Constraints with once

A constraint is dynamically evaluated and this imposes some penalty on performance. Sometimes, you don’t want or don’t need a constraint to be evaluated continuously, but only when your object is initialized. For this, you can use the once keyword. For example, the constraint ${parent.width} in the view in Listing 6.4 is probably only required to be initialized once, because the canvas width never changes throughout its lifetime.

Listing 6.4. A constraint that gets evaluated all the time

<canvas>
    <view y="0" bgcolor="0xbbccdd" height="4"
            width="${parent.width}">
</canvas>

In this case, you can use once for better performance, as shown in Listing 6.5:

Listing 6.5. A constraint that gets evaluated once

<canvas>
    <view y="0" bgcolor="0xbbccdd" height="4"
            width="$once{parent.width}">
</canvas>

If no keyword is present between $ and {, the value of always is implied. Using once can help your application perform better.