Android的设置界面实现比较简单,有时甚至只需要使用一个简单的xml文件即可.声明简单,但是如何从PreferenceScreen或者PreferenceCategory中删除一个Preference会简单么.为什么有些人写的就无法删除成功呢?本文将从Android源码实现来分析一下.
声明文件
<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
android:key="root">
<PreferenceCategory
android:key="theme"
android:title="Theme"
android:summary="Theme Settings"
>
<CheckBoxPreference
android:key="holo_theme"
android:title="Holo Theme"
android:summary="Use Holo Theme"
/>
</PreferenceCategory>
<CheckBoxPreference
android:key="rmcache"
android:title="Auto Clear Cache"
android:summary="Enable Auto Clear Cache "
/>
</PreferenceScreen> |
层级关系
删除Preference
- 删除key为rmcache的Preference,这个Preference是PreferenceScreen root的子节点.
PreferenceScreen screen = getPreferenceScreen();
CheckBoxPreference autoClearCheckboxPref = (CheckBoxPreference) screen.findPreference("rmcache");
screen.removePreference(autoClearCheckboxPref); |
- 删除key为holo_theme的Preference,其为PreferenceScreen root的孙子节点,非直接关系.
PreferenceCategory themePrefCategory = (PreferenceCategory) screen.findPreference("theme");
CheckBoxPreference holoCheckboxPref = (CheckBoxPreference)themePrefCategory.findPreference("holo_theme");
themePrefCategory.removePreference(holoCheckboxPref); |
为什么删除失败
很多人出现了删除失败的问题,主要原因是使用了非父亲节点来删除,比如这样
PreferenceScreen screen = getPreferenceScreen();
CheckBoxPreference holoCheckboxPref = (CheckBoxPreference)screen.findPreference("holo_theme");
screen.removePreference(holoCheckboxPref); |
PreferenceGroup删除实现,其实PreferenceScreen和PreferenceCategory都是PreferenceGroup的子类.
/**
* Removes a {@link Preference} from this group.
*
* @param preference The preference to remove.
* @return Whether the preference was found and removed.
*/
public boolean removePreference(Preference preference) {
final boolean returnValue = removePreferenceInt(preference);
notifyHierarchyChanged();
return returnValue;
}
private boolean removePreferenceInt(Preference preference) {
synchronized(this) {
preference.onPrepareForRemoval();
return mPreferenceList.remove(preference);
}
} |
而mPreferenceList中存放的都是当前PreferenceGroup的直接子Preference.
findPreference实现
findPreference查找不仅仅限于直接子Preference,会遍历其所有的子Preference.
所以代码中同样有root PreferenceGroup和直接父PreferenceGroup引用时,通常后者效率会高.
/**
* Finds a {@link Preference} based on its key. If two {@link Preference}
* share the same key (not recommended), the first to appear will be
* returned (to retrieve the other preference with the same key, call this
* method on the first preference). If this preference has the key, it will
* not be returned.
* <p>
* This will recursively search for the preference into children that are
* also {@link PreferenceGroup PreferenceGroups}.
*
* @param key The key of the preference to retrieve.
* @return The {@link Preference} with the key, or null.
*/
public Preference findPreference(CharSequence key) {
if (TextUtils.equals(getKey(), key)) {
return this;
}
final int preferenceCount = getPreferenceCount();
for (int i = 0; i < preferenceCount; i++) {
final Preference preference = getPreference(i);
final String curKey = preference.getKey();
if (curKey != null && curKey.equals(key)) {
return preference;
}
if (preference instanceof PreferenceGroup) {
final Preference returnedPreference = ((PreferenceGroup)preference)
.findPreference(key);
if (returnedPreference != null) {
return returnedPreference;
}
}
}
return null;
} |
findPreference和removePreference实现比较
为什么findPreference遍历所有的子节点,而removePreference不会,只会删除直接子Preference
原因有以下几点:
- findPreference支持遍历查找,减少了声明诸多的中间PreferenceGroup代码.而findPreference属于常用接口方法.
- removePreference调用较少.
- 当存在key相同的Preference时,如果removePreference不限定直接子Preference,那么无法准确删除哪一个.
其他
- Android的设计与实现
- Android系统源代码情景分析
