Android 自定义首选项值未保存

Question

I'm trying to get an existing Preference subclass to work that saves a color value. I didn't write the class, but it's supposed to be working on android-7 and above (I'm compiling to an android-9 target.) The full source code is available here . Here's where the preference is saved:

@Override
public void onColorChanged(int color) {
    if (isPersistent()) {
        boolean ret = persistInt(color);
    }
    // (update preview box, other stuff)
}

Using debug output I can tell that isPersistent() returns true , but persistInt() returns false . According to the Android documentation , persistInt() returns whether the preference is persistent; how can these return different values? (Note: setPersistent(true) is explicitly called from the constructor.)

In any case, the value is not saved. A call to getPersistedInt(defaultValue) returns the default value, even later in the same instance of the class. In the code below, getPersistedInt() is always called and always returns mDefaultValue .

public int getValue() {
    try {
        if (isPersistent()) {
            mValue = getPersistedInt(mDefaultValue);
        }
    } catch (ClassCastException e) {
        mValue = mDefaultValue;
    }

    return mValue;
}

Why is this, and how can I make sure the preference is persisted?

Answer 1

After lots of hopeless searching, I finally found the problem: the preference value was not assigned a key, due to a simple typo in my preferences XML file. (I used android.key instead of android:key .)

Since Android doesn't warn you when trying to persist a preference that has no key (but silently fails instead,) you should call the shouldPersist() function instead of isPersistent() before trying to persist a value, and perhaps log a warning if shouldPersist() returns false. For example:

@Override
public void onColorChanged(int color) {
    mValue = color;
    if (shouldPersist()) {
        persistInt(color);
    } else {
        if (isPersistent())
            Log.w("myapp", "shouldPersist() returned false. Check if this preference has a key.");
    }
    // (update preview box, other stuff)
}