Setting properties' values in MSBuild - msbuild

Question

Let's consider the below example.
There, I have:
target MAIN calls target t and then calls target tt.
target t calls target ttt, and target tt calls target tttt.
target t defines property aa, and target ttt modifies aa.
target tttt tries to print property aa's value.
In short, we have: MAIN -> {t -> {ttt->modifies aa, defines aa}, tt -> tttt -> prints aa}
But in target tttt, we can't "see" aa's updated value (by ttt)! How do I make that value visible to target tttt?
The whole script is as below:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="MAIN" >
<Target Name="MAIN" >
<CallTarget Targets="t" />
<CallTarget Targets="tt" />
</Target>
<Target Name="t">
<Message Text="t" />
<PropertyGroup>
<aa>1</aa>
</PropertyGroup>
<CallTarget Targets="ttt" />
</Target>
<Target Name="tt">
<Message Text="tt" />
<CallTarget Targets="tttt" />
</Target>
<Target Name="ttt">
<PropertyGroup>
<aa>122</aa>
</PropertyGroup>
<Message Text="ttt" />
</Target>
<Target Name="tttt">
<Message Text="tttt" />
<Message Text="tttt:$(aa)" />
</Target>
</Project>

Answer 1

As already said in an answer to another post you should model your MSBuild project with dependencies between your Targets rather than calling Targets one after another.
<Project DefaultTargets="tttt" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Target Name="t">
<Message Text="t" />
<PropertyGroup>
<aa>1</aa>
</PropertyGroup>
</Target>
<Target Name="tt" DependsOnTargets="t">
<Message Text="tt" />
</Target>
<Target Name="ttt" DependsOnTargets="t;tt">
<PropertyGroup>
<aa>122</aa>
</PropertyGroup>
<Message Text="ttt" />
</Target>
<Target Name="tttt" DependsOnTargets="t;tt;ttt">
<Message Text="tttt" />
<Message Text="tttt:$(aa)" />
</Target>
</Project>
An approach I use, is to define a Target as my final goal, putting it into the projects DefaultTargets.
Then add all the things that need to happen to achieve this goal.