在GIO中,为什么这些异步文件IO操作永远不会完成？(适用于C和Vala)

Question

我想执行异步文件IO操作.这是我在Vala中尝试做的简化版本:

void main(string[] args) {
    store_async();

    while(true)
        ;
}

async void store_async() {
    File file = File.new_for_path("settings.ini");

    stderr.printf("Checking if file exists...\n");
    if (!file.query_exists()) {
        stderr.printf("About to yield file.create_async...\n");
        try {
            yield file.create_async(FileCreateFlags.REPLACE_DESTINATION);
        } catch (Error err) {
            error("Error creating file: %s\n", err.message);
        }
        stderr.printf("Returned from file.create_async.\n");
    }

    string data = "hello\n";
    string new_etag;
    stderr.printf("About to yield file.replace_contents_async...\n");
    try {
        yield file.replace_contents_async(
            data.data,
            null,
            false,
            FileCreateFlags.NONE,
            null,
            out new_etag);
    } catch (Error err) {
        error("Error replacing contents: %s\n", err.message);
    }

    stderr.printf("Returned from file.replace_contents_async.\n");
}

当我运行这个程序,并且没有文件settings.ini存在时,settings.ini会创建并看到此​​输出:

Checking if file exists...
About to yield file.create_async...
(hangs)

如果settings.ini存在,则不会写任何内容,我看到此输出:

Checking if file exists...
About to yield file.create_async...
(hangs)

如果我尝试在C中重现问题,我会遇到类似的问题.这是我的C代码(它不会复制上面的整个vala示例,只是创建文件的部分):

#include <glib.h>
#include <gio/gio.h>
#include <stdio.h>

void create_callback(GObject *source_object, GAsyncResult *res, gpointer user_data);
void write_contents();

GFile* file;

void main(int argc, char** argv) {
    g_type_init();

    fprintf(stderr, "Before file_new_for_path\n");
    file = g_file_new_for_path("settings.ini");

    fprintf(stderr, "Before file_query_exists\n");
    if (!g_file_query_exists(file, NULL)) {
        fprintf(stderr, "Before file_create_async\n");
        g_file_create_async(
                file,
                G_FILE_CREATE_REPLACE_DESTINATION,
                G_PRIORITY_DEFAULT,
                NULL,
                create_callback,
                NULL);
        fprintf(stderr, "After file_create_async\n");
    } else {
        fprintf(stderr, "File already exists. Before write_contents\n");
        write_contents();
        fprintf(stderr, "File already exists. After write_contents\n");
    }

    while(TRUE)
        ;
}

void create_callback(GObject *source_object, GAsyncResult *res, gpointer user_data) {
    fprintf(stderr, "In create_callback. Before write_contents.\n");
    write_contents();
    fprintf(stderr, "In create_callback. After write_contents.\n");
}

void write_contents() {
    fprintf(stderr, "In write_contents\n");
}

当我运行这个例子时,我看到这个输出(假设settings.ini不存在):

Before file_new_for_path
Before file_query_exists
Before file_create_async
After file_create_async
(hangs)

换句话说,create_callback永远不会被称为.

我究竟做错了什么？为什么不g_file_create_async和g_file_replace_contents_async时,我打电话给他们曾完成？

Answer 1

glib中的异步实现要求您运行消息循环.如果没有,你的回调将如何以及何时被调用？

所以在Vala中添加:

var loop = new MainLoop();
loop.run();

而在C:

GMainLoop *loop = g_main_loop_new(NULL, FALSE);
g_main_loop_run(loop);
g_main_loop_unref(loop)