LVGL (9) Event 机制实现
Event Preview
LVGL 中包含了一个 Event 事件子系统，并定义了很多事件：
/*** Type of event being sent to the object.*/
typedef enum {LV_EVENT_ALL = 0,/** Input device events*/LV_EVENT_PRESSED, /**< The object has been pressed*/LV_EVENT_PRESSING, /**< The object is being pressed (called continuously while pressing)*/LV_EVENT_PRESS_LOST, /**< The object is still being pressed but slid cursor/finger off of the object */LV_EVENT_SHORT_CLICKED, /**< The object was pressed for a short period of time, then released it. Not called if scrolled.*/LV_EVENT_LONG_PRESSED, /**< Object has been pressed for at least `long_press_time`. Not called if scrolled.*/LV_EVENT_LONG_PRESSED_REPEAT, /**< Called after `long_press_time` in every `long_press_repeat_time` ms. Not called if scrolled.*/LV_EVENT_CLICKED, /**< Called on release if not scrolled (regardless to long press)*/LV_EVENT_RELEASED, /**< Called in every cases when the object has been released*/LV_EVENT_SCROLL_BEGIN, /**< Scrolling begins*/LV_EVENT_SCROLL_END, /**< Scrolling ends*/LV_EVENT_SCROLL, /**< Scrolling*/LV_EVENT_GESTURE, /**< A gesture is detected. Get the gesture with `lv_indev_get_gesture_dir(lv_indev_get_act());` */LV_EVENT_KEY, /**< A key is sent to the object. Get the key with `lv_indev_get_key(lv_indev_get_act());`*/LV_EVENT_FOCUSED, /**< The object is focused*/LV_EVENT_DEFOCUSED, /**< The object is defocused*/LV_EVENT_LEAVE, /**< The object is defocused but still selected*/LV_EVENT_HIT_TEST, /**< Perform advanced hit-testing*//** Drawing events*/LV_EVENT_COVER_CHECK, /**< Check if the object fully covers an area. The event parameter is `lv_cover_check_info_t *`.*/LV_EVENT_REFR_EXT_DRAW_SIZE, /**< Get the required extra draw area around the object (e.g. for shadow). The event parameter is `lv_coord_t *` to store the size.*/LV_EVENT_DRAW_MAIN_BEGIN, /**< Starting the main drawing phase*/LV_EVENT_DRAW_MAIN, /**< Perform the main drawing*/LV_EVENT_DRAW_MAIN_END, /**< Finishing the main drawing phase*/LV_EVENT_DRAW_POST_BEGIN, /**< Starting the post draw phase (when all children are drawn)*/LV_EVENT_DRAW_POST, /**< Perform the post draw phase (when all children are drawn)*/LV_EVENT_DRAW_POST_END, /**< Finishing the post draw phase (when all children are drawn)*/LV_EVENT_DRAW_PART_BEGIN, /**< Starting to draw a part. The event parameter is `lv_obj_draw_dsc_t *`. */LV_EVENT_DRAW_PART_END, /**< Finishing to draw a part. The event parameter is `lv_obj_draw_dsc_t *`. *//** Special events*/LV_EVENT_VALUE_CHANGED, /**< The object's value has changed (i.e. slider moved)*/LV_EVENT_INSERT, /**< A text is inserted to the object. The event data is `char *` being inserted.*/LV_EVENT_REFRESH, /**< Notify the object to refresh something on it (for the user)*/LV_EVENT_READY, /**< A process has finished*/LV_EVENT_CANCEL, /**< A process has been cancelled *//** Other events*/LV_EVENT_DELETE, /**< Object is being deleted*/LV_EVENT_CHILD_CHANGED, /**< Child was removed, added, or its size, position were changed */LV_EVENT_CHILD_CREATED, /**< Child was created, always bubbles up to all parents*/LV_EVENT_CHILD_DELETED, /**< Child was deleted, always bubbles up to all parents*/LV_EVENT_SCREEN_UNLOAD_START, /**< A screen unload started, fired immediately when scr_load is called*/LV_EVENT_SCREEN_LOAD_START, /**< A screen load started, fired when the screen change delay is expired*/LV_EVENT_SCREEN_LOADED, /**< A screen was loaded*/LV_EVENT_SCREEN_UNLOADED, /**< A screen was unloaded*/LV_EVENT_SIZE_CHANGED, /**< Object coordinates/size have changed*/LV_EVENT_STYLE_CHANGED, /**< Object's style has changed*/LV_EVENT_LAYOUT_CHANGED, /**< The children position has changed due to a layout recalculation*/LV_EVENT_GET_SELF_SIZE, /**< Get the internal size of a widget*/_LV_EVENT_LAST, /** Number of default events*/LV_EVENT_PREPROCESS = 0x80, /** This is a flag that can be set with an event so it's processedbefore the class default event processing */
