The NSRunLoop class declares the programmatic interface to objects that manage input sources. An NSRunLoop object processes input for sources such as mouse and keyboard events from the window system, NSPort objects, and NSConnection objects. An NSRunLoop object also processes NSTimer events.

NSRunLoop类声明了管理输入源（input sources）的可编程接口。一个NSRunLoop对象处理像鼠标和键盘事件这样的来自于窗口系统（ window system）、NSPort对象和 NSConnection对象的输入源（ input for source）。NSRunLoop也可以处理NSTimer事件。

Your application cannot either create or explicitly manage NSRunLoop objects. Each NSThread object, including the application’s main thread, has an NSRunLoop object automatically created for it as needed. If you need to access the current thread’s run loop, you do so with the class method currentRunLoop.

你的应用不能创建或管理NSRunLoop对象。每一个NSThread对象，包括应用的主线程（main thread）都有一个可以在需要时自动创建的NSRunLoop对象。如果你需要访问当前线程的run loop，你可以调用类方法currentRunLoop获得。

Note that from the perspective of NSRunloop, NSTimer objects are not "input"—they are a special type, and one of the things that means is that they do not cause the run loop to return when they fire.

从NSRunloop的角度来看，NSTimer对象不是输入（input）而是一种特殊的类型，这意味着当NSTimer对象被触发（fire）时，不会导致run loop返回（return）。

WARNING
The NSRunLoop class is generally not considered to be thread-safe and its methods should only be called within the context of the current thread. You should never try to call the methods of an NSRunLoop object running in a different thread, as doing so might cause unexpected results.

注意
NSRunLoop类一般不被认为是线程安全的，其方法只应该在当前线程的上下文中调用。你永远不要去试图调用另一个线程中正在运行的NSRunLoop对象，这样做将会导致不可预期的结果。

Accessing Run Loops and Modes
访问Run Loops和

• currentRunLoop
  Returns the NSRunLoop object for the current thread.

Declaration
OBJECTIVE-C

• (NSRunLoop *)currentRunLoop
  Return Value
  The NSRunLoop object for the current thread.

Discussion
If a run loop does not yet exist for the thread, one is created and returned.

Availability
Available in iOS 2.0 and later.
See Also
– currentMode

currentMode
Property
The receiver's current input mode. (read-only)

Declaration
OBJECTIVE-C
@property(readonly, copy) NSString *currentMode
Discussion
The receiver's current input mode. This method returns the current input mode only while the receiver is running; otherwise, it returns nil.

The current mode is set by the methods that run the run loop, such as acceptInputForMode:beforeDate: and runMode:beforeDate:.

Availability
Available in iOS 2.0 and later.
See Also

• currentRunLoop
  – limitDateForMode:
  – run
  – runUntilDate:

• limitDateForMode:
  Performs one pass through the run loop in the specified mode and returns the date at which the next timer is scheduled to fire.

Declaration
OBJECTIVE-C

• (NSDate *)limitDateForMode:(NSString *)mode
  Parameters
  mode
  The run loop mode to search. You may specify custom modes or use one of the modes listed in Run Loop Modes.
  Return Value
  The date at which the next timer is scheduled to fire, or nil if there are no input sources for this mode.

Discussion
The run loop is entered with an immediate timeout, so the run loop does not block, waiting for input, if no input sources need processing.

Availability
Available in iOS 2.0 and later.

• mainRunLoop
  Returns the run loop of the main thread.

Declaration
OBJECTIVE-C

• (NSRunLoop *)mainRunLoop
  Return Value
  An object representing the main thread’s run loop.

Availability
Available in iOS 2.0 and later.

• getCFRunLoop
  Returns the receiver's underlying CFRunLoop Reference object.

Declaration
OBJECTIVE-C

• (CFRunLoopRef)getCFRunLoop
  Return Value
  The receiver's underlying CFRunLoop Reference object.

Discussion
You can use the returned run loop to configure the current run loop using Core Foundation function calls. For example, you might use this function to set up a run loop observer.

Availability
Available in iOS 2.0 and later.
Managing Timers

• addTimer:forMode:
  Registers a given timer with a given input mode.

Declaration
OBJECTIVE-C

• (void)addTimer:(NSTimer *)aTimer forMode:(NSString *)mode
  Parameters
  aTimer
  The timer to register with the receiver.
  mode
  The mode in which to add aTimer. You may specify a custom mode or use one of the modes listed in Run Loop Modes.
  Discussion
  You can add a timer to multiple input modes. While running in the designated mode, the receiver causes the timer to fire on or after its scheduled fire date. Upon firing, the timer invokes its associated handler routine, which is a selector on a designated object.

