Declaring a custom android UI element using XML

Question

How do I declare an Android UI element using XML?

Answer 1

Addition to most voted answer.

obtainStyledAttributes()

I want to add some words about obtainStyledAttributes() usage, when we create custom view using android:xxx prdefined attributes. Especially when we use TextAppearance.
As was mentioned in "2. Creating constructors", custom view gets AttributeSet on its creation. Main usage we can see in TextView source code (API 16).

final Resources.Theme theme = context.getTheme();

// TextAppearance is inspected first, but let observe it later

TypedArray a = theme.obtainStyledAttributes(
            attrs, com.android.internal.R.styleable.TextView, defStyle, 0);

int n = a.getIndexCount();
for (int i = 0; i < n; i++) 
{
    int attr = a.getIndex(i);
    // huge switch with pattern value=a.getXXX(attr) <=> a.getXXX(a.getIndex(i))
}
a.recycle();

What we can see here?
obtainStyledAttributes(AttributeSet set, int[] attrs, int defStyleAttr, int defStyleRes)
Attribute set is processed by theme according to documentation. Attribute values are compiled step by step. First attributes are filled from theme, then values are replaced by values from style, and finally exact values from XML for special view instance replace others.
Array of requested attributes - com.android.internal.R.styleable.TextView
It is an ordinary array of constants. If we are requesting standard attributes, we can build this array manually.

What is not mentioned in documentation - order of result TypedArray elements.
When custom view is declared in attrs.xml, special constants for attribute indexes are generated. And we can extract values this way: a.getString(R.styleable.MyCustomView_android_text). But for manual int[] there are no constants. I suppose, that getXXXValue(arrayIndex) will work fine.

And other question is: "How we can replace internal constants, and request standard attributes?" We can use android.R.attr.* values.

So if we want to use standard TextAppearance attribute in custom view and read its values in constructor, we can modify code from TextView this way:

ColorStateList textColorApp = null;
int textSize = 15;
int typefaceIndex = -1;
int styleIndex = -1;

Resources.Theme theme = context.getTheme();

TypedArray a = theme.obtainStyledAttributes(attrs, R.styleable.CustomLabel, defStyle, 0);
TypedArray appearance = null;
int apResourceId = a.getResourceId(R.styleable.CustomLabel_android_textAppearance, -1);
a.recycle();
if (apResourceId != -1)
{
    appearance = 
        theme.obtainStyledAttributes(apResourceId, new int[] { android.R.attr.textColor, android.R.attr.textSize, 
            android.R.attr.typeface, android.R.attr.textStyle });
}
if (appearance != null)
{
    textColorApp = appearance.getColorStateList(0);
    textSize = appearance.getDimensionPixelSize(1, textSize);
    typefaceIndex = appearance.getInt(2, -1);
    styleIndex = appearance.getInt(3, -1);

    appearance.recycle();
}

Where CustomLabel is defined:

Maybe, I'm mistaken some way, but Android documentation on obtainStyledAttributes() is very poor.

Extending standard UI component

At the same time we can just extend standard UI component, using all its declared attributes. This approach is not so good, because TextView for instance declares a lot of properties. And it will be impossible to implement full functionality in overriden onMeasure() and onDraw().

But we can sacrifice theoretical wide reusage of custom component. Say "I know exactly what features I will use", and don't share code with anybody.

Then we can implement constructor CustomComponent(Context, AttributeSet, defStyle). After calling super(...) we will have all attributes parsed and available through getter methods.