1 LayoutInflate
Inflate的作用：将xml布局文件解析为对应的View对象
LayoutInflater layoutInflater = LayoutInflater.from(this);
View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);
参数与返回值.png
总体流程.png
xml布局文件是由树结构的多个子view组合而成，可想而知，解析过程肯定涉及到逐层递归。
源码如下：
/**
* 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();
//如果是头结点为Merge标签
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
//不是merge标签
{
// 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) {
// 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);
}
}
// 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.
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操作已省略
return result;
}
}
可以看到，LayoutInflater使用pull解析布局文件。先创建出根布局的View实例，然后递归查找这个View的子View，递归完成后将这个View添加到父布局。
其中，createViewFromTag方法，通过节点名以及属性，创建出view。createViewFromTag内部会调用createView()方法，最终通过反射创建出View实例并返回。
然后调用rInflateChildren()方法，最终会调用到rInflate()方法，其中又会递归调用rInflateChildren()。
/**
* Recursive method used to descend down the xml hierarchy and instantiate
* views, instantiate their children, and then call onFinishInflate().
* <p>
* <strong>Note:</strong> Default visibility so the BridgeInflater can
* override it.
*/
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
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)) {
parseRequestFocus(parser, parent);
} 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 {
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);
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
把整个布局文件都解析完成后就形成了一个完整的DOM结构，最终会把最顶层的根布局返回，至此inflate()过程全部结束。
2 View绘制流程
2.1 onMeasure()
public class MyView extends View {
......
//传入的宽高为父视图对子视图提供的参考大小
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(200, 200);
}
}
- 作用：测量视图的大小。
- 视图大小的控制是由父视图、布局文件、以及视图本身共同完成的，父视图会提供给子视图参考的大小，而开发人员可以在XML文件中指定视图的大小，然后视图本身会对最终的大小进行拍板。
- setMeasuredDimension()方法调用之后，我们才能使用getMeasuredWidth()和getMeasuredHeight()来获取视图测量出的宽高，以此之前调用这两个方法得到的值都会是0。
- 以下例子中，视图大小始终会是200*200。
2.2 onLayout()
//参数为父布局提供的参考位置
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
}
- 作用：父布局用来确定子View的摆放位置。相对于父布局的位置。
- 若在onLayout()中需要使用子view的宽高，则需要在onMeasure(）中先调用MeasureChild()进行测量。
- onLayout()之后，就可以调用getWidth()和getHeight()方法来获取宽高。
getWidth()为视图右边坐标减去左边坐标；getMeasuredWidth()为setMeasuredDimension()来设置的。
public class SimpleLayout extends ViewGroup {
public SimpleLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (getChildCount() > 0) {
View childView = getChildAt(0);
measureChild(childView, widthMeasureSpec, heightMeasureSpec);
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (getChildCount() > 0) {
View childView = getChildAt(0);
childView.layout(0, 0, childView.getMeasuredWidth(), childView.getMeasuredHeight());
}
}
}
2.3 onDraw()
- 作用：真正对视图进行绘制。
- onDraw（）函数中会传入Canvas画布对象，通过Paint对象在画布上进行绘制。
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
}
3 View重绘函数
invalidate():重新执行draw方法，不执行measure和layout。在UI线程调用。
postInvalifate():在非UI线程中使用。
requestLayout()：重新走一遍整个绘制流程。
4 Check异常点
1 根布局是Merge标签，却没有设置添加到父View
2 解析到include标签，但是parser深度为0，即include为根节点
3 解析子View的时候解析到Merge标签
