你真的懂么:Android中删除Preference

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

Preference Family Tree

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.

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