> For the complete documentation index, see [llms.txt](https://lightningminers.gitbook.io/chrome-extension-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lightningminers.gitbook.io/chrome-extension-book/index-2/nativemessage.md).
# 跨应用进程通信 - NativeMessage
NativeMessage 是 Chrome 提供的一个 Desktop App 可以与 Chrome 插件进行跨进程通信的机制，请正确的配置 manifest file 权限 `nativeMessaging`，并由插件启动桌面通信进程即可。
演示项目：[NativeMessage](https://github.com/welearnmore/chrome-extension-demos/tree/master/nativemessage)
> Chrome starts each native messaging host in a separate process and communicates with it using standard input (stdin) and standard output (stdout). The same format is used to send messages in both directions: each message is serialized using JSON, UTF-8 encoded and is preceded with 32-bit message length in native byte order. The maximum size of a single message from the native messaging host is 1 MB, mainly to protect Chrome from misbehaving native applications. The maximum size of the message sent to the native messaging host is 4 GB.
我们使用 Swift 实现了两个函数 SendMessage 和 RecvMessage ，来处理通信。但它 NativeMessage 还是有一些细节问题需要注意：
* 发送的必须是 JSON 字符串
* UTF-8
* 32-bit 长度
* 最大消息体不能超过 1MB
## 配置
在进程启动之前，必须将一份启动的配置清单，写入 Chrome 安装目录下的 NativeMessagingHosts，清单格式如下：
```javascript
{
"name": "com.google.chrome.tray.echo",
"description": "Chrome Native Messaging API Example Host",
"type": "stdio",
"path":"/Users/xiangwenwen/tray.app/Contents/MacOS/tray",
"allowed_origins": [
"chrome-extension://knldjmfmopnpolahpmmgbagdohdnhkik/"
]
}
```
## Connect
跨应用的进程通信必须由 Chrome Extension 来启动，Google 给我们提供了 connect 用于此，如：
```javascript
function sendNativeMessage() {
message = {"text": document.getElementById('input-text').value};
port.postMessage(message);
appendMessage("Sent message: " + JSON.stringify(message) + "");
}
function onNativeMessage(message) {
appendMessage("Received message: " + JSON.stringify(message) + "");
}
function onDisconnected() {
appendMessage("Failed to connect: " + chrome.runtime.lastError.message);
port = null;
updateUiState();
}
function connect() {
var hostName = "com.google.chrome.tray.echo";
appendMessage("Connecting to native messaging host " + hostName + "")
port = chrome.runtime.connectNative(hostName);
console.log(port)
port.onMessage.addListener(onNativeMessage);
port.onDisconnect.addListener(onDisconnected);
updateUiState();
}
```
`hostName` 是由你在配置清单中的 `name` 字段，当你在 Chrome 浏览器中执行上述的代码，Chrome 会在 NativeMessagingHosts 目录中找到你要跨进程通信的 Path ，并由 Chrome 主动调起将 Pipe 重定向到 Chrome。
## SendMessage 函数
得到即将发送字符串的长度，先将长度发送过去，最后再发送字符串。由于print打印的数据是写入在缓冲区，并未真正的完成发送，因此最后需要调用fflush确认：
```swift
func sendMessage(msg: String){
let data = "{\"echo\":\"\(msg)\"}"
let len = data.characters.count
print(Character(UnicodeScalar((UInt8)((len >> 0) & 0xff))), terminator:"")
print(Character(UnicodeScalar((UInt8)((len >> 8) & 0xff))), terminator:"")
print(Character(UnicodeScalar((UInt8)((len >> 16) & 0xff))), terminator:"")
print(Character(UnicodeScalar((UInt8)((len >> 24) & 0xff))), terminator:"")
print(data, terminator:"")
fflush(__stdoutp)
}
```
## RecvMessage
读取消息时也需要先获取前4个bytes，得到真实消息的长度，然后substring(from:)，从而得到真实的消息体。
```swift
func recvMessage(){
while true {
let message_stdin = FileHandle.standardInput
let message = String(data: message_stdin.availableData , encoding: .utf8)!
let start = message.index(message.startIndex, offsetBy: 4)
let text = message.substring(from: start)
DispatchQueue.main.async {
self.showMesg.stringValue = text
}
}
}
```
从官方文档上来看，这个API的细节处理暴露的并不是很多，调试还是挺困难的，有时候发生了错误，并不能明确快速的知道错误发生在哪里。整体的机制其实和socket编程挺类似的，前4个bytes存储的就是真实消息的长度，告诉对方，怎么获取到真实的消息。
