/**
* {@hide}
*
* <p>The state machine defined here is a hierarchical state machine which processes messages
* and can have states arranged hierarchically.</p>
*
* <p>A state is a <code>State</code> object and must implement
* <code>processMessage</code> and optionally <code>enter/exit/getName</code>.
* The enter/exit methods are equivalent to the construction and destruction
* in Object Oriented programming and are used to perform initialization and
* cleanup of the state respectively. The <code>getName</code> method returns the
* name of the state; the default implementation returns the class name. It may be
* desirable to have <code>getName</code> return the the state instance name instead,
* in particular if a particular state class has multiple instances.</p>
*
* <p>When a state machine is created, <code>addState</code> is used to build the
* hierarchy and <code>setInitialState</code> is used to identify which of these
* is the initial state. After construction the programmer calls <code>start</code>
* which initializes and starts the state machine. The first action the StateMachine
* is to the invoke <code>enter</code> for all of the initial state's hierarchy,
* starting at its eldest parent. The calls to enter will be done in the context
* of the StateMachine's Handler, not in the context of the call to start, and they
* will be invoked before any messages are processed. For example, given the simple
* state machine below, mP1.enter will be invoked and then mS1.enter. Finally,
* messages sent to the state machine will be processed by the current state;
* in our simple state machine below that would initially be mS1.processMessage.</p>
<pre>
mP1
/ \
mS2 mS1 ----> initial state
</pre>
* <p>After the state machine is created and started, messages are sent to a state
* machine using <code>sendMessage</code> and the messages are created using
* <code>obtainMessage</code>. When the state machine receives a message the
* current state's <code>processMessage</code> is invoked. In the above example
* mS1.processMessage will be invoked first. The state may use <code>transitionTo</code>
* to change the current state to a new state.</p>
*
* <p>Each state in the state machine may have a zero or one parent states. If
* a child state is unable to handle a message it may have the message processed
* by its parent by returning false or NOT_HANDLED. If a message is not handled by
* a child state or any of its ancestors, <code>unhandledMessage</code> will be invoked
* to give one last chance for the state machine to process the message.</p>
*
* <p>When all processing is completed a state machine may choose to call
* <code>transitionToHaltingState</code>. When the current <code>processingMessage</code>
* returns the state machine will transfer to an internal <code>HaltingState</code>
* and invoke <code>halting</code>. Any message subsequently received by the state
* machine will cause <code>haltedProcessMessage</code> to be invoked.</p>
*
* <p>If it is desirable to completely stop the state machine call <code>quit</code> or
* <code>quitNow</code>. These will call <code>exit</code> of the current state and its parents,
* call <code>onQuitting</code> and then exit Thread/Loopers.</p>
*
* <p>In addition to <code>processMessage</code> each <code>State</code> has
* an <code>enter</code> method and <code>exit</code> method which may be overridden.</p>
*
* <p>Since the states are arranged in a hierarchy transitioning to a new state
* causes current states to be exited and new states to be entered. To determine
* the list of states to be entered/exited the common parent closest to
* the current state is found. We then exit from the current state and its
* parent's up to but not including the common parent state and then enter all
* of the new states below the common parent down to the destination state.
* If there is no common parent all states are exited and then the new states
* are entered.</p>
*
* <p>Two other methods that states can use are <code>deferMessage</code> and
* <code>sendMessageAtFrontOfQueue</code>. The <code>sendMessageAtFrontOfQueue</code> sends
* a message but places it on the front of the queue rather than the back. The
* <code>deferMessage</code> causes the message to be saved on a list until a
* transition is made to a new state. At which time all of the deferred messages
* will be put on the front of the state machine queue with the oldest message
* at the front. These will then be processed by the new current state before
* any other messages that are on the queue or might be added later. Both of
* these are protected and may only be invoked from within a state machine.</p>
*
* <p>To illustrate some of these properties we'll use state machine with an 8
* state hierarchy:</p>
<pre>
mP0
/ \
mP1 mS0
/ \
mS2 mS1
/ \ \
mS3 mS4 mS5 ---> initial state
</pre>
* <p>After starting mS5 the list of active states is mP0, mP1, mS1 and mS5.
