前言
今天有用户反馈用我们的长按查词功能，直接就崩溃了，百分之百必现！第一反应是不可能，测试测了这么久都没问题，怎么一到你手上就崩了！！结果...用户发了一个小视频，还真就是崩了。多次尝试之后，发现在Android4.3手机上必崩。日志显示在RelativeLayout的measure方法报了空指正异常。
场景介绍
实际是一个PopupWindow的功能，相关代码如下：
contentView = (ViewGroup) LayoutInflater.from(context).inflate(R.layout.select_text_oprate_window, null);
contentView.measure(View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED), View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
mWindow = new PopupWindow(contentView, ViewGroup.LayoutParams.WRAP_CONTENT,ViewGroup.LayoutParams.WRAP_CONTENT, false);
就挂在了contentView.measure这里，同时contentView不为null。
看到这里蛋已经碎了一地。。。
源代码分析
View的源码中，有一个与父布局有关的参数：
/** * The layout parameters associated with this view and usedby the parent
* {@link android.view.ViewGroup} to determine how this view should be
* laid out.
* {@hide} */
protected ViewGroup.LayoutParams mLayoutParams;
这个参数表示子布局对父布局的请求（期望能够以某种形式布局，布局的最终形式由整个View层次树决定）
这个参数的相关方法：
/**
* Get the LayoutParams associated with this view. All views should have
* layout parameters. These supply parameters to the <i>parent</i> of this
* view specifying how it should be arranged. There are many subclasses of
* ViewGroup.LayoutParams, and these correspond to the different subclasses
* of ViewGroup that are responsible for arranging their children.
*
* This method may return null if this View is not attached to a parent
* ViewGroup or {@link #setLayoutParams(android.view.ViewGroup.LayoutParams)}
* was not invoked successfully. When a View is attached to a parent
* ViewGroup, this method must not return null.
*
* @return The LayoutParams associated with this view, or null if no
* parameters have been set yet */@ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
public ViewGroup.LayoutParams getLayoutParams() {
return mLayoutParams;
}
//**
* Set the layout parameters associated with this view. These supply
* parameters to the <i>parent</i> of this view specifying how it should be
* arranged. There are many subclasses of ViewGroup.LayoutParams, and these
* correspond to the different subclasses of ViewGroup that are responsible
* for arranging their children.
*
* @param params The layout parameters for this view, cannot be null
*/
public void setLayoutParams(ViewGroup.LayoutParams params) {
if (params == null) {
throw new NullPointerException("Layout parameters cannot be null");
}
mLayoutParams = params;
resolveLayoutParams();
if (mParent instanceof ViewGroup) {
((ViewGroup) mParent).onSetLayoutParams(this, params);
}
requestLayout();
}
可能是这个参数的问题，因为我们在使用的时候没有为contentView指定attach的父布局。
LayoutInflater类中inflate方法：
/**
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
* <em><strong>Important</strong></em> For performance
* reasons, view inflation relies heavily on pre-processing of XML files
* that is done at build time. Therefore, it is not currently possible to
* use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
*
* @param parser XML dom node containing the description of the view
* hierarchy.
* @param root Optional view to be the parent of the generated hierarchy (if
* <em>attachToRoot</em> is true), or else simply an object that
* provides a set of LayoutParams values for root of the returned
* hierarchy (if <em>attachToRoot</em> is false.)
* @param attachToRoot Whether the inflated hierarchy should be attached to
* the root parameter? If false, root is only used to create the
* correct subclass of LayoutParams for the root view in the XML.
* @return The root View of the inflated hierarchy. If root was supplied and
* attachToRoot is true, this is root; otherwise it is the root of
* the inflated XML file.
*/
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;
}
}
这个方法中params参数，当root!=null的时候，调用root.generateLayoutParams产生一个布局参数。attachToRoot==false的时候，调用temp.setLayoutParams(params);似的布局参数生效；attachToRoot==true的时候，调用root.addView(temp, params)。
综上，root!=null的时候params参数会被设置；但如果root==null，那么params==null,也就是我们报空指针异常的原因。
