Develop Reference

Android custom image view using built-in attributes

It's my very first question here, so please go easy on me ;)
I've built my custom View class extending ImageView.
public class CustomImageView extends ImageView {
// ...
}
I have created a set of custom parameters for it in the shape of a <declare-styleable> item in the attrs.xml file.
<declare-styleable name="CustomImageView">
<attr name="angle" format="integer"/>
</declare-styleable>
I've figured out how to access (i.e. read from within the class and set from within the layout) these values.
TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.CustomImageView, 0, 0);
try {
a.getInt(R.styleable.CustomImageView_angle, 0);
} finally {
a.recycle();
}
So far, so easy. All of the above are directly taken from the guide.
However, I could not figure out how to access the inherited attributes of the ImageView class. Specifically, I want to read what was set as the src attribute of the ImageView. I'm assuming I have to use a different value for the second parameter of the obtainStyledAttributes(...) call, but I don't know what to use there and this obviously does not work:
a = context.getTheme().obtainStyledAttributes(attrs, ImageView, 0, 0);
So, how do I access the built-in attributes of my super class?
How do I get the int value (drawable res id) that was set for the android:src attribute?
Thanks for your help!

How do I get the int value (drawable res id) that was set for the
android:src attribute?
Use getAttributeResourceValue to get id of drawable :
public CustomImageView(Context context, AttributeSet attrs) {
super(context, attrs);
String android_schemas = "http://schemas.android.com/apk/res/android";
int srcId = attrs.getAttributeResourceValue(android_schemas, "src", -1);
}

Alter AttributeSet values in android

I am having custom view which will take attribute set(xml value) as constructor value
public CustomView(Context context) // No Attributes in this one.
{
super(context);
this(context, null, 0);
}
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
this(context, attrs, 0)
}
public CustomView(Context context, AttributeSet attrs, int default_style) {
super(context, attrs, default_style);
readAttrs(context, attrs, defStyle);
init();
}
In Fragment class i am setting the view as
CustomView customView = (CustomView) view.findViewById(R.id.customView);
where custom view contains various value such as height,width,padding etc.
i want to modify those values based on required condition and set it back to custom view.
I placed setting width height code in onDraw method and called invalidte view.
But above method will set the every time if i called invalidate method in CustomView class.
how to overcome this so that i can pass modified attribute set value in constructor only.?
Edit: I need to modify the view values(initialize with new values) which is set during attribute constructor so that i will get a refreshed view with a new values.
Override #OnDraw or 'Invalidate' is not a good function for me where inside invalidate i have written the methods which will execute in each second interval.

I see that your CustomView can have multiple attributes and you want to modify some of these attributes based on some condition and pass this in the constructor.
Few best practices while designing a custom view:
If you have custom attributes, make sure that you expose them via setters and getters. In your setter method, call invalidate();
Don't try modifying any attributes inside onDraw() or onMeasure() methods.
Try your best to avoid writing Custom constructors for your Custom View.
So the ideal way to solve your problem is to instantiate your CustomView and then modify the attributes, either externally (in your Activity or Fragment), or have a method inside the CustomView.java and then invoke it externally. Doing this will still give you the same result you are looking for.

So lets say you declared your custom attributes like this for view named StarsView
<declare-styleable name="StarsView">
<attr name="stars" format="integer" />
<attr name="score" format="float" />
</declare-styleable>
And you want to read attributes from something like this
<my.package..StarsView
app:stars="5"
app:score="4.6"
You do just this in constructor
public StarsView(Context context, AttributeSet attrs) {
super(context, attrs);
if(attrs != null) {
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.StarsView, defStyleAttr, 0);
stars = Tools.MathEx.clamp(1, 10, a.getInt(R.styleable.StarsView_stars, 5));
score = (int)Math.floor(a.getFloat(R.styleable.StarsView_score, stars) * 2f);
a.recycle(); // its important to call recycle after we are done
}
}

It's probably not the solution you were hoping for, but put a FrameLayout in your xml instead of the CustomView, and then create your CustomView programmatically with the FrameLayout as it's parent

How to require a view with specified id

I want to write a custom view that requires some button with a speicific id to be present (to be able to be found by id), as in ListActivity android.R.id.list must be present, how do i do the same with a custom id of my own?
I plan on reusing this view in an Android lib for several use cases, but all of them must declare some specific view with specific id so that i can find it by id in the lib code and manipulate it for later use in using Applications...

