在我们想XML布局文件转换为View对象的时候.我们都会使用LayoutInflate对象.顾名思义咋一眼就能看出来他是布局填充器.那么接下来看看LayoutInfalte的使用
总体分为
- LayoutInfalter的获取方式
- 加载布局过程中LayoutParams的丢失
LayoutInfalter的获取方式
首先我们看看获取LayoutInfalte的加载方式
- getLayoutInflater();
- LayoutInflater.from(context);
- Context.getSystemService(LAYOUT_INFLATER_SERVICE);
关于1方法是在Activity中使用的.那么我们依次看看他们具体的实现流程
/**
* Convenience for calling
* {@link android.view.Window#getLayoutInflater}.
*/
public LayoutInflater getLayoutInflater() {
return getWindow().getLayoutInflater();
}
Activity.class,显然他是调用了Window类中的getLayoutInflate,那么我们接着看Window中是如何实现的
/**
* Quick access to the {@link LayoutInflater} instance that this Window
* retrieved from its Context.
*
* @return LayoutInflater The shared LayoutInflater.
*/
public abstract LayoutInflater getLayoutInflater();
是一个抽象方法.那么我们就要找他的实现类.那么Window的实现类是什么?已知的实现类是PhoneWindow.class.那么我们去看看PhoneWindow是如何实现的
private LayoutInflater mLayoutInflater;
public PhoneWindow(Context context) {
super(context);
mLayoutInflater = LayoutInflater.from(context);
}
/**
* Return a LayoutInflater instance that can be used to inflate XML view layout
* resources for use in this Window.
*
* @return LayoutInflater The shared LayoutInflater.
*/
@Override
public LayoutInflater getLayoutInflater() {
return mLayoutInflater;
}
那么显然我们看到的便是PhoneWindow在初始化的时候通过调用LayoutInfalter类中的from(context)初始化布局加载器.那么请挥挥鼠标移到我们刚刚说的三个方法.是不是惊奇的发现了搞了半天是在调方法二.让我们接下来看看LayoutInfalte类
/**
* Obtains the LayoutInflater from the given context.
*/
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
看完之后是否顿时又发现了什么?没错就是调用方法三.那么搞了半天说白就是根据一个上下文加载布局的过程中.最终的方法调用方法还是直接调用Context.getSystemService(LAYOUT_INFLATER_SERVICE); 获取布局加载器
好了布局加载器的获取方法暂告一段落.个人习惯来说比较建议使用第三种方式.毕竟少走不少流程.
加载布局过程中LayoutParams的丢失
下面直接贴上Demo
XML文件:
布局文件:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" > <ListView
android:id="@+id/listView1"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView> </LinearLayout>
ListView内容布局,这里我们把每个内容项设置为100dip的高度
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="100dip"
android:orientation="horizontal" > <TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="内容一"
android:textSize="30sp"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="内容二"
android:textSize="30sp"/> </LinearLayout>
代码部分:
public class MainActivity extends Activity {
private ListView listView;
private CustomAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listView = (ListView) findViewById(R.id.listView1);
adapter = new CustomAdapter(this);
listView.setAdapter(adapter);
}
}
CustomAdapter.java
class CustomAdapter extends BaseAdapter {
private LayoutInflater inflater;
public CustomAdapter(Context context) {
this.inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public int getCount() {
return 10;
}
@Override
public Object getItem(int position) {
return null;
}
@Override
public long getItemId(int position) {
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if(convertView == null) {
convertView = inflater.inflate(R.layout.list_item, null);
}
return convertView;
}
}
那么我们现在运行看看效果
貌似我们并没有看到我们想要的效果.每个内容项并没有30dip的效果.那么问题出在哪里呢?请看CustomAdapter中的代码的以下片段
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if(convertView == null) {
convertView = inflater.inflate(R.layout.list_item, null);
}
return convertView;
}
在这里inflate(R.layout.list_item, null);我们并没有指定根布局,那么出现的状况如上图.让我们用工具(hierarchyviewer.bat)看看
惊奇的发现了.我们原来的XML布局文件中指定的layout_height竟然变成了wrap_content了,那么接下来我们先看看另外一种情况
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if(convertView == null) {
convertView = inflater.inflate(R.layout.list_item, parent, false); //修改了这里
}
return convertView;
}
先看看效果图.
奇怪的是我们想要的效果发生了100dip的高度.那么我们通过工具再看看
显而易见我们的布局参数没有丢失了.那么原因到底是什么呢?为什么会出现布局参数丢失呢?其实问题就是出现在inflate()方法的参数上
我们来看看源码,inflate()一共有四个重载方法,以下按照依次调用顺序
- inflate(int resource, ViewGroup root); >> inflate(int resource, ViewGroup root, boolean attachToRoot) ; >> inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot);
- inflate(XmlPullParser parser, ViewGroup root) >> inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot);
那么我们直接看最后调用的方法
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
final String name = parser.getName();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, attrs, false);
} else {
// Temp is the root view that was found in the xml
View temp;
if (TAG_1995.equals(name)) {
temp = new BlinkLayout(mContext, attrs);
} else {
temp = createViewFromTag(root, name, attrs);
}
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp
rInflate(parser, temp, attrs, true);
if (DEBUG) {
System.out.println("-----> done inflating children");
}
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (IOException e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
return result;
}
}
代码显然有点长.我们挑重点看.
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
我们可以看到只要root不为空且attachToRoot取反为true.那么temp就设置布局参数,temp一个临时变量View,继续走
if (root != null && attachToRoot) {
root.addView(temp, params);
}
当root != null && attachToRoot往所在的父布局添加一个带参数的temp(View).最后
return result 而 result = root;
同理使用两个参数的便能知道并没有添加布局参数上到根布局.从而导致了默认的使用了WRAP_CONTENT.而不是我们指定的布局参数值
到此LayoutInfalte的使用告一段落.有错误的希望大家提出.菜鸟一枚第一次写博客.难免出现各种错误.大家见谅见谅!