Layers（图层）

Order of creation（图层顺序）

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By default, LVGL draws new objects on top of old objects.

For example, assume we added a button to a parent object named button1 and then another button named button2. Then button1 (with its child object(s)) will be in the background and can be covered by button2 and its children.

默认情况下，LVGL 在旧对象之上绘制新对象。

例如，假设我们向名为 button1 的父对象添加了一个按钮，然后添加了另一个名为 button2 的按钮。然后 button1（及其子对象）将在背景中并且可以被 button2 及其子对象覆盖。

../_images/layers.png

/*Create a screen*/
lv_obj_t * scr = lv_obj_create(NULL, NULL);
lv_scr_load(scr);          /*Load the screen*/

/*Create 2 buttons*/
lv_obj_t * btn1 = lv_btn_create(scr, NULL);         /*Create a button on the screen*/
lv_btn_set_fit(btn1, true, true);                   /*Enable to automatically set the size according to the content*/
lv_obj_set_pos(btn1, 60, 40);              	   /*Set the position of the button*/

lv_obj_t * btn2 = lv_btn_create(scr, btn1);         /*Copy the first button*/
lv_obj_set_pos(btn2, 180, 80);                    /*Set the position of the button*/

/*Add labels to the buttons*/
lv_obj_t * label1 = lv_label_create(btn1, NULL);	/*Create a label on the first button*/
lv_label_set_text(label1, "Button 1");          	/*Set the text of the label*/

lv_obj_t * label2 = lv_label_create(btn2, NULL);  	/*Create a label on the second button*/
lv_label_set_text(label2, "Button 2");            	/*Set the text of the label*/

/*Delete the second label*/
lv_obj_del(label2);

Bring to the foreground（前台展示）

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There are 2 explicit way to bring an object to the foreground:

  • Use lv_obj_move_foreground(obj) to explicitly tell the library to bring an object to the foreground. Similarly, use lv_obj_move_background(obj) to move to the background.

  • When lv_obj_set_parent(obj, new_parent) is used, obj will be on the foreground on the new_parent.

有两种显式方法可以将对象置于前台：

  • 使用 lv_obj_move_foreground(obj) 明确告诉库将一个对象带到前台。同样，使用lv_obj_move_background(obj) 移动到背景。

  • 当使用 lv_obj_set_parent(obj, new_parent) 时，obj 将位于 new_parent 的前台。

Top and sys layers（顶层和系统层）

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LVGL uses two special layers named as layer_top and layer_sys. Both are visible and common on all screens of a display. They are not, however, shared among multiple physical displays. The layer_top is always on top of the default screen (lv_scr_act()), and layer_sys is on top of layer_top.

The layer_top can be used by the user to create some content visible everywhere. For example, a menu bar, a pop-up, etc. If the click attribute is enabled, then layer_top will absorb all user click and acts as a modal.

LVGL 使用名为“layer_top”和“layer_sys”的两个特殊层。 两者在显示器的所有屏幕上都是可见的和通用的。 然而，它们不会在多个物理显示器之间共享。 layer_top 始终位于默认屏幕的顶部（lv_scr_act()），而 layer_sys 位于 layer_top 的顶部。

用户可以使用 layer_top 来创建一些随处可见的内容。例如，菜单栏、弹出窗口等。如果启用了click 属性，那么layer_top 将吸收所有用户点击并充当模态。

lv_obj_set_click(lv_layer_top(), true);
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The layer_sys is also used for similar purposes on LVGL. For example, it places the mouse cursor above all layers to be sure it's always visible.

layer_sys 也用于 LVGL 的类似目的。例如，它将鼠标光标放在所有图层上方以确保它始终可见。