Just do what the ListActivity does.
Check for the ID in your custom view and throw an exception if it does not exist in the layout.
Snippet from ListActivity Source:
#Override
public void onContentChanged() {
super.onContentChanged();
View emptyView = findViewById(com.android.internal.R.id.empty);
mList = (ListView)findViewById(com.android.internal.R.id.list);
if (mList == null) {
throw new RuntimeException(
"Your content must have a ListView whose id attribute is " +
"'android.R.id.list'");
}

The best way to do this is in a flexible way is to use a custom attribute. It allows for any id to be the required id, but it also uses the dev tools to enforce that a valid id is used.
Declare that your custom view is style-able with a custom attribute in an attrs.xml file like this:
<resources>
<declare-styleable name="MyView">
<attr name="required_view_id" format="integer" />
</declare-styleable>
</resources>
You can then refer to the attribute from a layout file like below. Pay special attention to the header where the "app" namespace is defined. You can use any name you want for your custom attribute namespace, but you have to declare it to use any of your custom attributes when defining the views later. Note the custom attribute on MyView.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<View
android:id="#+id/the_id_of_the_required_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<com.full.package.to.MyView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:required_view_id="#id/the_id_of_the_required_view" />
</LinearLayout>
Now you need to make sure this view is present in your custom view class. You can required that your custom attribute is set by overriding certain constructors. You'll also need to actually verify the presence of the required view at some point. Here's a rough idea:
public class MyView extends View {
private int mRequiredId;
public MyView(Context context, AttributeSet attrs) {
super(context, attrs);
checkForRequiredViewAttr(context, attrs);
}
public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
checkForRequiredViewAttr(context, attrs);
}
// Verify that the required id attribute was set
private void checkForRequiredViewAttr(Context context, AttributeSet attrs) {
TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.MyView, 0, 0);
mRequiredId = a.getResourceId(R.styleable.MyView_required_view_id, -1);
a.recycle();
if (mRequiredId == -1) {
throw new RuntimeException("No required view id attribute was set");
}
}
// This allows the custom view to be programmatically instantiated, so long as
// the required id is manually set before adding it to a layout
public void setRequiredId(int id) {
mRequiredId = id;
}
// Check for the existence of a view with the required id
#Override
protected void onAttachedToWindow() {
View root = getRootView();
View requiredView = root.findViewById(mRequiredId);
if (requiredView == null) {
throw new RuntimeException(
String.format("Cannot find view in layout with id of %s", mRequiredId));
}
super.onAttachedToWindow();
}
}
Using onAttachedToWindow to check for the required view may not be good enough for your purposes. It won't, for example, prevent the required view from being removed. Finding a view in a layout isn't a cheap operation, especially for complex layouts, so you shouldn't constantly check for it.

Defining custom attrs

I need to implement my own attributes like in com.android.R.attr
Found nothing in official documentation so I need information about how to define these attrs and how to use them from my code.

