版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/zly921112/article/details/84788413
前言
说到View的绘制大部分人都能说上一两句，但细节实在太多如果没系统的去看很难吃透。近期我专门抽了一周多的时间读了绘制相关的源码，这里准备用三篇博客做一个系统的讲述，目录如下。
- View绘制源码浅析(一）布局的加载
- View绘制源码浅析(二）布局的测量、布局、绘制
- View绘制源码浅析(三）
requestLayout、invalidate与postInvalidate三者的区别
本文的源码基于API27。
疑问
布局加载最重要的就是setContentView()方法了，只需要我们传入一个布局id即可完成布局的加载，但实际上这里是有几个疑问的。
- 如何根据xml创建View的。
- 如何读取xml中View相关属性的。
- 创建的View添加到了哪。
接下来我们带着这些问题再去看源码，避免迷失方向。
setContentView()
我们先从setContentView()这个布局加载的入口开始，看看究竟如何加载布局的。
//MainActivity.java
public class MainActivity extends AppCompatActivity {//继承appCompatActivity
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);//布局加载的入口
}
}
//AppCompatActivity.java
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);//拿到Activity的委托对象调用setContentView()
}
//AppCompatActivity.java
@NonNull
public AppCompatDelegate getDelegate() {//获取Activity委托对象
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
//AppCompatDelegate.java
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {//创建Activity委托对象
return create(activity, activity.getWindow(), callback);//这里将activity.getWindow()传入。
}
//AppCompatDelegate.java
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {//根据不同的版本创建不同的Activity委托对象
if (Build.VERSION.SDK_INT >= 24) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (Build.VERSION.SDK_INT >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else {
return new AppCompatDelegateImplV14(context, window, callback);
}
}
//AppCompatDelegateImplV9.java
//最终是调到v9的setContentView方法
@Override
public void setContentView(int resId) {
ensureSubDecor();//确保SubDecor相关布局初始化完成
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);//找到id为content的view
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);//通过LayoutInflater直接把我们的布局添加到id为content的布局上
mOriginalWindowCallback.onContentChanged();
}
由于继承的是appCompatActivity这个兼容的Activity所以是根据不同的api版本创建不同的AppCompatDelegate实现类以兼容老逻辑。setContentView()最终是调到了AppCompatDelegateImplV9的setContentView()，接下来具体实现分为两步。
- 通过
ensureSubDecor()方法确保SubDecor相关布局初始化完成。 - 找到
SubDecor中id为content的布局，将我们自己的布局inflater到content上。
这里说明下，SubDecor不是DecorView，只是一个变量名为subDecor的ViewGroup不过这里充当DecorView的角色，不要混淆了。
这里先说第一步ensureSubDecor()
//AppCompatDelegateImplV9.java
private void ensureSubDecor() {//确保SubDecor的创建
if (!mSubDecorInstalled) {//如果没有创建SubDecor
mSubDecor = createSubDecor();//创建SubDecor
...
}
}
private ViewGroup createSubDecor() {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);//拿到AppCompat相关的主题属性
//根据主题中的属性执行对应的Feature方法
if (a.getBoolean(R.styleable.AppCompatTheme_windowNoTitle, false)) {
requestWindowFeature(Window.FEATURE_NO_TITLE);//我们比较熟悉的FEATURE_NO_TITLE Feature
} else if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBarOverlay, false)) {
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionModeOverlay, false)) {
requestWindowFeature(FEATURE_ACTION_MODE_OVERLAY);
}
mIsFloating = a.getBoolean(R.styleable.AppCompatTheme_android_windowIsFloating, false);
a.recycle();
mWindow.getDecorView();//确保DecorView的创建
final LayoutInflater inflater = LayoutInflater.from(mContext);//用来填充SubDecor的inflater
ViewGroup subDecor = null;//subDecor布局
//接下来是根据主题属性初始化不同的subDecor布局
if (!mWindowNoTitle) {
if (mIsFloating) {
// If we're floating, inflate the dialog title decor
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_dialog_title_material, null);
// Floating windows can never have an action bar, reset the flags
mHasActionBar = mOverlayActionBar = false;
} else if (mHasActionBar) {
/**
* This needs some explanation. As we can not use the android:theme attribute
* pre-L, we emulate it by manually creating a LayoutInflater using a
* ContextThemeWrapper pointing to actionBarTheme.
