@linux1s1s
2019-02-14T10:29:27.000000Z
字数 5203
阅读 2834
AndroidView
2015-04
Android遵循MVC框架,Android的项目中UI是通过XML开发的,业务逻辑通过Java实现,所以项目中才会有java目录用于存放业务逻辑代码,而res目录用于存放XML文件,而这部分XML是如何转换成UI的?这个问题比较大,这里仅仅简单说一下:先看一下我们熟知的 setContentView(...)
public class MainActivity extends ActionBarActivity{
@Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
我们继续追踪下去:(注意此处的ActionBarActivity引用的是v7包)
public class ActionBarActivity extends FragmentActivity implements Callback, SupportParentable, DelegateProvider {
ActionBarActivityDelegate mImpl;
public ActionBarActivity() {
}
public void setContentView(int layoutResID) {
this.mImpl.setContentView(layoutResID);
}
}
为了兼容API level 7以上即Android2.1 而设计的,很明显这里ActionBarActivityDelegate是个兼容代理类,进入到这个类里面看一下setContentView实现,发现是个抽象方法,那么找一下实现类即可,这里一共有两个实现类,我们列举一下:
ActionBarActivityDelegateICS 实现类
class ActionBarActivityDelegateICS extends ActionBarActivityDelegate {
public void setContentView(int resId) {
this.mActivity.superSetContentView(resId);
}
}
ActionBarActivityDelegateBase 实现类
class ActionBarActivityDelegateBase extends ActionBarActivityDelegate implements Callback, android.support.v7.internal.view.menu.MenuBuilder.Callback {
public void setContentView(int resId) {
this.ensureSubDecor();
ViewGroup contentParent = (ViewGroup)this.mActivity.findViewById(16908290);
contentParent.removeAllViews();
this.mActivity.getLayoutInflater().inflate(resId, contentParent);
this.mActivity.onSupportContentChanged();
}
}
可以发现上面这个类最终走向:inflate(...) 方法。
接着我们来继续追踪一下ActionBarActivityDelegateICS这类的实现
public class PhoneWindow extends Window implements MenuBuilder.Callback {
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
}
代码比较长,我们重点关注一下Line 18 最终走到 inflate方法。
所以 这个 setContentView(...) 最终都是要调用 LayoutInflate.inflate()这个方法,接下来我们看看这个方法吧
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
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, attrs, false, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, attrs, false);
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
rInflate(parser, temp, attrs, true, 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 (IOException 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;
}
}
从这里我们就可以清楚地看出,LayoutInflater 其实就是使用Android提供的pull解析方式来解析布局文件的。不熟悉pull解析方式的朋友可以网上搜一下,教程很多,我就不细讲了,这里我们注意看下第41行,调用了 createViewFromTag() 这个方法,并把节点名和参数传了进去。看到这个方法名,我们就应该能猜到,它是用于根据节点名来创建View对象的。确实如此,在 createViewFromTag() 方法的内部又会去调用 createView()方法,然后使用反射的方式创建出View的实例并返回。
当然上面只是创建了一个View,其他的View会通过第63行的 rInflate(parser, temp, attrs, true, true) 方法循环遍历这个根布局下的所有子View。
这里额外提高了setContentView(...)这个方法,多说一句:
Android界面显示的原理要比我们所看到的东西复杂得多。任何一个Activity中显示的界面其实主要都由两部分组成,标题栏和内容布局。标题栏就是在很多界面顶部显示的那部分内容,比如刚刚我们的那个例子当中就有标题栏,可以在代码中控制让它是否显示。而内容布局就是一个FrameLayout,这个布局的id叫作content,我们调用setContentView()方法时所传入的布局其实就是放到这个FrameLayout中的,这也是为什么这个方法名叫作setContentView(),而不是叫setView()。来一张图大概说明一下:
接下来就是View的三部曲了:onMeasure()、onLayout()、onDraw()过程,然后才出现在屏幕上面。