Currently the best documentation is the source. You can take a look at it here (attrs.xml).
You can define attributes in the top <resources> element or inside of a <declare-styleable> element. If I'm going to use an attr in more than one place I put it in the root element. Note, all attributes share the same global namespace. That means that even if you create a new attribute inside of a <declare-styleable> element it can be used outside of it and you cannot create another attribute with the same name of a different type.
An <attr> element has two xml attributes name and format. name lets you call it something and this is how you end up referring to it in code, e.g., R.attr.my_attribute. The format attribute can have different values depending on the 'type' of attribute you want.
reference - if it references another resource id (e.g, "#color/my_color", "#layout/my_layout")
color
boolean
dimension
float
integer
string
fraction
enum - normally implicitly defined
flag - normally implicitly defined
You can set the format to multiple types by using |, e.g., format="reference|color".
enum attributes can be defined as follows:
<attr name="my_enum_attr">
<enum name="value1" value="1" />
<enum name="value2" value="2" />
</attr>
flag attributes are similar except the values need to be defined so they can be bit ored together:
<attr name="my_flag_attr">
<flag name="fuzzy" value="0x01" />
<flag name="cold" value="0x02" />
</attr>
In addition to attributes there is the <declare-styleable> element. This allows you to define attributes a custom view can use. You do this by specifying an <attr> element, if it was previously defined you do not specify the format. If you wish to reuse an android attr, for example, android:gravity, then you can do that in the name, as follows.
An example of a custom view <declare-styleable>:
<declare-styleable name="MyCustomView">
<attr name="my_custom_attribute" />
<attr name="android:gravity" />
</declare-styleable>
When defining your custom attributes in XML on your custom view you need to do a few things. First you must declare a namespace to find your attributes. You do this on the root layout element. Normally there is only xmlns:android="http://schemas.android.com/apk/res/android". You must now also add xmlns:whatever="http://schemas.android.com/apk/res-auto".
Example:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:whatever="http://schemas.android.com/apk/res-auto"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<org.example.mypackage.MyCustomView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
whatever:my_custom_attribute="Hello, world!" />
</LinearLayout>
Finally, to access that custom attribute you normally do so in the constructor of your custom view as follows.
public MyCustomView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.MyCustomView, defStyle, 0);
String str = a.getString(R.styleable.MyCustomView_my_custom_attribute);
//do something with str
a.recycle();
}
The end. :)

Qberticus's answer is good, but one useful detail is missing. If you are implementing these in a library replace:
xmlns:whatever="http://schemas.android.com/apk/res/org.example.mypackage"
with:
xmlns:whatever="http://schemas.android.com/apk/res-auto"
Otherwise the application that uses the library will have runtime errors.

The answer above covers everything in great detail, apart from a couple of things.
First, if there are no styles, then the (Context context, AttributeSet attrs) method signature will be used to instantiate the preference. In this case just use context.obtainStyledAttributes(attrs, R.styleable.MyCustomView) to get the TypedArray.
Secondly it does not cover how to deal with plaurals resources (quantity strings). These cannot be dealt with using TypedArray. Here is a code snippet from my SeekBarPreference that sets the summary of the preference formatting its value according to the value of the preference. If the xml for the preference sets android:summary to a text string or a string resouce the value of the preference is formatted into the string (it should have %d in it, to pick up the value). If android:summary is set to a plaurals resource, then that is used to format the result.
// Use your own name space if not using an android resource.
final static private String ANDROID_NS =
"http://schemas.android.com/apk/res/android";
private int pluralResource;
private Resources resources;
private String summary;
public SeekBarPreference(Context context, AttributeSet attrs) {
// ...
TypedArray attributes = context.obtainStyledAttributes(
attrs, R.styleable.SeekBarPreference);
pluralResource = attrs.getAttributeResourceValue(ANDROID_NS, "summary", 0);
if (pluralResource != 0) {
if (! resources.getResourceTypeName(pluralResource).equals("plurals")) {
pluralResource = 0;
}
}
if (pluralResource == 0) {
summary = attributes.getString(
R.styleable.SeekBarPreference_android_summary);
}
attributes.recycle();
}
#Override
public CharSequence getSummary() {
int value = getPersistedInt(defaultValue);
if (pluralResource != 0) {
return resources.getQuantityString(pluralResource, value, value);
}
return (summary == null) ? null : String.format(summary, value);
}
This is just given as an example, however, if you want are tempted to set the summary on the preference screen, then you need to call notifyChanged() in the preference's onDialogClosed method.