*/
TypedValue outValue = new TypedValue();
mContext.getTheme().resolveAttribute(R.attr.actionBarTheme, outValue, true);
Context themedContext;
if (outValue.resourceId != 0) {
themedContext = new ContextThemeWrapper(mContext, outValue.resourceId);
} else {
themedContext = mContext;
}
// Now inflate the view using the themed context and set it as the content view
subDecor = (ViewGroup) LayoutInflater.from(themedContext)
.inflate(R.layout.abc_screen_toolbar, null);
mDecorContentParent = (DecorContentParent) subDecor
.findViewById(R.id.decor_content_parent);
mDecorContentParent.setWindowCallback(getWindowCallback());
/**
* Propagate features to DecorContentParent
*/
if (mOverlayActionBar) {
mDecorContentParent.initFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (mFeatureProgress) {
mDecorContentParent.initFeature(Window.FEATURE_PROGRESS);
}
if (mFeatureIndeterminateProgress) {
mDecorContentParent.initFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
}
}
} else {
if (mOverlayActionMode) {
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_screen_simple_overlay_action_mode, null);
} else {
subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
}
if (Build.VERSION.SDK_INT >= 21) {
// If we're running on L or above, we can rely on ViewCompat's
// setOnApplyWindowInsetsListener
ViewCompat.setOnApplyWindowInsetsListener(subDecor,
new OnApplyWindowInsetsListener() {
@Override
public WindowInsetsCompat onApplyWindowInsets(View v,
WindowInsetsCompat insets) {
final int top = insets.getSystemWindowInsetTop();
final int newTop = updateStatusGuard(top);
if (top != newTop) {
insets = insets.replaceSystemWindowInsets(
insets.getSystemWindowInsetLeft(),
newTop,
insets.getSystemWindowInsetRight(),
insets.getSystemWindowInsetBottom());
}
// Now apply the insets on our view
return ViewCompat.onApplyWindowInsets(v, insets);
}
});
} else {
// Else, we need to use our own FitWindowsViewGroup handling
((FitWindowsViewGroup) subDecor).setOnFitSystemWindowsListener(
new FitWindowsViewGroup.OnFitSystemWindowsListener() {
@Override
public void onFitSystemWindows(Rect insets) {
insets.top = updateStatusGuard(insets.top);
}
});
}
}
if (subDecor == null) {//检查SubDecor是否创建
throw new IllegalArgumentException(
"AppCompat does not support the current theme features: { "
+ "windowActionBar: " + mHasActionBar
+ ", windowActionBarOverlay: "+ mOverlayActionBar
+ ", android:windowIsFloating: " + mIsFloating
+ ", windowActionModeOverlay: " + mOverlayActionMode
+ ", windowNoTitle: " + mWindowNoTitle
+ " }");
}
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);//找到subDecor中id为action_bar_activity_content的布局
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);//找到PhoneWindow中id为content的布局
if (windowContentView != null) {
while (windowContentView.getChildCount() > 0) {//将windowContentView的子布局全部添加到subDecor中id为action_bar_activity_content的布局上
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
windowContentView.setId(View.NO_ID);//清除PhoneWindow中id为content的布局id
contentView.setId(android.R.id.content);//给subDecor中id为action_bar_activity_content的布局设置上新的id为content以假乱真
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
// Now set the Window's content view with the decor
mWindow.setContentView(subDecor);//将subDecor设置到window上
return subDecor;
}
这里先拿到主题中的属性，然后根据主题中的属性设置对应的Feature，并根据条件创建对应的subDecor。接下来拿到PhoneWindow中id为content的View，把它的子布局全部添加到subDecor中id为action_bar_activity_content的布局上，然后将windowContentView的id移除，给subDecor中id为action_bar_activity_content的布局设置上新的id为content以假乱真充当ContentView的角色，最后将SubDecor通过mWindow.setContentView(subDecor)设置到window上。
那么经过ensureSubDecor()方法后我们就完成了DecorView和SubDecor的初始化并通过mWindow.setContentView(subDecor)将SubDecor添加到了DecorView上。完成了SubDecor和DecorView的关联。
在回到我们之前的setContentView()
//AppCompatDelegateImplV9.java
//最终是调到v9的setContentView方法
@Override
public void setContentView(int resId) {
ensureSubDecor();//确保SubDecor相关布局初始化完成
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);//找到id为content的view
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);//通过LayoutInflater直接把我们的布局添加到id为content的布局上
mOriginalWindowCallback.onContentChanged();
}
完成SubDecor初始化后，我们通过mSubDecor.findViewById(android.R.id.content)找到contentParent，然后直接LayoutInflater.from(mContext).inflate(resId, contentParent)将布局添加到了contentParent上完成了布局的添加。
那么对于第一和第二个问题则必须在LayoutInflater.inflate()中寻找答案了，而第三个问题我们已经可以回答了
创建的View添加到了哪？
答：添加到了id为
android.R.id.content的view上。
LayoutInflater.inflate()
接下来我们看下inflate()是如何将我们的布局添加到id为content的view上的。
//LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
...
