首先我们定义状态类：
//================== State.h ==================
@interface State : NSObject<ActionProtocol>
{
@protected Coder *_coder;
}
- (instancetype)initWithCoder:(Coder *)coder;
@end
//================== State.m ==================
@implementation State
- (instancetype)initWithCoder:(Coder *)coder{
self = [super init];
if (self) {
_coder = coder;
}
return self;
}
@end状态类持有一个coder，也就是程序员的实例，并遵循了ActionProtocol：
//================== ActionProtocol.h ==================
@protocol ActionProtocol <NSObject>
@optional;
- (void)wakeUp;
- (void)fallAsleep;
- (void)startCoding;
- (void)startEating;
@endActionProtocol定义了程序员的一些动作，这些动作是程序员的日常活动，也是触发状态切换的动作，因此State也需要遵循这个协议，因为它的子类需要实现这些操作。
接下来我们看一下State的子类，根据上面说的四种状态，我们定义下面四个状态子类：
StateAwake:
//================== StateAwake.h ==================
@interface StateAwake : State
@end
@implementation StateAwake
- (void)wakeUp{
NSLog(@"Already awake, can not change state to awake again");
}
- (void)startCoding{
NSLog(@"Change state from awake to coding");
[_coder setState:(State *)[_coder stateCoding]];
}
- (void)startEating{
NSLog(@"Change state from awake to eating");
[_coder setState:(State *)[_coder stateEating]];
}
- (void)fallAsleep{
NSLog(@"Change state from awake to sleeping");
[_coder setState:(State *)[_coder stateSleeping]];
}
@endStateSleeping:
//================== StateSleeping.h ==================
@interface StateSleeping : State
@end
//================== StateSleeping.m ==================
@implementation StateSleeping
- (void)wakeUp{
NSLog(@"Change state from sleeping to awake");
[_coder setState:(State *)[_coder stateAwake]];
}
- (void)startCoding{
NSLog(@"Already sleeping, can not change state to coding");
}
- (void)startEating{
NSLog(@"Already sleeping, can change state to eating");
}
- (void)fallAsleep{
NSLog(@"Already sleeping, can not change state to sleeping again");
}
@endStateEating:
//================== StateEating.h ==================
@interface StateEating : State
@end
//================== StateEating.m ==================
@implementation StateEating
- (void)wakeUp{
NSLog(@"Already awake, can not change state to awake again");
}
- (void)startCoding{
NSLog(@"New idea came out! change state from eating to coding");
[_coder setState:(State *)[_coder stateCoding]];
}
- (void)startEating{
NSLog(@"Already eating, can not change state to eating again");
}
- (void)fallAsleep{
NSLog(@"Too tired, change state from eating to sleeping");
[_coder setState:(State *)[_coder stateSleeping]];
}
@end"StateCoding":
//================== StateCoding.h ==================
@interface StateCoding : State
@end
//================== StateCoding.m ==================
@implementation StateCoding
- (void)wakeUp{
NSLog(@"Already awake, can not change state to awake again");
}
- (void)startCoding{
NSLog(@"Already coding, can not change state to coding again");
}
- (void)startEating{
NSLog(@"Too hungry, change state from coding to eating");
[_coder setState:(State *)[_coder stateEating]];
}
- (void)fallAsleep{
NSLog(@"Too tired, change state from coding to sleeping");
[_coder setState:(State *)[_coder stateSleeping]];
}
@end从上面的类可以看出，在有些状态之间的转换是失效的，有些是可以的。
比如相同状态的切换是无效的；从 sleeping无法切换到coding，但是反过来可以，因为可能写代码累了就直接睡了。
下面我们看一下程序员类的实现：
//================== Coder.h ==================
@interface Coder : NSObject<ActionProtocol>
@property (nonatomic, strong) StateAwake *stateAwake;
@property (nonatomic, strong) StateCoding *stateCoding;
@property (nonatomic, strong) StateEating *stateEating;
@property (nonatomic, strong) StateSleeping *stateSleeping;
- (void)setState:(State *)state;
@end
//================== Coder.m ==================
@implementation Coder
{
State *_currentState;
}
- (instancetype)init{
self = [super init];
if (self) {
_stateAwake = [[StateAwake alloc] initWithCoder:self];
_stateCoding = [[StateCoding alloc] initWithCoder:self];
_stateEating = [[StateEating alloc] initWithCoder:self];
_stateSleeping = [[StateSleeping alloc] initWithCoder:self];
_currentState = _stateAwake;
}
return self;
}
- (void)setState:(State *)state{
_currentState = state;
}
- (void)wakeUp{
[_currentState wakeUp];
}
- (void)startCoding{
[_currentState startCoding];
}
- (void)startEating{
[_currentState startEating];
}
- (void)fallAsleep{
[_currentState fallAsleep];
}
@end从上面的代码我们可以看到，程序员类持有一个当前的状态的实例，在初始化后默认的状态为awake，并对外提供一个setState:的方法来切换状态。而且在初始化方法里，我们实例化了所有的状态，目的是在切换状态中时使用，详见具体状态类的方法：
- (void)startEating{
NSLog(@"Too hungry, change state from coding to eating");
[_coder setState:(State *)[_coder stateEating]];
}上面这段代码有点绕，可能需要多看几遍源码才能理解（这里面
[_coder stateEating]是调用了coder的一个get方法，返回了stateEating这个实例）。
最后，在程序员的动作方法里面，实际上调用的是当前状态对应的方法（这也就是为何程序员类和状态类都要遵循ActionProtocol的原因）。
这样，我们的状态类，状态子类，程序员类都声明好了。我们看一下如何使用：
Coder *coder = [[Coder alloc] init];
//change to awake.. failed
[coder wakeUp];//Already awake, can not change state to awake again
//change to coding
[coder startCoding];//Change state from awake to coding
//change to sleep
[coder fallAsleep];//Too tired, change state from coding to sleeping
//change to eat...failed
[coder startEating];//Already sleeping, can change state to eating
//change to wake up
[coder wakeUp];//Change state from sleeping to awake
//change wake up...failed
[coder wakeUp];//Already awake, can not change state to awake again
//change to eating
[coder startEating];//Change state from awake to eating
//change to coding
[coder startCoding];//New idea came out! change state from eating to coding
//change to sleep
[coder fallAsleep];//Too tired, change state from coding to sleeping在上面的代码里，我们实例化了一个程序员类，接着不断调用一些触发状态改变的方法。我们把每次状态切换的日至输出注释到了代码右侧，可以看到在一些状态的切换是不允许的：
- 比如从上到下的第一个
[coder wakeUp]：因为程序员对象初始化后默认是awake状态，所以无法切换到相同的状态 - 比如从上到下的第一个
[coder startEating]:在睡觉时是无法直接切换到eating状态；而在后面wake以后，再执行[coder startEating]就成功了。
从上面的例子可以看出，使用状态模式不需要去写if-else，而且如果今后想添加一个状态，只需要再创建一个状态子类，并在新的状态子类添加好对所有状态的处理，并在之前的状态子类中添加上对新状态的处理即可。即便我们修改了之前定义好的状态子类，但是这样也总比使用庞大的if-else要方便多。