Why can't set "defaultTextFormat" directly? - actionscript-3

Question

for example,we can set graphics of a shape directly(without creating an external Graphics variable):
var my_shape:Shape=new Shape();
my_shape.graphics.beginFill(0);
but that's not same as defaultTextFormat
the below code doesn't work:
var my_text:TextField=new TextField();
my_text.defaultTextFormat.size=47;
typing dot after defaultTextFormat,the code hint of text format appears and there is no compiler error but still doesn't work
we must create an external TextFormat variable:
var my_text:TextField=new TextField();
var my_format:TextFormat=new TextFormat();
my_format.size=47;
my_text.defaultTextFormat=my_format;
but why can't set directly?
I don't like a lot of variables.
after that,explain the difference between textFormat and Graphics.
Thanks for your help.

Answer 1

When you access/read a TextField's defaultTextFormat property (which is what's happening in the line my_text.defaultTextFormat.size=47;), you end up getting a whole new object returned. Eg, it creates a new TextFormat and returns that.
Here is an example to illustrate:
var tf:TextFormat = new TextFormat();
textField.defaultTextFormat = tf;
trace(tf == textField.defaultTextFormat) //false
The TextField doesn't know anything about the TextFormat it returns from defaultTextFormat. So when you change it, it doesn't update anything automatically because it has no scope inside the TextField that generated it.
In order to see the change, you have to reassign the whole object, and
then reassign the text (if you've already set the text).
This unfortunately means you'll have do it like in your second example.
It's probably some kind of an efficiency thing under the hood to help prevent memory leaks and the like.
Here are some examples to consider further:
var txt:TextField = new TextField();
addChild(txt);
var tf:TextFormat;
txt.text = "hi"; //default formatting;
tf = txt.defaultTextFormat; //get the default formatting, which actually returns a brand new object
tf.color = 0xFF0000; //make it red;
//nothing has changed visually
txt.defaultTextFormat = tf; //this won't update it either
//nothing has changed visually
txt.text = txt.text; //now that we've 'changed' the text, you'll see red

Answer 2

my_text.defaultTextFormat = my_format;
my_format is the defaultTextFormat of your textField my_text. defaultTextFormat is a property of your TextField (which value is my_format).
my_format.size = 47;
47 is the size of your TextFormat my_format. size is a property of your TextFormat (which value is 47).
my_text.defaultTextFormat.size = 47;
...but size has never been a property of a defaultTextFormat.
So you cannot put properties directly on the defaultTextFormat. What
you need to do is to make a text format, set the properties, THEN set
defaultTextFormat = myTextFormat.
Adobe help about defaultTextFormat.