inflate

概述

inflate方法在activity的onCreate中，在Listview的Adapter的getView中，在增加view的时候，我们都经常用到inflate方法比如

LayoutInflater.from(getContext()).inflate(resId, head)

这个就是把resId指定的layout渲染之后加入到head中。

在使用inflate的使用，我们会经常发现宽高等布局信息失效，这是怎么回事呢？

详解

LayoutInflater的inflate方法主要有2个重载

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}

两参的本质是调用三参的方法，在看LayoutInflater.from(getContext()).inflate(btnResId, head)

会做哪些事情，首先他根据xml文件用反射创建出View树，这样xml文件对应出了一个View。然后根据View的布局参数和head 的类型创建出LayoutParams，然后我们根据把View添加到head内，添加的参数是LayoutParams。

简单的说，做3件事，创建View，生成LayoutParams，添加View到根布局。

Inflate的核心代码如下所示。

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
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, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, 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 against its context.
rInflateChildren(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 (Exception 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;
}

Trace.traceEnd(Trace.TRACE_TAG_VIEW);

return result;
}
}

分析参数情况

1、三参函数，root为null，此时只做第一件事情，把xml渲染成View，然后返回

2、三参函数，root不为null，attachToRoot为true，此时和2参的函数一样，做3件事，返回root的view。

3、三参函数,root不为null，attachToRoot为false，此时把xml渲染成view，生成LayoutParams并设置，但并不添加view，返回的是xml渲染出的view。

 分析具体案例

根据前面的参数情况，我们再来分析案例，

案例1

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View view = View.inflate(this, R.layout.layout_menu_item, null);//返回的view是xml对应的view
layout.addView(view);

这对应第一种参数情况，这样layoutparamete是没有的，包括layout_width和layout_height

案例2

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View view = View.inflate(this, R.layout.layout_menu_item, layout);

这个对应低2种参数情况，返回的view是layout，而不是xml对应的view

案例3

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View v1 = LayoutInflater.from(this).inflate(R.layout.layout_menu_item, null);
layout.addView(v1, 200, 200);

这个对应第一种参数情况，宽高为200，200

案例4

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View v1 = LayoutInflater.from(this).inflate(R.layout.layout_menu_item, layout, false);
layout.addView(v1);

对应第三种参数情况，返回的v1是渲染好的xml对应的view而不是root

案例5

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View v1 = LayoutInflater.from(this).inflate(R.layout.layout_menu_item, layout, true);

对应第二种情况返回root

案例6

BaseAdapter中往往有这样的代码

public View getView(int position, View convertView, ViewGroup parent) {

ViewHolder holder = null;
if (convertView == null) {
holder = new ViewHolder();
convertView = mInflater.inflate(R.layout.listview_demo3_item, null);
holder.img = (ImageView) convertView.findViewById(R.id.img);
holder.title = (TextView) convertView.findViewById(R.id.title);
。。。。

我们会发现layout_menu_item的跟布局的宽高是无效的，那如果我想要跟布局的宽高有效，该怎么办呢？看看inflate那行代码。

用convertView = mInflater.inflate(R.layout.listview_demo3_item,parent,false);

这样就可以了。

总结

1.需要把view按照宽高信息（本质是布局信息）添加到view，得用

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View root = View.inflate(this, R.layout.layout_menu_item, layout);

这时候返回的是root。
2.需要把view按照宽高信息（本质是布局信息）添加到view，并且想要返回view而不是root，得用

LinearLayout layout = (LinearLayout)findViewById(R.id.container);
View v1 = LayoutInflater.from(this).inflate(R.layout.layout_menu_item, layout, false);
layout.addView(v1);

3.setContentView的另一种写法，为了得到content这个viewGroup

rootView = (FrameLayout) LayoutInflater.from(this).inflate(getResId(), null);
setContentView(rootView);