GSource 创建时各函数的用处

1.prepare:

进入睡眠之前，在g_main_context_prepare里，mainloop调用所有Source的prepare函数，计算最小的timeout时间，该时间决定下一次睡眠的时间。

2.check:

poll被唤醒后，在g_main_context_check里，mainloop调用所有Source的check函数，检查是否有Source已经准备好了。如果poll是由于错误或者超时等原因唤醒的，就不必进行dispatch了。

3.dispatch:

当有Source准备好了，在g_main_context_dispatch里，mainloop调用所有Source的dispatch函数，去分发消息。

4.finalize:

在Source被移出时，mainloop调用该函数去销毁Source。



实例验证：

1.创建两个GSource,一个2秒醒一次，一个3秒醒一次。

#include <stdio.h>
    #include <stdlib.h>
    #include <stdarg.h>
    #include <string.h>

    #include <glib.h>

    typedef struct _DemoSource DemoSource;
    struct _DemoSource
    {
      GSource               source;
      int   interval;
      GTimeVal              last;
    };

    DemoSource *s1, *s2;
    const char* cur_source_name = NULL;

    static glong my_time_val_difference (const GTimeVal *compare, const GTimeVal *now)
    {
      return (compare->tv_sec - now->tv_sec) * 1000 + (compare->tv_usec - now->tv_usec) / 1000;
    }

    static const char* get_source_str( GSource* source_ )
    {
            if( (unsigned int)source_ == (unsigned int)s1 ) return "s1";
            else if( (unsigned int)source_ == (unsigned int)s2 ) return "s2";

            return "unkown";
    }

    static gboolean prepare_func( GSource *source_, gint *timeout )
    {
            DemoSource* source = (DemoSource*)source_;
            GTimeVal now, target;
            glong diff;

            cur_source_name = get_source_str( source_ );

            g_source_get_current_time( source_, &now );

            target = source->last;
            g_time_val_add( &target, source->interval );

            diff = my_time_val_difference( &now, &target );

            printf("-----%s Enter -----\n", __FUNCTION__);

            if( diff >= 0 )
            {
                    *timeout = 0;
                    printf( "Source %s need to dispatch right now. D:%d\n", cur_source_name, diff );
                    return TRUE;
            }
            else
            {
                    *timeout = -diff;
                    printf( "Source %s need to sleep %d ms.\n", cur_source_name, *timeout );
                    return FALSE;
            }
            printf("-----%s Exit-----\n", __FUNCTION__);
    }

    static gboolean check_func( GSource* source_ )
    {
            DemoSource* source = (DemoSource*)source_;
            GTimeVal now, target;
            glong diff;

            cur_source_name = get_source_str( source_ );

            g_source_get_current_time( source_, &now );

            target = source->last;
            g_time_val_add( &target, source->interval );

            diff = my_time_val_difference( &now, &target );

            printf("-----%s Enter -----\n", __FUNCTION__);
            if( diff >= 0 )
            {
                    printf("Yes. Source %s need to dispatch.\n", cur_source_name );
                    return TRUE;
            }
            else
            {
                    printf("No. Source %s want to sleep.\n", cur_source_name );
                    return FALSE;
            }
            printf("-----%s Exit-----\n", __FUNCTION__);
    }

    static gboolean dispatch_func(GSource *source_, GSourceFunc callback, gpointer user_data)
    {
            cur_source_name = get_source_str( source_ );
            printf("-----%s Enter -----\n", __FUNCTION__);
            DemoSource* source = (DemoSource*)source_;
            g_get_current_time( &source->last );
            printf("Source %s dispatched.\n", cur_source_name);
            printf("-----%s Exit-----\n", __FUNCTION__);
    }

    static void finalize_func( GSource* source_ )
    {
            cur_source_name = get_source_str( source_ );
            printf("-----%s Enter -----\n", __FUNCTION__);
            printf("Source %s finalized.\n", cur_source_name );
            printf("-----%s Exit-----\n", __FUNCTION__);
    }

    SourceFuncs source_funcs =
    {
            prepare_func,
            check_func,
            dispatch_func,
            finalize_func,
    };

    int main( int argc, char** argv )
    {
            GMainLoop* loop;

            loop = g_main_loop_new( NULL, TRUE );

            s1 = (DemoSource*)g_source_new( &source_funcs, sizeof(DemoSource) );
            s2 = (DemoSource*)g_source_new( &source_funcs, sizeof(DemoSource) );
            s1->interval = 2000*1000;               //ms
            s2->interval = 3000*1000;

            g_get_current_time( &s1->last );
            g_get_current_time( &s2->last );

            g_source_attach( (GSource*)s1, NULL );
            g_source_attach( (GSource*)s2, NULL );

            printf( "s1 addr:0x%x\n", (unsigned int)s1 );
            printf( "s2 addr:0x%x\n", (unsigned int)s2 );

            g_main_loop_run( loop );

            return 0;
    }

2.编译后的输出结果：

s1 addr:0x804ae98
s2 addr:0x804b500
-----prepare_func Enter -----
Source s1 need to sleep 2000 ms.
-----prepare_func Enter -----
Source s2 need to sleep 3000 ms.


-----check_func Enter -----
Yes. Source s1 need to dispatch.
-----check_func Enter -----
No. Source s2 want to sleep.
-----dispatch_func Enter -----
Source s1 dispatched.
-----dispatch_func Exit-----
-----prepare_func Enter -----
Source s1 need to sleep 2000 ms.
-----prepare_func Enter -----
Source s2 need to sleep 1000 ms.


-----check_func Enter -----
No. Source s1 want to sleep.
-----check_func Enter -----
Yes. Source s2 need to dispatch.
-----dispatch_func Enter -----
Source s2 dispatched.
-----dispatch_func Exit-----
-----prepare_func Enter -----
Source s1 need to sleep 1000 ms.
-----prepare_func Enter -----
Source s2 need to sleep 3000 ms.



-----check_func Enter -----
Yes. Source s1 need to dispatch.
-----check_func Enter -----
No. Source s2 want to sleep.
-----dispatch_func Enter -----
Source s1 dispatched.
-----dispatch_func Exit-----
-----prepare_func Enter -----
Source s1 need to sleep 2000 ms.
-----prepare_func Enter -----
Source s2 need to sleep 2000 ms.

3.结果分析：

（1）首先2个source的prepare函数被调用。一个要呼2秒，一个要呼3秒

（2）2秒后2个source的check函数被调用。s1确实是要dispatch，s2回答不关我事，不要叫我

（3）s1被dispatch。

（4）两个source的prepare函数再次调用。一个要呼2秒，一个再继续呼1秒

（5）再次check

（6）再次dispatch。

循环往复

4.个人理解：

mainloop相当于管家，先问下(prepare())诸位大概什么时候用餐(dispatch())。统计后就设一个闹钟，到时提醒。但是由于没记下各自的时间，所以让大家再确认一下是不是自己(check())，如果check返回true，那么就被dispatch()了。

或者由于外界事件，比如冰淇淋在大家呼呼的过程中送来了，管家就叫大家check()一下。虽然没到时间，但是如果check()返回true，那么也直接dispatch()了