWindowMap) {
            ......

            mPolicy.adjustWindowParamsLw(win.mAttrs);
            ......
        }
        ......

        return res;
    }

mPolicy是标记为final的成员变量:

final WindowManagerPolicy mPolicy = PolicyManager.makeNewWindowManager();

继续看PolicyManager.makeNewWindowManager:

public final class PolicyManager {
    private static final String POLICY_IMPL_CLASS_NAME =
        "com.android.internal.policy.impl.Policy";

    private static final IPolicy sPolicy;

    static {
        // Pull in the actual implementation of the policy at run-time
        try {
            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
            sPolicy = (IPolicy)policyClass.newInstance();
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
        } catch (InstantiationException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        }
    }

    // Cannot instantiate this class
    private PolicyManager() {}

    ......
    public static WindowManagerPolicy makeNewWindowManager() {
        return sPolicy.makeNewWindowManager();
    }
    ......
}

这里sPolicy是com.android.internal.policy.impl.Policy对象, 再看看它的makeNewWindowManager方法返回的是什么:

public WindowManagerPolicy makeNewWindowManager() {
    return new PhoneWindowManager();
}

现在我们知道mPolicy实际上是PhoneWindowManager, 那么

int res = mPolicy.checkAddPermission(attrs, appOp);

实际调用的代码是:

    @Override
    public int checkAddPermission(WindowManager.LayoutParams attrs, int[] outAppOp) {
        int type = attrs.type;

        outAppOp[0] = AppOpsManager.OP_NONE;

        if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW
                || type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {
            return WindowManagerGlobal.ADD_OKAY;
        }
        String permission = null;
        switch (type) {
            case TYPE_TOAST:
                // XXX right now the app process has complete control over
                // this...  should introduce a token to let the system
                // monitor/control what they are doing.
                break;
            case TYPE_DREAM:
            case TYPE_INPUT_METHOD:
            case TYPE_WALLPAPER:
            case TYPE_PRIVATE_PRESENTATION:
                // The window manager will check these.
                break;
            case TYPE_PHONE:
            case TYPE_PRIORITY_PHONE:
            case TYPE_SYSTEM_ALERT:
            case TYPE_SYSTEM_ERROR:
            case TYPE_SYSTEM_OVERLAY:
                permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;
                outAppOp[0] = AppOpsManager.OP_SYSTEM_ALERT_WINDOW;
                break;
            default:
                permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;
        }
        if (permission != null) {
            if (mContext.checkCallingOrSelfPermission(permission)
                    != PackageManager.PERMISSION_GRANTED) {
                return WindowManagerGlobal.ADD_PERMISSION_DENIED;
            }
        }
        return WindowManagerGlobal.ADD_OKAY;
    }

我截取的是4.4_r1的代码, 我们最关心的部分其实一直没有变, 那就是TYPE_TOAST根本没有做权限检查, 直接break出去了, 最后返回WindowManagerGlobal.ADD_OKAY.

不需要权限显示悬浮窗的原因已经找到了, 接着刚才addWindow方法的分析, 继续看下面一句:

mPolicy.adjustWindowParamsLw(win.mAttrs);

也就是PhoneWindowManager.adjustWindowParamsLw, 注意这里我给出了三个版本的实现, 一个是2.0到2.3.7实现的版本, 一个是4.0.1到4.3.1实现的版本, 一个是4.4实现的版本:

//Android 2.0 - 2.3.7 PhoneWindowManager
    public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {
        switch (attrs.type) {
            case TYPE_SYSTEM_OVERLAY:
            case TYPE_SECURE_SYSTEM_OVERLAY:
            case TYPE_TOAST:
                // These types of windows can't receive input events.
                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                        | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
                break;
        }
    }

//Android 4.0.1 - 4.3.1 PhoneWindowManager
    public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {
        switch (attrs.type) {
            case TYPE_SYSTEM_OVERLAY:
            case TYPE_SECURE_SYSTEM_OVERLAY:
            case TYPE_TOAST:
                // These types of windows can't receive input events.
                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                        | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
                attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;
                break;
        }
    }


//Android 4.4 PhoneWindowManager
    @Override
    public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {
        switch (attrs.type) {
            case TYPE_SYSTEM_OVERLAY:
            case TYPE_SECURE_SYSTEM_OVERLAY:
                // These types of windows can't receive input events.
                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                        | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
                attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;
                break;
        }
    }

grepcode上没有3.x的代码, 我也没查具体是什么, 没必要考虑3.x.
可以看到, 在4.0.1以前, 当我们使用TYPE_TOAST, Android会偷偷给我们加上FLAG_NOT_FOCUSABLE和FLAG_NOT_TOUCHABLE, 4.0.1开始, 会额外再去掉FLAG_WATCH_OUTSIDE_TOUCH, 这样真的是什么事件都没了. 而4.4开始, TYPE_TOAST被移除了, 所以从4.4开始, 使用TYPE_TOAST的同时还可以接收触摸事件和按键事件了, 而4.4以前只能显示出来, 不能交互.

API level 18及以下使用TYPE_TOAST无法接收触摸事件的原因也找到了.

尾声

原文发的时候很多事情没搞清楚, 后来文章编辑了十几次, 加上这篇文章, 基本上把所有的疑问都搞明白了. 嗯, 关于这个神奇的悬浮窗的事情应该到这里就结束了.

本人水平有限, 如有错误, 欢迎指正, 以免误导他人