final XmlResourceParser parser = res.getLayout(resource);//根据资源id创建解析器
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
...
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) {//拿到第一个节点
// Empty
}
if (type != XmlPullParser.START_TAG) {//如果不是开始标签报错
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
final String name = parser.getName();//拿到节点名字
if (TAG_MERGE.equals(name)) {//merge单独处理
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 {//通用逻辑处理
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);//创建xml中根布局
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);//根据父布局创建xml中根布局的lp，因为自己的lp是跟父布局有关的。
if (!attachToRoot) {//当满足root不为null并且attachToRoot为false则直接将lp设置到temp上
// 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);//inflate temp的子布局，具体实现就是递归的把view添加到temp上
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) {//如果root不为null 并且 attachToRoot为true则直接add到root上
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) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return result;
}
}
它是先通过res.getLayout(resource)从布局id拿到解析器对象XmlResourceParser，然后通过createViewFromTag()方法创建根布局，再root.generateLayoutParams(attrs)创建根布局的lp，之后根据root和attachToRoot这两个条件判断是在addView()还是在setLayoutParams()给根布局加上lp，这中间还有一步rInflateChildren()将xml中的子布局依次添加到根布局上。
这中间有些细节值得深究下
createViewFromTag()如何创建布局的（解答第一个问题）root.generateLayoutParams(attrs)如何读取lp相关属性的（解答第二个问题）
我们先看root.generateLayoutParams(attrs)，之所以要这样是因为每个view的lp的创建都是跟父布局有关的，比如root是LinearLayout那么创建的就是LinearLayout.LayoutParams并初始化LinearLayout独有的weight和gravity属性，而如果root是RelativeLayout那么创建的是RelativeLayout.LayoutParams并初始化跟各个布局的约束关系。
这里我们可以看下LinearLayout.generateLayoutParams()方法
//LinearLayout.java
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LinearLayout.LayoutParams(getContext(), attrs);
}
//LinearLayout.LayoutParams 内部类
public static class LayoutParams extends ViewGroup.MarginLayoutParams {
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);//这里还调用了父类MarginLayoutParams的构造方
TypedArray a =
c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.LinearLayout_Layout);
weight = a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight, 0);//获取xml中weight属性
gravity = a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity, -1);//获取layout_gravity属性
a.recycle();
}
}
//ViewGroup.MarginLayoutParams 内部类 初始化margin属性
public static class MarginLayoutParams extends ViewGroup.LayoutParams {
public MarginLayoutParams(Context c, AttributeSet attrs) {
super();
TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_MarginLayout);
setBaseAttributes(a,
R.styleable.ViewGroup_MarginLayout_layout_width,
R.styleable.ViewGroup_MarginLayout_layout_height);//调用父类ViewGroup.LayoutParams的setBaseAttributes()读取最基础的宽高属性
//下面就是读取xml各种margin属性
int margin = a.getDimensionPixelSize(
com.android.internal.R.styleable.ViewGroup_MarginLayout_layout_margin, -1);
if (margin >= 0) {//如果margin大于0则直接用margin属性
leftMargin = margin;
topMargin = margin;
rightMargin= margin;
bottomMargin = margin;
} else {//分别读取left、top、right、bottom的margin属性
int horizontalMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginHorizontal, -1);
int verticalMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginVertical, -1);
if (horizontalMargin >= 0) {
leftMargin = horizontalMargin;
rightMargin = horizontalMargin;
} else {
leftMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginLeft,
UNDEFINED_MARGIN);
if (leftMargin == UNDEFINED_MARGIN) {
mMarginFlags |= LEFT_MARGIN_UNDEFINED_MASK;
leftMargin = DEFAULT_MARGIN_RESOLVED;
}
rightMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginRight,
UNDEFINED_MARGIN);
if (rightMargin == UNDEFINED_MARGIN) {
mMarginFlags |= RIGHT_MARGIN_UNDEFINED_MASK;
