在撸代码的过程中经常遇到创建 View 视图,创建视图有四种方式,但是这四种创建方式到底有什么不同呢?以前只知道能创建,但是不是这四种创建方式的区别,今天让我们一起去解析一下这几种创建方式。
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view 创建方式
在android API中有下列几种创建方式
view创建方法.jpg
点进去看源码可以知道这四个方法最终调用同一个方法
* @param parser xml解析器
* @param root 父布局
* @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.
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
// 最终都会调用这一个方法中
}
- 重点分析inflate()这个方法,上源码
这个方法分为两个部分(标号为 1 ,2 处),下面会详细介绍
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
// 获取子布局的属性
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
//将父布局赋值给 result
View result = root;
try {
....省略....
// 获取布局节点的名称
final String name = parser.getName();
// 1. 如果根布局标签是"merge"
if (TAG_MERGE.equals(name)) {
// 满足root!=null&&attachToRoot=true才行,也就是说"merge"无法独立存在,必须要添加到ViewGroup中
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 {
// 2.根布局标签不是"merge",调用createViewFromTag()把根布局的View创建出来
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
// 2.1如果父布局不为空
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
//从attrs中获取子布局的宽高
params = root.generateLayoutParams(attrs);
//如果attachToRoot ->false 把父布局的参数设置给新建的view
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
// 2.2如果父布局不为空 并且 attachToRoot true 时 将创建的temp 添加到父布局中
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.
// 2.3如果父布局为空 或者 attachToRoot ->false 创建的temp布局 作为结果返回
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
......省略
} finally {
......省略
}
// 3.返回结果
return result;
}
}
当跟布局标签是“merge”时 会走标号1,跟布局标签不是“merge”时走标号2。(“merge”标签到底起什么作用呢?请看 《性能优化之布局优化篇二 使用<merge>标签 》)
- 部分一( 如果根布局标签是"merge")
// 如果根布局标签是"merge"
if (TAG_MERGE.equals(name)) {
// 满足root!=null&&attachToRoot=true才行,也就是说"merge"无法独立存在,必须要添加到ViewGroup中
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
//会执行这个方法
rInflate(parser, root, inflaterContext, attrs, false);
}
//将子布局中的控件实例化 添加到父布局中
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
//创建一个view
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
//添加到父布局容器中
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
//这时候通知父控件执行onFinishInflate方法,而此时,也紧紧是将所有的子控件实例化到内存中,也就是可以通过getChildAt()来获取相应的子控件实例了。
parent.onFinishInflate();
}
}
是merge标签,则会创建子控件并且添加到parent 父布局中
- 部分二( 如果根布局标签不是"merge")
else {
// 根布局标签不是"merge",调用createViewFromTag()把根布局的View创建出来
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
//如果父布局不为空
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
//从attrs中获取子布局的宽高
params = root.generateLayoutParams(attrs);
//如果attachToRoot ->false 把父布局的参数设置给新建的view
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
//如果父布局不为空 并且 attachToRoot true 时 将创建的temp 添加到父布局中
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.
//如果父布局为空 或者 attachToRoot ->false 创建的temp布局 作为结果返回
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
......省略
} finally {
......省略
}
return result;
}
}
如果根布局不是merage标签
1.首先会将根布局创建出来 View temp = createViewFromTag(root, name, inflaterContext, attrs);
2.判断父布局parent是否为空,不为空,会从attrs中获取根布局的宽高 parmas,然后再判断 attachToRoot,
如果 attachToRoot==false ,会将 parmas 设置给创建的 temp;
3.循环创建根布局里的子控件,添加到temp根布局;
4.如果 parent != null 并且 attachToRoot ==true时,将temp 添加到父布局中;
5.如果 parent == null 或者 attachToRoot ==false时, 将temp作为结果result返回;
总结
1.两个参数时,根据 (root != null) 值 给 attachToRoot 赋值
2.一般情况来说,我们一般不会在布局中使用“merage"标签
3.parent !=null 时,从attrs中获取宽高 根据 attachToRoot = false 时 给根布局设置parmas(宽高)
4.parent != null 且 attachToRoot == true 时 将根布局 temp 添加到父布局 parent 中
5.parent ==null 或者 attachToRoot == false 时 将根布局 temp 作为结果result返回
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