The traditional approach is full of boilerplate code and clumsy resource handling. That's why I made the Spyglass framework. To demonstrate how it works, here's an example showing how to make a custom view that displays a String title.
Step 1: Create a custom view class.
public class CustomView extends FrameLayout {
private TextView titleView;
public CustomView(Context context) {
super(context);
init(null, 0, 0);
}
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
init(attrs, 0, 0);
}
public CustomView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(attrs, defStyleAttr, 0);
}
#RequiresApi(21)
public CustomView(
Context context,
AttributeSet attrs,
int defStyleAttr,
int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
init(attrs, defStyleAttr, defStyleRes);
}
public void setTitle(String title) {
titleView.setText(title);
}
private void init(AttributeSet attrs, int defStyleAttr, int defStyleRes) {
inflate(getContext(), R.layout.custom_view, this);
titleView = findViewById(R.id.title_view);
}
}
Step 2: Define a string attribute in the values/attrs.xml resource file:
<resources>
<declare-styleable name="CustomView">
<attr name="title" format="string"/>
</declare-styleable>
</resources>
Step 3: Apply the #StringHandler annotation to the setTitle method to tell the Spyglass framework to route the attribute value to this method when the view is inflated.
#HandlesString(attributeId = R.styleable.CustomView_title)
public void setTitle(String title) {
titleView.setText(title);
}
Now that your class has a Spyglass annotation, the Spyglass framework will detect it at compile-time and automatically generate the CustomView_SpyglassCompanion class.
Step 4: Use the generated class in the custom view's init method:
private void init(AttributeSet attrs, int defStyleAttr, int defStyleRes) {
inflate(getContext(), R.layout.custom_view, this);
titleView = findViewById(R.id.title_view);
CustomView_SpyglassCompanion
.builder()
.withTarget(this)
.withContext(getContext())
.withAttributeSet(attrs)
.withDefaultStyleAttribute(defStyleAttr)
.withDefaultStyleResource(defStyleRes)
.build()
.callTargetMethodsNow();
}
That's it. Now when you instantiate the class from XML, the Spyglass companion interprets the attributes and makes the required method call. For example, if we inflate the following layout then setTitle will be called with "Hello, World!" as the argument.
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:width="match_parent"
android:height="match_parent">
<com.example.CustomView
android:width="match_parent"
android:height="match_parent"
app:title="Hello, World!"/>
</FrameLayout>
The framework isn't limited to string resources has lots of different annotations for handling other resource types. It also has annotations for defining default values and for passing in placeholder values if your methods have multiple parameters.
Have a look at the Github repo for more information and examples.

if you omit the format attribute from the attr element, you can use it to reference a class from XML layouts.
example from attrs.xml.
Android Studio understands that the class is being referenced from XML
i.e.
Refactor > Rename works
Find Usages works
and so on...
don't specify a format attribute in .../src/main/res/values/attrs.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
<declare-styleable name="MyCustomView">
....
<attr name="give_me_a_class"/>
....
</declare-styleable>
</resources>
use it in some layout file .../src/main/res/layout/activity__main_menu.xml
<?xml version="1.0" encoding="utf-8"?>
<SomeLayout
xmlns:app="http://schemas.android.com/apk/res-auto">
<!-- make sure to use $ dollar signs for nested classes -->
<MyCustomView
app:give_me_a_class="class.type.name.Outer$Nested/>
<MyCustomView
app:give_me_a_class="class.type.name.AnotherClass/>
</SomeLayout>
parse the class in your view initialization code .../src/main/java/.../MyCustomView.kt
class MyCustomView(
context:Context,
attrs:AttributeSet)
:View(context,attrs)
{
// parse XML attributes
....
private val giveMeAClass:SomeCustomInterface
init
{
context.theme.obtainStyledAttributes(attrs,R.styleable.ColorPreference,0,0).apply()
{
try
{
// very important to use the class loader from the passed-in context
giveMeAClass = context::class.java.classLoader!!
.loadClass(getString(R.styleable.MyCustomView_give_me_a_class))
.newInstance() // instantiate using 0-args constructor
.let {it as SomeCustomInterface}
}
finally
{
recycle()
}
}
}

HERE is the official documentation for creating custom attributes and Views

How do I use obtainStyledAttributes(int []) with internal Themes of Android

So I have looked around and found out that android.R.styleable is no longer part of the SDK even though it is still documented here.
That wouldn't really be an issue if it was clearly documented what the alternative is. For example the AOSP Calendar App is still using the android.R.styleable
// Get the dim amount from the theme
TypedArray a = obtainStyledAttributes(com.android.internal.R.styleable.Theme);
lp.dimAmount = a.getFloat(android.R.styleable.Theme_backgroundDimAmount, 0.5f);
a.recycle();
So how would one get the backgroundDimAmount without getting the int[] from android.R.styleable.Theme?
What do I have to stick into obtainStyledAttributes(int []) in order to make it work with the SDK?