* So the order of calling processMessage when a message is received is mS5,
* mS1, mP1, mP0 assuming each processMessage indicates it can't handle this
* message by returning false or NOT_HANDLED.</p>
*
* <p>Now assume mS5.processMessage receives a message it can handle, and during
* the handling determines the machine should change states. It could call
* transitionTo(mS4) and return true or HANDLED. Immediately after returning from
* processMessage the state machine runtime will find the common parent,
* which is mP1. It will then call mS5.exit, mS1.exit, mS2.enter and then
* mS4.enter. The new list of active states is mP0, mP1, mS2 and mS4. So
* when the next message is received mS4.processMessage will be invoked.</p>
*
* <p>Now for some concrete examples, here is the canonical HelloWorld as a state machine.
* It responds with "Hello World" being printed to the log for every message.</p>
<pre>
class HelloWorld extends StateMachine {
HelloWorld(String name) {
super(name);
addState(mState1);
setInitialState(mState1);
}
public static HelloWorld makeHelloWorld() {
HelloWorld hw = new HelloWorld("hw");
hw.start();
return hw;
}
class State1 extends State {
@Override public boolean processMessage(Message message) {
log("Hello World");
return HANDLED;
}
}
State1 mState1 = new State1();
}
void testHelloWorld() {
HelloWorld hw = makeHelloWorld();
hw.sendMessage(hw.obtainMessage());
}
</pre>
* <p>A more interesting state machine is one with four states
* with two independent parent states.</p>
<pre>
mP1 mP2
/ \
mS2 mS1
</pre>
* <p>Here is a description of this state machine using pseudo code.</p>
<pre>
state mP1 {
enter { log("mP1.enter"); }
exit { log("mP1.exit"); }
on msg {
CMD_2 {
send(CMD_3);
defer(msg);
transitionTo(mS2);
return HANDLED;
}
return NOT_HANDLED;
}
}
INITIAL
state mS1 parent mP1 {
enter { log("mS1.enter"); }
exit { log("mS1.exit"); }
on msg {
CMD_1 {
transitionTo(mS1);
return HANDLED;
}
return NOT_HANDLED;
}
}
state mS2 parent mP1 {
enter { log("mS2.enter"); }
exit { log("mS2.exit"); }
on msg {
CMD_2 {
send(CMD_4);
return HANDLED;
}
CMD_3 {
defer(msg);
transitionTo(mP2);
return HANDLED;
}
return NOT_HANDLED;
}
}
state mP2 {
enter {
log("mP2.enter");
send(CMD_5);
}
exit { log("mP2.exit"); }
on msg {
CMD_3, CMD_4 { return HANDLED; }
CMD_5 {
transitionTo(HaltingState);
return HANDLED;
}
return NOT_HANDLED;
}
}
</pre>
* <p>The implementation is below and also in StateMachineTest:</p>
<pre>
class Hsm1 extends StateMachine {
public static final int CMD_1 = 1;
public static final int CMD_2 = 2;
public static final int CMD_3 = 3;
public static final int CMD_4 = 4;
public static final int CMD_5 = 5;
public static Hsm1 makeHsm1() {
log("makeHsm1 E");
Hsm1 sm = new Hsm1("hsm1");
sm.start();
log("makeHsm1 X");
return sm;
}
Hsm1(String name) {
super(name);
log("ctor E");
// Add states, use indentation to show hierarchy
addState(mP1);
addState(mS1, mP1);
addState(mS2, mP1);
addState(mP2);
// Set the initial state
setInitialState(mS1);
log("ctor X");
}
class P1 extends State {
@Override public void enter() {
log("mP1.enter");
}
@Override public boolean processMessage(Message message) {
