如何为Android指定null作为XML属性？

Question

What do I need to put in for an attribute if I want to put a null there?

For example, this piece of code:

<?xml version="1.0" encoding="utf-8"?>
<transition xmlns:android="http://schemas.android.com/apk/res/android" >

    <item android:drawable="@drawable/ef_gradient_blue" />
    <item android:drawable="@null" />

</transition>

But this won't work, as my app crashes when it tries to load this drawable. I need it to be null, so it can make a transition from fully opaque to completely see through.

In my Java, I load it like this:

mTransitionBlue = (TransitionDrawable)mProjectResources.getDrawable(R.drawable.transition_blue);

But I get a runtime exception from this, saying it can't find the transition_blue.xml, which is the XML specified before. So how do I achieve this?

Answer 1

You should use a transparent drawable there (android lib has it predefined):

<transition xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/ef_gradient_blue" />
    <item android:drawable="@android:color/transparent" />
</transition>

the @android:color/transparent 's value is #00000000 . It's important to have the first two digits 0, since they define the alpha value of the color (the rest is red, green and blue).
So you can use

<item android:drawable="#00000000" />

as well, or redefine it among your colors.

For reference you could take a look at the Android Developers 2D Graphics and Transition Drawables articles.