sqlite中如何用触发器执行取消和重做逻辑（2）
作者：tamsyn 来源：www.sqlite.com.cn
下面的demonstration代码包含一个status_refresh程序，它激活Undo and Redo按钮， 根据没做的和要重做的事来选菜单。你需要重新定义这个程序，用来涉及特定的Undo和Redo按钮，为你的应用线则进入菜单。这里所提供的执行只是一个例子。
demonstration代码假定sqlite数据库是用一个名为"db"的句柄打开的。对于一个内置内存的数据库来说，合适的指令应如下：
sqlite3 db :memory:
这里是demonstration代码：
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# 每件事都在一个私有的有名称的空间里进行
namespace eval ::undo {
# proc: ::undo::activate TABLE ...
# title: Start up the undo/redo system
#
# 参数应是在一个或多个数据库表格（在数据库中和句柄"db"相关联）
# 它们的变化被记录下来是为了撤销和重做的目的。
proc activate {args} {
variable _undo
if {$_undo(active)} return
eval _create_triggers db $args
set _undo(undostack) {}
set _undo(redostack) {}
set _undo(active) 1
set _undo(freeze) -1
_start_interval
}
# proc: ::undo::deactivate
# title: Halt the undo/redo system and delete the undo/redo stacks
#
proc deactivate {} {
variable _undo
if {!$_undo(active)} return
_drop_triggers db
set _undo(undostack) {}
set _undo(redostack) {}
set _undo(active) 0
set _undo(freeze) -1
}
# proc: ::undo::freeze
# title: Stop accepting database changes into the undo stack
#
# 当这个例行程序被启用直到下一步被解冻前，新的数据库的变化将不被记录在撤销存储栈。
#
proc freeze {} {
variable _undo
if {![info exists _undo(freeze)]} return
if {$_undo(freeze)>=0} {error "recursive call to ::undo::freeze"}
set _undo(freeze) [db one {SELECT coalesce(max(seq),0) FROM undolog}]
}
# proc: ::undo::unfreeze
# title: Begin accepting undo actions again.
#
proc unfreeze {} {
variable _undo
if {![info exists _undo(freeze)]} return
if {$_undo(freeze)<0} {error "called ::undo::unfreeze while not frozen"}
db eval "DELETE FROM undolog WHERE seq>$_undo(freeze)"
set _undo(freeze) -1
}
# proc: ::undo::event
# title: Something undoable has happened
#
# 当一个不可撤销的操作发生的时候调用这个程序。在下一个空闲时刻之前激活::undo::barrier。
#
proc event {} {
variable _undo
if {$_undo(pending)==""} {
set _undo(pending) [after idle ::undo::barrier]
}
}
# proc: ::undo::barrier
# title: Create an undo barrier right now.
#
proc barrier {} {
variable _undo
catch {after cancel $_undo(pending)}
set _undo(pending) {}
if {!$_undo(active)} {
refresh
return
}
set end [db one {SELECT coalesce(max(seq),0) FROM undolog}]
if {$_undo(freeze)>=0 && $end>$_undo(freeze)} {set end $_undo(freeze)}
set begin $_undo(firstlog)
_start_interval
if {$begin==$_undo(firstlog)} {
refresh
return
}
lappend _undo(undostack) [list $begin $end]
set _undo(redostack) {}
refresh
}
# proc: ::undo::undo
# title: Do a single step of undo
#
proc undo {} {
_step undostack redostack
}
# proc: ::undo::redo
# title: Redo a single step
#
proc redo {} {
_step redostack undostack
}
# proc: ::undo::refresh
# title: Update the status of controls after a database change
#
# 基于现行数据库的状态，为了合理的进行控制，撤销模块在任何撤销和重做后调用这个例行程序。
这个模块通过激活所有顶级有名称的空间中的status_refresh模块来工作。
#
proc refresh {} {
set body {}
foreach ns [namespace children ::] {
if {[info proc ${ns}::status_refresh]==""} continue
append body ${ns}::status_refresh/n
}
proc ::undo::refresh {} $body
refresh
}
# proc: ::undo::reload_all
# title: Redraw everything based on the current database
#
# 为了使屏幕在数据库内容的基础上被完全重新绘图，撤销模块在任何的撤销和重做后都调用这个
# 例行程序。通过在每个顶级的有名称的空间调用"reload" 模块而不是::undo来完成这个程序。
proc reload_all {} {
set body {}
foreach ns [namespace children ::] {
if {[info proc ${ns}::reload]==""} continue
append body ${ns}::reload/n
}
proc ::undo::reload_all {} $body
reload_all
}