boolean retVal;
log("mP1.processMessage what=" + message.what);
switch(message.what) {
case CMD_2:
// CMD_2 will arrive in mS2 before CMD_3
sendMessage(obtainMessage(CMD_3));
deferMessage(message);
transitionTo(mS2);
retVal = HANDLED;
break;
default:
// Any message we don't understand in this state invokes unhandledMessage
retVal = NOT_HANDLED;
break;
}
return retVal;
}
@Override public void exit() {
log("mP1.exit");
}
}
class S1 extends State {
@Override public void enter() {
log("mS1.enter");
}
@Override public boolean processMessage(Message message) {
log("S1.processMessage what=" + message.what);
if (message.what == CMD_1) {
// Transition to ourself to show that enter/exit is called
transitionTo(mS1);
return HANDLED;
} else {
// Let parent process all other messages
return NOT_HANDLED;
}
}
@Override public void exit() {
log("mS1.exit");
}
}
class S2 extends State {
@Override public void enter() {
log("mS2.enter");
}
@Override public boolean processMessage(Message message) {
boolean retVal;
log("mS2.processMessage what=" + message.what);
switch(message.what) {
case(CMD_2):
sendMessage(obtainMessage(CMD_4));
retVal = HANDLED;
break;
case(CMD_3):
deferMessage(message);
transitionTo(mP2);
retVal = HANDLED;
break;
default:
retVal = NOT_HANDLED;
break;
}
return retVal;
}
@Override public void exit() {
log("mS2.exit");
}
}
class P2 extends State {
@Override public void enter() {
log("mP2.enter");
sendMessage(obtainMessage(CMD_5));
}
@Override public boolean processMessage(Message message) {
log("P2.processMessage what=" + message.what);
switch(message.what) {
case(CMD_3):
break;
case(CMD_4):
break;
case(CMD_5):
transitionToHaltingState();
break;
}
return HANDLED;
}
@Override public void exit() {
log("mP2.exit");
}
}
@Override
void onHalting() {
log("halting");
synchronized (this) {
this.notifyAll();
}
}
P1 mP1 = new P1();
S1 mS1 = new S1();
S2 mS2 = new S2();
P2 mP2 = new P2();
}
</pre>
* <p>If this is executed by sending two messages CMD_1 and CMD_2
* (Note the synchronize is only needed because we use hsm.wait())</p>
<pre>
Hsm1 hsm = makeHsm1();
synchronize(hsm) {
hsm.sendMessage(obtainMessage(hsm.CMD_1));
hsm.sendMessage(obtainMessage(hsm.CMD_2));
try {
// wait for the messages to be handled
hsm.wait();
} catch (InterruptedException e) {
loge("exception while waiting " + e.getMessage());
}
}
</pre>
* <p>The output is:</p>
<pre>
D/hsm1 ( 1999): makeHsm1 E
D/hsm1 ( 1999): ctor E
D/hsm1 ( 1999): ctor X
D/hsm1 ( 1999): mP1.enter
D/hsm1 ( 1999): mS1.enter
D/hsm1 ( 1999): makeHsm1 X
D/hsm1 ( 1999): mS1.processMessage what=1
D/hsm1 ( 1999): mS1.exit
D/hsm1 ( 1999): mS1.enter
D/hsm1 ( 1999): mS1.processMessage what=2
D/hsm1 ( 1999): mP1.processMessage what=2
D/hsm1 ( 1999): mS1.exit
D/hsm1 ( 1999): mS2.enter
D/hsm1 ( 1999): mS2.processMessage what=2
D/hsm1 ( 1999): mS2.processMessage what=3
D/hsm1 ( 1999): mS2.exit
D/hsm1 ( 1999): mP1.exit
D/hsm1 ( 1999): mP2.enter
D/hsm1 ( 1999): mP2.processMessage what=3
D/hsm1 ( 1999): mP2.processMessage what=4
D/hsm1 ( 1999): mP2.processMessage what=5
D/hsm1 ( 1999): mP2.exit
D/hsm1 ( 1999): halting
</pre>
*/
