queryUsageStats得到空list

long time = System.currentTimeMillis();
List<UsageStats> stats = mUsageStatsManager.queryUsageStats(UsageStatsManager.INTERVAL_DAILY, time - 1000*30, time);
发现 stats.size()==0
这种情况一般是缺少权限
1：判断当前应用是否被授权为有权查看使用情况的应用

public boolean checkMonitorAppPermission() {
        try {
            Object object = getSystemService(Context.APP_OPS_SERVICE);
            if (object == null) {
                return false;
            }
            Class localClass = object.getClass();
            Class[] arrayOfClass = new Class[3];
            arrayOfClass[0] = Integer.TYPE;
            arrayOfClass[1] = Integer.TYPE;
            arrayOfClass[2] = String.class;
            Method method = localClass.getMethod("checkOp", arrayOfClass);
            if (method == null) {
                return false;
            }
            Object[] arrayOfObject1 = new Object[3];
            arrayOfObject1[0] = 43;
            arrayOfObject1[1] = Integer.valueOf(Binder.getCallingUid());
            arrayOfObject1[2] = getPackageName();
            int result = ((Integer) method.invoke(object, arrayOfObject1)).intValue();
            Log.e("result", "========");
            Log.e("result1", String.valueOf(result));
            if (result == 0) {
                return true;
            } else {
                return false;
            }
        } catch (Exception ex) {

        }
        return false;
    }

2:如果没授权则跳转授权

  Intent intent = new Intent(Settings.ACTION_USAGE_ACCESS_SETTINGS);
  startActivity(intent);

授权后就可以得到最近活跃的应用信息了，问题解决！