问题
使用View.inflate()加载RecyclerView的item布局时,导致item内容无法居中显示.
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View inflate = View.inflate(parent.getContext(), R.layout.rv_item_dance_indicator, null);
return new ViewHolder(inflate);
}
当时脑子抽风了,想偷个懒,用这种方式加载item布局,结果发现item竟然不能居中显示,一直以为是item布局的问题,这里把布局文件也贴一下.
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:paddingEnd="12dp"
android:paddingStart="12dp"
>
<ImageView
android:layout_gravity="center_vertical"
android:id="@+id/iv_indicator"
android:src="@drawable/indicator_selector"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
tools:ignore="ContentDescription" />
</FrameLayout>
各种尝试还是找不到原因,真无语了,怎么就居中不了呢?
最后还是百度大法,搜索关键字recyclerview item不能居中显示,出来一大堆,说什么的都有,比如更改一下根布局,改成线性布局啊,一看有不靠谱,最后还是找到一点有用的信息,原文地址
解决方案
动手一试,改成如下代码,果然成了,可喜可贺.
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View inflate = LayoutInflater.from(parent.getContext())
.inflate(R.layout.rv_item_dance_indicator, parent, false);
return new ViewHolder(inflate);
}
当然学习不能不能只知其然而不知其所以然,抱着这个心态来瞄一瞄源码,这两种加载方式的区别到底在哪,以后到底该怎么使用?
源码分析
LayoutInflater.from
/**
* 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;
}
调用系统服务得到一个布局加载器.
LayoutInflater.inflate
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}
//传入布局资源id,并获得一个xml解析器,用以解析xml布局资源
final XmlResourceParser parser = res.getLayout(resource);
try {
//调用inflate开始解析xml
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
inflate方法接收3个参数
-
int resource需要加载的布局资源文件 -
ViewGroup root父布局 -
boolean attachToRoot是否需要添加进父布局
然后调用内部的inflate方法
inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
....
//这里只贴关键代码
// Temp is the root view that was found in the xml
//解析xml标签并返回一个view
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
//root是最初传进来的父view
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
//如果父view不为空,则会生成相应的布局参数
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
//如果attachToRoot为false,为view设置布局参数
// 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.
//递归加载子view
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.
//如果父view不为空且attachToRoot为true,则将生成的view添加到父view中
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;
}
这里可以看出你传不传root很重要,如果传了,会为加载的view设置布局参数,并且attchToRoot为ture还会将加载的布局添加到父view中.
在看一下generateLayoutParams方法
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LayoutParams(getContext(), attrs);
}
public LayoutParams(Context c, AttributeSet attrs) {
TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
setBaseAttributes(a,
R.styleable.ViewGroup_Layout_layout_width,
R.styleable.ViewGroup_Layout_layout_height);
a.recycle();
}
这里是实例化一个布局参数并读取xml中设置的宽高进行初始化,所以只有当你传入了父view,xml中定义layout_width和layout_height才有效.否则给的应该是一个默认值.应该都是wrap_content,这里只是做一个猜测.
View.inflate()
public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}
还是调用 LayoutInflater.from(context),并传了两个参数,继续跟进去
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
这里又调用了3个参数的inflate方法.如果root为空,第三个参数为false,否则为true(即添加到父布局内部).
总结
以后不管在什么情况下加载布局,一律使用LayoutInflator.inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
有父布局则传父布局,需要加入到父布局第三个参数传true,否则传false.
