Modeless and modality(模态与非模态)
A modal window (often a dialog) requires the end user to respond before continuing to work in the current application. A modeless window requires no such response. When you create a window, you specify its modality by setting the Modal property to Yes or No. The default is No (modeless).
modal windows are modal only with respect to other windows in the same application.(模态窗口仅仅针对于同一个应用程序的其他窗口) That is, when Oracle Forms displays a modal window, the end user is still allowed to work in windows owned by other applications.
HOW TO CREATE A STACKED CANVASES
To convert an existing content canvas to a stacked canvas, simply change its Canvas Type property from Content to Stacked.
作用：1、Display items or boilerplate graphics that end users need to see only in certain situations. (help text、用户调用时候出现的图片等)
2、Display headers that display constant information such as company names, Dates, and status information. (重复利用 stacked_canvases，避免重复复制单元)
3、Create a block with items that scroll dynamically in the window where they are displayed, much like a spreadsheet.(电子表格超出屏幕的扩展)-Note that stacked canvases can have their own scroll bars, independent of the window's scroll bars.
Forms 里面的基本对象：
FORM.: 可以有多个Window。
Window: 只能有一个CANVAS，但是还可以包括其他的类型的CANVAS，比如CANVAS_STACKED,这 种Canvas类型将在后面的章节中讲到。
Canvas: Canvas is a surface where visual objects,such as graphics and items,are arranged.
和Block没有直接的对应关系，而是直接和Item对应。
一个Canvas上面可以有来自不同的Block的多个Item.
是我们对Item进行位置调整的平台。
Block: Logical owner of items,items in one block are logically related
There are two main tupes of blocks:data block and control block
Item: Form中最基本的组织元素，我们设计的最小单元
Parameter：相当于全局变量，其作用是传递参数，Oracle标准的Form也有单独建一个Block（多 个Item来传递参数的例子。
Record Group: 主要应用是可以提供给LOV与list item,ftree作为数据源
A record group is internal Form Builder data structure that has a column/row
framework similar to a database table
LOV: List Of Value,会直接对应到Text Item上面，点击后会弹出给用户选择数据。
Attached Libraries:存放的是Oracle标准的一些PLL文件。
Property Classes:Oracle标准的一些属性类，我们在建立Window、Canvas、Data Block等都会选 择一个属性类以继承一些属性和方法
在我们的开发过程中，经常用到的有FORM. Trigger、Window、Canvas、Data Block(Item)、LOV(List of Value)、Record Group、Parameters。其中Window、Canvas、Data Block、Item是四个最基本的Object，缺一不可。
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/26470172/viewspace-758442/ ，如需转载，请注明出处，否则将追究法律责任。
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