} lv_event_code_t;事件主要分为了几类：
- Input 事件：点击，长按，滚动，焦点，等等；
- Draw 事件：Main 绘制开始，绘制中，完成绘制，Post 绘制等；
- Special 事件：刷新事件等；
- Others：Object 被删除，Child 被删除，等等；
LVGL Event 贯穿了整个 LVGL，是其中不可或缺的一部分，它可以将事件有效的通知到组件，应用层可以通过注册事件回调，来监测并响应事件；当然 LVGL 内部的绘制流程，也会通过 Event 机制，通知到各个组件，进行绘制等操作；
LVGL 中事件的发送，靠的是一个叫 lv_event_sent 的函数；
lv_res_t lv_event_send(lv_obj_t * obj, lv_event_code_t event_code, void * param)
{if(obj == NULL) return LV_RES_OK;LV_ASSERT_OBJ(obj, MY_CLASS);lv_event_t e;e.target = obj;e.current_target = obj;e.code = event_code;e.user_data = NULL;e.param = param;e.deleted = 0;e.stop_bubbling = 0;e.stop_processing = 0;/*Build a simple linked list from the objects used in the events*It's important to know if this object was deleted by a nested event*called from this `event_cb`.*/e.prev = event_head;event_head = &e;/*Send the event*/lv_res_t res = event_send_core(&e);/*Remove this element from the list*/event_head = e.prev;return res;
}首先，它组装了一个 lv_event_t 的结构，并将他初始化；
然后通过 event_send_core 来传递 Event 到事件处理中心；event 发送和处理之间是同步调用，也就是 event_sent_core 中即有发送，也处理了接收，并调用了对应的监听某 Event 的 obj 注册下去的回调函数；
Event callback
要看明白 event_send_core 函数，需要先了解 obj event 的回调函数在哪定义的，obj 的定义如下：
可以看到有 2 个叫做 event callback 的地方：
lv_obj_t
|--> lv_obj_class_t|--> void (*event_cb)另一个是：
lv_obj_t
|--> lv_obj_spec_attr_t|--> _lv_event_dsc_t|--> lv_event_cb_t同样是 event callback 这两个有啥区别呢？
我们来看使用场景：
我给一个 button 添加一个按键的响应，那么代码如下：
lv_obj_t * btn = lv_btn_create(lv_scr_act());
lv_obj_add_event_cb(btn, my_event_cb, LV_EVENT_CLICKED, NULL); /*Assign an event callback*/...static void my_event_cb(lv_event_t * event)
{printf("Clicked\n");
}使用 lv_obj_add_event_cb 函数，为 button 指定了它收到 Event (被点击事件) 后的响应函数为 my_event_cb；
这个 lv_obj_add_event_cb 函数的实现为：
struct _lv_event_dsc_t * lv_obj_add_event_cb(lv_obj_t * obj, lv_event_cb_t event_cb, lv_event_code_t filter,void * user_data)
{LV_ASSERT_OBJ(obj, MY_CLASS);lv_obj_allocate_spec_attr(obj);obj->spec_attr->event_dsc_cnt++;obj->spec_attr->event_dsc = lv_mem_realloc(obj->spec_attr->event_dsc,obj->spec_attr->event_dsc_cnt * sizeof(lv_event_dsc_t));LV_ASSERT_MALLOC(obj->spec_attr->event_dsc);obj->spec_attr->event_dsc[obj->spec_attr->event_dsc_cnt - 1].cb = event_cb;obj->spec_attr->event_dsc[obj->spec_attr->event_dsc_cnt - 1].filter = filter;obj->spec_attr->event_dsc[obj->spec_attr->event_dsc_cnt - 1].user_data = user_data;return &obj->spec_attr->event_dsc[obj->spec_attr->event_dsc_cnt - 1];
}可以看到，用户层通过 lv_obj_add_event_cb 函数调用注册下去的响应回调，是通过插入到对象 obj 的 obj->spec_attr->event_dsc 这个地方的，也就是：
lv_obj_t
|--> lv_obj_spec_attr_t|--> _lv_event_dsc_t|--> lv_event_cb_t我们在创建一个 label 的时候，使用 lv_label_create：
#define MY_CLASS &lv_label_classconst lv_obj_class_t lv_label_class = {.constructor_cb = lv_label_constructor,.destructor_cb = lv_label_destructor,.event_cb = lv_label_event,.width_def = LV_SIZE_CONTENT,.height_def = LV_SIZE_CONTENT,.instance_size = sizeof(lv_label_t),.base_class = &lv_obj_class
};/*********************** MACROS**********************//*********************** GLOBAL FUNCTIONS**********************/lv_obj_t * lv_label_create(lv_obj_t * parent)
