Android中的布局加载器LayoutInflater


概述

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)

主要是从源码角度理解下面三段代码的区别:

mInflater.inflate(R.layout.item, null);
mInflater.inflate(R.layout.item, parent ,false);
mInflater.inflate(R.layout.item, parent ,true);

第一句:直接返回了布局,不正确处理布局参数
第二句:返回布局,并能正确处理了布局参数
第三句:返回布局,能正确处理了布局参数,并将布局添加到parent中

源码

从下面使用代码开始:

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LayoutInflater layoutInflater = LayoutInflater.from(HomeAcivity.this);
layoutInflater.inflate(R.layout.layout_rv_photo_item, null);
layoutInflater.inflate(R.layout.layout_rv_photo_item,
(ViewGroup)findViewById(android.R.id.content),
false);
layoutInflater.inflate(R.layout.layout_rv_photo_item,
(ViewGroup)findViewById(android.R.id.content),
true);

最后会都会来到这里:

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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;
//result是根布局
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)
//第二句的关键
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.
//第三句话的关键
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.
//parent为空或者第三个参数为false,都是直接返回第一个参数设置的布局,
//否则返回parent
if (root == null || !attachToRoot) {
result = temp;
}
}

} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;

Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

return result;
}
}
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