xml - Why doesn't `android:foreground` attribute work?

Question

Take a look at this small android app:

MainActivity.java:

package io.github.gsaga.toucheventtest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

activity_main:

<ImageView android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:foreground="@drawable/ic_launcher_background"
    xmlns:android="http://schemas.android.com/apk/res/android" />

The image pointed to by android:foreground isn't displayed, but it appears if I change foreground to src or background in activity_main.xml. This code seems to follow the instructions described here:

https://developer.android.com/reference/android/view/View.html#attr_android:foreground

Why doesn't the android:foreground tag work in above code?

NOTE:

minSdkVersion is 19, I'm running this app on Android 5.1 (API level 22)

Answer 1

Short answer

This is due to a bug which existing in Android since API level 23.

More details on the behavior

Here is the list of all the XML attributes and corresponding methods related to setting foreground drawables to views with the API level they are introduced through FrameLayout. However, these are later moved into View in API level 23.

╔════════════════════════════╦═════════════════════════════════════════════════╦═════════════╗
║       XML attribute        ║                     Method                      ║   Added in  ║
║                            ║                                                 ║ (API level) ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foreground         ║ setForeground(Drawable)                         ║ 1           ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundGravity  ║ setForegroundGravity(int gravity)               ║ 1           ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundTint     ║ setForegroundTintMode(PorterDuff.Mode tintMode) ║ 21          ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundTintMode ║ setForegroundTintMode(PorterDuff.Mode tintMode) ║ 21          ║
╚════════════════════════════╩═════════════════════════════════════════════════╩═════════════╝

• Android doc says setForeground(Drawable) is added in API 1 and setForegroundTintList (ColorStateList tint) and setForegroundTintMode (PorterDuff.Mode tintMode) are added in API level 21 to View. Actually they were there in FrameLayout until it moved in API 23.

• In API level < 23, you will get a warning even though it is not required. You can just suppress it. See this.

Now take a look at how these properties work on different versions.

╔═══════════╦══════════════════╦══════════════════╗
║ API level ║      By code     ║     Using XML    ║
╠═══════════╬══════════════════╬══════════════════╣
║ <23       ║ FrameLayout only ║ FrameLayout only ║
╠═══════════╬══════════════════╬══════════════════╣
║ >=23      ║ FrameLayout only ║ All views        ║
╚═══════════╩══════════════════╩══════════════════╝

The cause of the bug

When these properties moved to View in API level 23, they did some strange modifications on it which can be called a bug. While loading properties from XML, it checks whether the View is a FrameLayout which is not present inside the methods we can use for the same purpose.

View constructor, API level 23:

case R.styleable.View_foreground:
    if (targetSdkVersion >= Build.VERSION_CODES.M || this instanceof FrameLayout) {
        setForeground(a.getDrawable(attr));
    }
    break;