{LV_LOG_INFO("begin");lv_obj_t * obj = lv_obj_class_create_obj(MY_CLASS, parent);lv_obj_class_init_obj(obj);return obj;
}可以看到，静态的 lv_obj_class_t lv_label_class中，它的 event_cb 被赋值为了 lv_label_event 函数；
这里为什么我不拿 lv_btn_create 打比方呢，因为 button 的这个 class 没有赋值 event_cb；
所以，得出了结论就是，这两个 Event callback，一个是面向用户的，一个是面向 LVGL 系统的！
Event Core
接着看 event_send_core 的实现，这里分为几部分来分析源码；
首先，获取 input 系统的句柄，调用 feedback_cb 来处理触摸，按键这些事件；
static lv_res_t event_send_core(lv_event_t * e)
{EVENT_TRACE("Sending event %d to %p with %p param", e->code, (void *)e->current_target, e->param);/*Call the input device's feedback callback if set*/lv_indev_t * indev_act = lv_indev_get_act();if(indev_act) {if(indev_act->driver->feedback_cb) indev_act->driver->feedback_cb(indev_act->driver, e->code);if(e->stop_processing) return LV_RES_OK;if(e->deleted) return LV_RES_INV;}.........return res;
}其次，获取被应用层注册了的 obj 的回调 (e->current_target 就是构建 lv_event_t 的时候传入的 obj)，然后判断应用层指定的 event code 是否满足 EVENT PREPROCESS，如果是，那么进行调用应用层指定的 event_dsc->cb，
static lv_res_t event_send_core(lv_event_t * e)
{
........lv_res_t res = LV_RES_OK;lv_event_dsc_t * event_dsc = res == LV_RES_INV ? NULL : lv_obj_get_event_dsc(e->current_target, 0);uint32_t i = 0;while(event_dsc && res == LV_RES_OK) {if(event_dsc->cb && ((event_dsc->filter & LV_EVENT_PREPROCESS) == LV_EVENT_PREPROCESS)&& (event_dsc->filter == (LV_EVENT_ALL | LV_EVENT_PREPROCESS) ||(event_dsc->filter & ~LV_EVENT_PREPROCESS) == e->code)) {e->user_data = event_dsc->user_data;event_dsc->cb(e);if(e->stop_processing) return LV_RES_OK;/*Stop if the object is deleted*/if(e->deleted) return LV_RES_INV;}i++;event_dsc = lv_obj_get_event_dsc(e->current_target, i);}
........return res;
}接着，通过调用 lv_obj_event_base 处理 obj 的 lv_obj_class_t 中的 event_cb 被静态初始化的部分：
static lv_res_t event_send_core(lv_event_t * e)
{
....res = lv_obj_event_base(NULL, e);....return res;
}lv_res_t lv_obj_event_base(const lv_obj_class_t * class_p, lv_event_t * e)
{const lv_obj_class_t * base;if(class_p == NULL) base = e->current_target->class_p;else base = class_p->base_class;/*Find a base in which call the ancestor's event handler_cb if set*/while(base && base->event_cb == NULL) base = base->base_class;if(base == NULL) return LV_RES_OK;if(base->event_cb == NULL) return LV_RES_OK;/*Call the actual event callback*/e->user_data = NULL;base->event_cb(base, e);lv_res_t res = LV_RES_OK;/*Stop if the object is deleted*/if(e->deleted) res = LV_RES_INV;return res;
}再接着，处理应用层指定的 event code 相等的部分，即，比较 event code 等于当前的 e->code 的时候，才调用 callback；循环的取并执行
static lv_res_t event_send_core(lv_event_t * e)
{
......event_dsc = res == LV_RES_INV ? NULL : lv_obj_get_event_dsc(e->current_target, 0);i = 0;while(event_dsc && res == LV_RES_OK) {if(event_dsc->cb && ((event_dsc->filter & LV_EVENT_PREPROCESS) == 0)&& (event_dsc->filter == LV_EVENT_ALL || event_dsc->filter == e->code)) {e->user_data = event_dsc->user_data;event_dsc->cb(e);if(e->stop_processing) return LV_RES_OK;/*Stop if the object is deleted*/if(e->deleted) return LV_RES_INV;}i++;event_dsc = lv_obj_get_event_dsc(e->current_target, i);}......return res;
}最后，处理 bubbled 的部分，
Bubbled 是冒泡的意思，Event 冒泡指的是该 obj 的 Event 可以向它的 parent 进行传递，如果当前 obj 是支持 Bubble 的话，那么它会向它的 parent (如果有 parent)进行传递，直到某一个 parent 不支持 bubble 为止；
static lv_res_t event_send_core(lv_event_t * e)
{
........if(res == LV_RES_OK && e->current_target->parent && event_is_bubbled(e)) {e->current_target = e->current_target->parent;res = event_send_core(e);if(res != LV_RES_OK) return LV_RES_INV;}return res;
}
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