The receiver retains aTimer. To remove a timer from all run loop modes on which it is installed, send an invalidate message to the timer.

Availability
Available in iOS 2.0 and later.
Managing Ports

• addPort:forMode:
  Adds a port as an input source to the specified mode of the run loop.

Declaration
OBJECTIVE-C

• (void)addPort:(NSPort *)aPort forMode:(NSString *)mode
  Parameters
  aPort
  The port to add to the receiver.
  mode
  The mode in which to add aPort. You may specify a custom mode or use one of the modes listed in Run Loop Modes.
  Discussion
  This method schedules the port with the receiver. You can add a port to multiple input modes. When the receiver is running in the specified mode, it dispatches messages destined for that port to the port’s designated handler routine.

Availability
Available in iOS 2.0 and later.
See Also
– removePort:forMode:

• removePort:forMode:
  Removes a port from the specified input mode of the run loop.

Declaration
OBJECTIVE-C

• (void)removePort:(NSPort *)aPort forMode:(NSString *)mode
  Parameters
  aPort
  The port to remove from the receiver.
  mode
  The mode from which to remove aPort. You may specify a custom mode or use one of the modes listed in Run Loop Modes.
  Discussion
  If you added the port to multiple input modes, you must remove it from each mode separately.

Availability
Available in iOS 2.0 and later.
See Also
– addPort:forMode:

Running a Loop

• run
  Puts the receiver into a permanent loop, during which time it processes data from all attached input sources.

Declaration
OBJECTIVE-C

• (void)run
  Discussion
  If no input sources or timers are attached to the run loop, this method exits immediately; otherwise, it runs the receiver in the NSDefaultRunLoopMode by repeatedly invoking runMode:beforeDate:. In other words, this method effectively begins an infinite loop that processes data from the run loop’s input sources and timers.

Manually removing all known input sources and timers from the run loop is not a guarantee that the run loop will exit. OS X can install and remove additional input sources as needed to process requests targeted at the receiver’s thread. Those sources could therefore prevent the run loop from exiting.

If you want the run loop to terminate, you shouldn't use this method. Instead, use one of the other run methods and also check other arbitrary conditions of your own, in a loop. A simple example would be:

BOOL shouldKeepRunning = YES; // global
NSRunLoop *theRL = [NSRunLoop currentRunLoop];
while (shouldKeepRunning && [theRL runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]);
where shouldKeepRunning is set to NO somewhere else in the program.

Availability
Available in iOS 2.0 and later.
See Also
– runUntilDate:

• runMode:beforeDate:
  Runs the loop once, blocking for input in the specified mode until a given date.

Declaration
OBJECTIVE-C

• (BOOL)runMode:(NSString *)mode beforeDate:(NSDate *)limitDate
  Parameters
  mode
  The mode in which to run. You may specify custom modes or use one of the modes listed in Run Loop Modes.
  limitDate
  The date until which to block.
  Return Value
  YES if the run loop ran and processed an input source or if the specified timeout value was reached; otherwise, NO if the run loop could not be started.

Discussion
If no input sources or timers are attached to the run loop, this method exits immediately and returns NO; otherwise, it returns after either the first input source is processed or limitDate is reached. Manually removing all known input sources and timers from the run loop does not guarantee that the run loop will exit immediately. OS X may install and remove additional input sources as needed to process requests targeted at the receiver’s thread. Those sources could therefore prevent the run loop from exiting.

NOTE
A timer is not considered an input source and may fire multiple times while waiting for this method to return

Availability
Available in iOS 2.0 and later.
See Also
– run
– runUntilDate:

• runUntilDate:
  Runs the loop until the specified date, during which time it processes data from all attached input sources.

Declaration
OBJECTIVE-C

• (void)runUntilDate:(NSDate *)limitDate
  Parameters
  limitDate
  The date up until which to run.
  Discussion
  If no input sources or timers are attached to the run loop, this method exits immediately; otherwise, it runs the receiver in the NSDefaultRunLoopMode by repeatedly invoking runMode:beforeDate: until the specified expiration date.

Manually removing all known input sources and timers from the run loop is not a guarantee that the run loop will exit. OS X can install and remove additional input sources as needed to process requests targeted at the receiver’s thread. Those sources could therefore prevent the run loop from exiting.

