1 """The tasks module provides a simple light-weight alternative to threads.
3 When you have a long-running job you will want to run it in the background,
4 while the user does other things. There are four ways to do this:
6 - Use a new thread for each task.
7 - Use callbacks from an idle handler.
8 - Use a recursive mainloop.
11 Using threads causes a number of problems. Some builds of pygtk/python don't
12 support them, they introduce race conditions, often lead to many subtle
13 bugs, and they require lots of resources (you probably wouldn't want 10,000
14 threads running at once). In particular, two threads can run at exactly the
15 same time (perhaps on different processors), so you have to be really careful
16 that they don't both try to update the same variable at the same time. This
17 requires lots of messy locking, which is hard to get right.
19 Callbacks work within a single thread. For example, you open a dialog box and
20 then tell the system to call one function if it's closed, and another if the
21 user clicks OK, etc. The function that opened the box then returns, and the
22 system calls one of the given callback functions later. Callbacks only
23 execute one at a time, so you don't have to worry about race conditions.
24 However, they are often very awkward to program with, because you have to
25 save state somewhere and then pass it to the functions when they're called.
27 A recursive mainloop only works with nested tasks (you can create a
28 sub-task, but the main task can't continue until the sub-task has
29 finished). We use these for, eg, rox.alert() boxes since you don't
30 normally want to do anything else until the box is closed, but it is not
31 appropriate for long-running jobs.
33 Tasks use python's generator API to provide a more pleasant interface to
34 callbacks. See the Task class (below) for more information.
37 from __future__
import generators
42 # The list of Blockers whose event has happened, in the order they were
47 """A Blocker object starts life with 'happened = False'. Tasks can
48 ask to be suspended until 'happened = True'. The value is changed
49 by a call to trigger().
53 kettle_boiled = tasks.Blocker()
57 print "Add tea leaves"
59 print "Pour water into cup"
61 yield tasks.TimeoutBlocker(120)
65 tasks.Task(make_tea())
68 print "Kettle boiled!"
69 kettle_boiled.trigger()
71 You can also yield a list of Blockers. Your function will resume
72 after any one of them is triggered. Use blocker.happened to
73 find out which one(s). Yielding a Blocker that has already
74 happened is the same as yielding None (gives any other Tasks a
75 chance to run, and then continues).
79 self
.happened
= False # False until event triggered
80 self
._rox
_lib
_tasks
= {} # Tasks waiting on this blocker
83 """The event has happened. Note that this cannot be undone;
84 instead, create a new Blocker to handle the next occurance
86 if self
.happened
: return # Already triggered
88 #assert self not in _run_queue # XXX: Slow
91 _run_queue
.append(self
)
93 def add_task(self
, task
):
94 """Called by the schedular when a Task yields this
95 Blocker. If you override this method, be sure to still
96 call this method with Blocker.add_task(self)!"""
97 self
._rox
_lib
_tasks
[task
] = True
99 def remove_task(self
, task
):
100 """Called by the schedular when a Task that was waiting for
101 this blocker is resumed."""
102 del self
._rox
_lib
_tasks
[task
]
104 class IdleBlocker(Blocker
):
105 """An IdleBlocker blocks until a task starts waiting on it, then
106 immediately triggers. An instance of this class is used internally
107 when a Task yields None."""
108 def add_task(self
, task
):
109 """Also calls trigger."""
110 Blocker
.add_task(self
, task
)
113 class TimeoutBlocker(Blocker
):
114 """Triggers after a set number of seconds. rox.toplevel_ref/unref
115 are called to prevent the app quitting while a TimeoutBlocker is
117 def __init__(self
, timeout
):
118 """Trigger after 'timeout' seconds (may be a fraction)."""
119 Blocker
.__init
__(self
)
121 g
.timeout_add(long(timeout
* 1000), self
._timeout
)
127 class InputBlocker(Blocker
):
128 """Triggers when os.read(stream) would not block."""
131 def __init__(self
, stream
):
132 Blocker
.__init
__(self
)
133 self
._stream
= stream
135 def add_task(self
, task
):
136 Blocker
.add_task(self
, task
)
137 if self
._tag
is None:
138 self
._tag
= g
.input_add(self
._stream
, g
.gdk
.INPUT_READ
,
139 lambda src
, cond
: self
.trigger())
141 def remove_task(self
, task
):
142 Blocker
.remove_task(self
, task
)
143 if not self
._rox
_lib
_tasks
:
144 g
.input_remove(self
._tag
)
147 _idle_blocker
= IdleBlocker()
150 """Create a new Task when you have some long running function to
151 run in the background, but which needs to do work in 'chunks'.
152 Example (the first line is needed to enable the 'yield' keyword in
155 from __future__ import generators
156 from rox import tasks
158 for x in range(start, start + 5):
162 tasks.Task(my_task(0))
163 tasks.Task(my_task(10))
167 Yielding None gives up control of the processor to another Task,
168 causing the sequence printed to be interleaved. You can also yield a
169 Blocker (or a list of Blockers) if you want to wait for some
170 particular event before resuming (see the Blocker class for details).
173 def __init__(self
, iterator
, name
= None):
174 """Call iterator.next() from a glib idle function. This function
175 can yield Blocker() objects to suspend processing while waiting
176 for events. name is used only for debugging."""
177 assert iterator
.next
, "Object passed is not an iterator!"
178 self
.next
= iterator
.next
180 # Block new task on the idle handler...
181 _idle_blocker
.add_task(self
)
182 self
._rox
_blockers
= (_idle_blocker
,)
185 # Remove from our blockers' queues
186 for blocker
in self
._rox
_blockers
:
187 blocker
.remove_task(self
)
190 new_blockers
= self
.next()
191 except StopIteration:
196 rox
.report_exception()
198 if new_blockers
is None:
199 # Just give up control briefly
200 new_blockers
= (_idle_blocker
,)
202 if isinstance(new_blockers
, Blocker
):
203 # Wrap a single yielded blocker into a list
204 new_blockers
= (new_blockers
,)
205 # Are we blocking on something that already happened?
206 for blocker
in new_blockers
:
208 new_blockers
= (_idle_blocker
,)
210 # Add to new blockers' queues
211 for blocker
in new_blockers
:
212 blocker
.add_task(self
)
213 self
._rox
_blockers
= new_blockers
216 if self
.name
is None:
218 return "[Task '%s']" % self
.name
220 # Must append to _run_queue right after calling this!
222 assert not _run_queue
224 g
.idle_add(_handle_run_queue
)
226 def _handle_run_queue():
233 if next
is _idle_blocker
:
234 # Since this blocker will never run again, create a
235 # new one for future idling.
236 _idle_blocker
= IdleBlocker()
238 tasks
= next
._rox
_lib
_tasks
.keys()
239 #print "Resume", tasks