Objective-C : Documentation : Syntax : Messages

Messages are enclosed by square brackets and are formatted:

[reciever message];

The reciever of the message must be of type Class(a class) or id(an object).

Example:

[dog rollover];

Arguments

A message can have any number of arguments. Arguments are seperated by colons and each argument may have a name associated with it.

Examples:

Message with single argument:

[dog fetch:stick];

Message with two arguments:

[dog bring:paper to:me];

Argument names are optional:

[aPoint set:3 :6 :5];

You can even do things like:

[dog barkAt:aCat ifYouSeeOne];

Messages within Messages

Just as C functions can be used as arguments in other C functions,

messages and expressions can also be placed within messages.

For example:

[[myAunt phone] setTo:[myUncle phone]];

Another example:

[[vector origin] setXValue:( cos(25.0) + pi )/2];

Selectors

Selectors are the run-time system's identifier for a method. The SEL data type is used for selectors.

The sel_get_uid() function can be used to get a method's selector from it's name:

SEL myselector = sel_get_uid("bark");

The - perform: method is used to send a message to an object using a selector.

[dog perform:sel_get_uid("bark")];

is the same as:

[dog bark];

Arguments can also be sent:

[dog perform:sel_get_uid("fetch:") with:stick];

is the same as:

[dog fetch:stick];

Selectors are usefull for changing which message is sent to an object at runtime:

strcpy(astring, "bark");

[dog perform:sel_get_uid(astring)];

Or:

SEL myselector = sel_get_uid("bark");

[dog perform:myselector];

This is especially usefull for sending messages to a list of objects:

[viewList makeObjectsPerform:sel_get_uid("draw")];