Service能够创建界面（addView）吗？

一个Service能够创建界面（addView）吗？

一个app，只有Service，没有Activity，能够通过WindowManager调用addView()添加可视界面吗？

答案是可以，但是能够创建的界面类型（WindowManager.LayoutParams.type）不多，且大多需要android.permission.INTERNAL_SYSTEM_WINDOW权限，这个权限只能授予system app。

在没有Activity的进程中创建显示View，存在两个问题：

1. 线程，安卓只能在主线程操作UI界面。
2. token，在启动Activity时实例化ActivityRecord对象创建token（ActivityRecord继承WindowToken类）。

第一个问题好解决，Service生命周期方法onStartCommand()运行在主线程，可以在此方法中定义Handler，service线程通过handler发送消息跨线程通信，在主线程操作UI界面。

    Handler handler;
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                switch (msg.what) {
                    case 1:
                        CursorLocationView cursorLocationView = new CursorLocationView(BleService.this);
                        params.type = 2015;		//WindowManager.LayoutParams.TYPE_SECURE_SYSTEM_OVERLAY
                        mWindowManager.addView(cursorLocationView, params);
                        break;
                }
            }
        };
    }

第二个问题需要解决token问题，WindowManagerService.addWindow()方法中在检查到token==null的情况时通过unprivilegedAppCanCreateTokenWith()检查属性：WindowManager.LayoutParams.type，如果不是特权类型的话返回false。

//WindowManagerService.java
	private boolean unprivilegedAppCanCreateTokenWith(WindowState parentWindow,
            int callingUid, int type, int rootType, IBinder tokenForLog, String packageName) {
        if (rootType >= FIRST_APPLICATION_WINDOW && rootType <= LAST_APPLICATION_WINDOW) {
            ProtoLog.w(WM_ERROR, "Attempted to add application window with unknown token "
                    + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (rootType == TYPE_INPUT_METHOD) {
            ProtoLog.w(WM_ERROR, "Attempted to add input method window with unknown token "
                    + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (rootType == TYPE_VOICE_INTERACTION) {
            ProtoLog.w(WM_ERROR,
                    "Attempted to add voice interaction window with unknown token "
                            + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (rootType == TYPE_WALLPAPER) {
            ProtoLog.w(WM_ERROR, "Attempted to add wallpaper window with unknown token "
                    + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (rootType == TYPE_QS_DIALOG) {
            ProtoLog.w(WM_ERROR, "Attempted to add QS dialog window with unknown token "
                    + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (rootType == TYPE_ACCESSIBILITY_OVERLAY) {
            ProtoLog.w(WM_ERROR,
                    "Attempted to add Accessibility overlay window with unknown token "
                            + "%s.  Aborting.", tokenForLog);
            return false;
        }
        if (type == TYPE_TOAST) {
            // Apps targeting SDK above N MR1 cannot arbitrary add toast windows.
            if (doesAddToastWindowRequireToken(packageName, callingUid, parentWindow)) {
                ProtoLog.w(WM_ERROR, "Attempted to add a toast window with unknown token "
                        + "%s.  Aborting.", tokenForLog);
                return false;
            }
        }
        return true;
    }

可以看到以下类型是需要token的，都和用户界面交互强相关:

• FIRST_APPLICATION_WINDOW~LAST_APPLICATION_WINDOW
• TYPE_INPUT_METHOD
• TYPE_VOICE_INTERACTION
• TYPE_WALLPAPER
• TYPE_QS_DIALOG
• TYPE_ACCESSIBILITY_OVERLAY
• TYPE_TOAST（appInfo.targetSdkVersion < Build.VERSION_CODES.O 时例外）

其余特权类型在token=null的情况下，创建新的Token对象：

//WindowManagerService.java
public int addWindow(Session session, IWindow client, int seq,
            LayoutParams attrs, int viewVisibility, int displayId, Rect outFrame,
            Rect outContentInsets, Rect outStableInsets,
            DisplayCutout.ParcelableWrapper outDisplayCutout, InputChannel outInputChannel,
            InsetsState outInsetsState, InsetsSourceControl[] outActiveControls,
            int requestUserId) {
    		...
            if (token == null) {
                if (!unprivilegedAppCanCreateTokenWith(parentWindow, callingUid, type,
                        rootType, attrs.token, attrs.packageName)) {
                    return WindowManagerGlobal.ADD_BAD_APP_TOKEN;
                }
                
                // 特权类型！！！！
                if (hasParent) {
                    // Use existing parent window token for child windows.
                    token = parentWindow.mToken;
                } else {
                    final IBinder binder = attrs.token != null ? attrs.token : client.asBinder();
                    token = new WindowToken(this, binder, type, false, displayContent,
                            session.mCanAddInternalSystemWindow, isRoundedCornerOverlay);
                }
            }
}

一些权限类型：