Availability
Available in iOS 2.0 and later.
See Also
– run

• acceptInputForMode:beforeDate:
  Runs the loop once or until the specified date, accepting input only for the specified mode.

Declaration
OBJECTIVE-C

• (void)acceptInputForMode:(NSString *)mode beforeDate:(NSDate *)limitDate
  Parameters
  mode
  The mode in which to run. You may specify custom modes or use one of the modes listed in Run Loop Modes.
  limitDate
  The date up until which to run.
  Discussion
  If no input sources or timers are attached to the run loop, this method exits immediately; otherwise, it runs the run loop once, returning as soon as one input source processes a message or the specifed time elapses.

NOTE
A timer is not considered an input source and may fire multiple times while waiting for this method to return

Manually removing all known input sources and timers from the run loop is not a guarantee that the run loop will exit. OS X can install and remove additional input sources as needed to process requests targeted at the receiver’s thread. Those sources could therefore prevent the run loop from exiting.

Availability
Available in iOS 2.0 and later.
See Also
– runMode:beforeDate:

Scheduling and Canceling Messages

• performSelector:target:argument:order:modes:
  Schedules the sending of a message on the current run loop.

Declaration
OBJECTIVE-C

• (void)performSelector:(SEL)aSelector target:(id)target argument:(id)anArgument order:(NSUInteger)order modes:(NSArray<NSString *> *)modes
  Parameters
  aSelector
  A selector that identifies the method to invoke. This method should not have a significant return value and should take a single argument of type id.
  target
  The object that defines the selector in aSelector.
  anArgument
  The argument to pass to the method when it is invoked. Pass nil if the method does not take an argument.
  order
  The priority for the message. If multiple messages are scheduled, the messages with a lower order value are sent before messages with a higher order value.
  modes
  An array of input modes for which the message may be sent. You may specify custom modes or use one of the modes listed in Run Loop Modes.
  Discussion
  This method sets up a timer to perform the aSelector message on the current thread’s run loop at the start of the next run loop iteration. The timer is configured to run in the modes specified by the modes parameter. When the timer fires, the thread attempts to dequeue the message from the run loop and perform the selector. It succeeds if the run loop is running and in one of the specified modes; otherwise, the timer waits until the run loop is in one of those modes.

This method returns before the aSelector message is sent. The receiver retains the target and anArgument objects until the timer for the selector fires, and then releases them as part of its cleanup.

Use this method if you want multiple messages to be sent after the current event has been processed and you want to make sure these messages are sent in a certain order.

Availability
Available in iOS 2.0 and later.
See Also
– cancelPerformSelector:target:argument:

• cancelPerformSelector:target:argument:
  Cancels the sending of a previously scheduled message.

Declaration
OBJECTIVE-C

• (void)cancelPerformSelector:(SEL)aSelector target:(id)target argument:(id)anArgument
  Parameters
  aSelector
  The previously-specified selector.
  target
  The previously-specified target.
  anArgument
  The previously-specified argument.
  Discussion
  You can use this method to cancel a message previously scheduled using the performSelector:target:argument:order:modes: method. The parameters identify the message you want to cancel and must match those originally specified when the selector was scheduled. This method removes the perform request from all modes of the run loop.

Availability
Available in iOS 2.0 and later.

• cancelPerformSelectorsWithTarget:
  Cancels all outstanding ordered performs scheduled with a given target.

Declaration
OBJECTIVE-C

• (void)cancelPerformSelectorsWithTarget:(id)target
  Parameters
  target
  The previously-specified target.
  Discussion
  This method cancels the previously scheduled messages associated with the target, ignoring the selector and argument of the scheduled operation. This is in contrast to cancelPerformSelector:target:argument:, which requires you to match the selector and argument as well as the target. This method removes the perform requests for the object from all modes of the run loop.

Availability
Available in iOS 2.0 and later.
Constants
Run Loop Modes
NSRunLoop defines the following run loop mode.

Declaration
OBJECTIVE-C
extern NSString* const NSDefaultRunLoopMode;
extern NSString* const NSRunLoopCommonModes;
Constants
NSDefaultRunLoopMode
The mode to deal with input sources other than NSConnection objects.

This is the most commonly used run-loop mode.

Available in iOS 2.0 and later.
NSRunLoopCommonModes
Objects added to a run loop using this value as the mode are monitored by all run loop modes that have been declared as a member of the set of “common" modes; see the description of CFRunLoopAddCommonMode for details.

Available in iOS 2.0 and later.