The CustomView API demo shows how to retrieve styled attributes. The code for the view is here:
https://github.com/android/platform_development/blob/master/samples/ApiDemos/src/com/example/android/apis/view/LabelView.java
The styleable array used to retrieve the text, color, and size is defined in the <declare-styleable> section here:
https://github.com/android/platform_development/blob/master/samples/ApiDemos/res/values/attrs.xml#L24
You can use <declare-styleable> to define any list of attributes that you want to retrieve as a group, containing both your own and ones defined by the platform.
As far as these things being in the documentation, there is a lot of java doc around the styleable arrays that makes them useful to have in the documentation, so they have been left there. However as the arrays change, such as new attributes being added, the values of the constants can change, so the platform ones can not be in the SDK (and please do not use any tricks to try to access them). There should be no need to use the platform ones anyway, because they are each there just for the implementation of parts of the framework, and it is trivial to create your own as shown here.

In the example, they left out the reference to the Context 'c':
public ImageAdapter(Context c) {
TypedArray a = c.obtainStyledAttributes(R.styleable.GalleryPrototype);
mGalleryItemBackground = a.getResourceId(
R.styleable.GalleryPrototype_android_galleryItemBackground, 0);
a.recycle();
return mGalleryItemBackground;
}
Changing obtainStyledAttributes to c.obtainStyledAttributes should work

Example of pulling out standard attribute (background) in a custom view which has its own default style. In this example the custom view PasswordGrid extends GridLayout. I specified a style for PasswordGrid which sets a background image using the standard android attribute android:background.
public class PasswordGrid extends GridLayout {
public PasswordGrid(Context context) {
super(context);
init(context, null, 0);
}
public PasswordGrid(Context context, AttributeSet attrs) {
super(context, attrs, R.attr.passwordGridStyle);
init(context, attrs, 0);
}
public PasswordGrid(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
init(context, attrs, defStyle);
}
private void init(Context context, AttributeSet attrs, int defStyle) {
if (!isInEditMode()) {
TypedArray stdAttrs = context.obtainStyledAttributes(attrs,
new int[] { android.R.attr.background }, // attribute[s] to access
defStyle,
R.style.PasswordGridStyle); // Style to access
// or use any style available in the android.R.style file, such as
// android.R.style.Theme_Holo_Light
if (stdAttrs != null) {
Drawable bgDrawable = stdAttrs.getDrawable(0);
if (bgDrawable != null)
this.setBackground(bgDrawable);
stdAttrs.recycle();
}
}
}
Here is part of my styles.xml file:
<declare-styleable name="passwordGrid">
<attr name="drawOn" format="color|reference" />
<attr name="drawOff" format="color|reference" />
<attr name="pathWidth" format="integer" />
<attr name="pathAlpha" format="integer" />
<attr name="pathColor" format="color" />
</declare-styleable>
<style name="PasswordGridStyle" parent="#android:style/Widget.GridView" >
<!-- Style custom attributes. -->
<item name="drawOff">#drawable/ic_more</item>
<item name="drawOn">#drawable/ic_menu_cut</item>
<item name="pathWidth">31</item>
<item name="pathAlpha">129</item>
<item name="pathColor">#color/green</item>
<!-- Style standard attributes -->
<item name="android:background">#drawable/pattern_bg</item>
</style>

This appears to be a bug in the SDK. I have filed an issue on it, which you may wish to star so as to receive updates on it.
As a worksaround, you can use reflection to access the field:
Class clazz=Class.forName("android.R$styleable");
int i=clazz.getField("Theme_backgroundDimAmount").getInt(clazz);