是以下错误中的加号吗：android：icon =“ @ + drawable /

Question

I have been looking at this Android drawing app on github:

https://github.com/valerio-bozzolan/AcrylicPaint

In an XML file defining the layout for a menu, all the android:icon lines have a plus sign before the drawable . None of the drawable examples I've found on stackoverflow or elsewhere have that plus sign. And Android Studio 2.2.3 is giving me a "Cannot resolve symbol" error for each of them. If I remove the plus sign, that error goes away. But the app seems to work fine with the plus signs, too. Are the plus signs just plain wrong? Or is there something going on here that I don't understand?

The file is /app/res/menu/main.xml, if you want to look at it. Here's a screenshot of some sample code, with errors in red text. Thanks for any help you can provide.

Answer 1

Sloppy Mistake .You must use @drawable instead of @+drawable .

android:icon="@drawable/

Shows you to access drawables that you keep inside your app's drawable folder.

Answer 2

So the plus sign looks the mystery in your case. Let me give a detailed explanation of the plus sign.

1. When we give an id to any view, it is in general new and that id reference does not exist in R.id class.
   So when you use something like findViewById(R.id.view) , you can not access that particular id since it is not available.
   So to notify the complier that a new reference has to be added in R.id file we declare id as android:@+id/view , here + means add this id to R.id reference.

2. But in case of drawables, styles, colors, etc., you first create them in their respective folders which is accessible by the compiler (say Gradle) and is automatically reference in their respective class like R.drawable, R.style, etc.
   So when we refer them in xml, we don't use + sign as that reference is already created during their respective declarations.

Answer 3

Only id is coded as android:id="@+id/dummy"

For drawable and other things you need not have + in it

It can be used like android:icon="@drawable/yourImage"

android:style="@style/myStyle"

Answer 4

Unlike android:id="@+id/aSampleId" , you cannot generate images on the fly. If you want to access the drawable folder then you need to have an image in the folder itself.

Suppose you have an image in the drawable folder drawable/anImage.png , then you would need to call on it with android:icon="@drawable/anImage"