Android7.0需要fileprovider宣告，
Android8.0需要新增 申明許可權

<uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />

if (Build.VERSION.SDK_INT > Build.VERSION_CODES.M) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {//新增8.0應用單個的安裝許可權判斷
        activity?.let {
            if (it.packageManager.canRequestPackageInstalls()) { //使用此方法判斷該應用有沒有開啟允許安裝未知應用的功能
                install(it, File(filePath))
            } else {
                alert { //此處使用了anko功能 所以直接使用alert
                    message = "點選確定,設定允許未知來源應用安裝開關即可立即升級更新"
                    negativeButton("確定") {
                        val intent = Intent(Settings.ACTION_MANAGE_UNKNOWN_APP_SOURCES)
                        startActivityForResult(intent, 999)
                        it.dismiss()
                    }
                    positiveButton("取消") {
                        it.dismiss()
                    }
                }.show()
            }
        }
    } else {
        activity?.apply {
            install(this, File(filePath))
        }
    }

} else {
    installAPP(Uri.parse("file://" + filePath), context)
 }




 private fun installAPP(data: Uri, context: Context) {
    val promptInstall = Intent(Intent.ACTION_VIEW)
            .setDataAndType(data, "application/vnd.android.package-archive")
    // FLAG_ACTIVITY_NEW_TASK 可以保證安裝成功時可以正常開啟 app
    promptInstall.flags = Intent.FLAG_ACTIVITY_NEW_TASK
    context.startActivity(promptInstall)
}

fun install(context: Context, downloadfile: File) {
    val intent = Intent(Intent.ACTION_VIEW)
    val apkUri = getUriForFile(context, BuildConfig.APPLICATION_ID + ".fileprovider", downloadfile)
    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK
    intent.addFlags(Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
    intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
    intent.setDataAndType(apkUri, "application/vnd.android.package-archive")
    context.startActivity(intent)

}

 override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        if (requestCode == 999) {
            activity?.let { //嚴格來講此處應該再次判斷是否已經開啟許可權
                install(it, File(filePath))
            }
        }
    }