rightMargin = DEFAULT_MARGIN_RESOLVED;
}
}
startMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginStart,
DEFAULT_MARGIN_RELATIVE);
endMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginEnd,
DEFAULT_MARGIN_RELATIVE);
if (verticalMargin >= 0) {
topMargin = verticalMargin;
bottomMargin = verticalMargin;
} else {
topMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginTop,
DEFAULT_MARGIN_RESOLVED);
bottomMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginBottom,
DEFAULT_MARGIN_RESOLVED);
}
if (isMarginRelative()) {
mMarginFlags |= NEED_RESOLUTION_MASK;
}
}
final boolean hasRtlSupport = c.getApplicationInfo().hasRtlSupport();
final int targetSdkVersion = c.getApplicationInfo().targetSdkVersion;
if (targetSdkVersion < JELLY_BEAN_MR1 || !hasRtlSupport) {
mMarginFlags |= RTL_COMPATIBILITY_MODE_MASK;
}
// Layout direction is LTR by default
mMarginFlags |= LAYOUT_DIRECTION_LTR;
a.recycle();
}
}
//ViewGroup.LayoutParams 内部类
public static class LayoutParams {
protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {//读取宽高
width = a.getLayoutDimension(widthAttr, "layout_width");
height = a.getLayoutDimension(heightAttr, "layout_height");
}
}
可以看到LinearLayout.LayoutParams有两层继承关系，LinearLayout.LayoutParams负责拿到LinearLayout的特有属性，ViewGroup.MarginLayoutParams负责拿到margin属性，ViewGroup.LayoutParams负责拿到宽高。一般的ViewGroup基本都是实现自己的lp然后继承ViewGroup.MarginLayoutParams。
这里可以回答我们第二个问题
如何读取xml中View相关属性的？
通过
root.generateLayoutParams(attrs)根据root不同创建不同的LayoutParams读取xml中相关属性
接下来看createViewFromTag()如何创建布局的
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);//调用创建重载方法
}
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
try {
View view;
//优先尝试通过Factory创建View
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
//其次通过mPrivateFactory创建，一般情况下是为null的
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
//上面两种方式都创建失败了
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {//如果view的名字中不带'.'，其实就是系统控件，类似TextView这种我们在xml写的时候不是全类名。
view = onCreateView(parent, name, attrs);//创建系统控件
} else {//全类名控件的创建分支，一般都是自定义控件
view = createView(name, null, attrs);//创建自定义控件
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
代码中可以看出布局的创建是先用Factory2和Factory，如果创建失败在通过mPrivateFactory，如果还是失败则通过onCreateView()或者createView()方法创建。Factory我们稍后说，mPrivateFactory一般为null我们暂且忽略。
我们先看onCreateView()或者createView()
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {//创建系统控件
return onCreateView(name, attrs);
}
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);//给系统控件加上全类名前缀"android.view."
}
//最终都是通过createView创建View
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);//获取缓存的Constructor
Class<? extends View> clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
if (constructor == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);//拿到name对应的class
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);//拿到View的构造方法
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);//缓存
} else {
// If we have a filter, apply it to cached constructor
if (mFilter != null) {
// Have we seen this name before?
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// New class -- remember whether it is allowed
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, attrs);
}
}
}
Object lastContext = mConstructorArgs[0];
if (mConstructorArgs[0] == null) {
// Fill in the context if not already within inflation.
mConstructorArgs[0] = mContext;
}
Object[] args = mConstructorArgs;
args[1] = attrs;
final View view = constructor.newInstance(args);//通过反射创建View
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
} catch (NoSuchMethodException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassCastException e) {
// If loaded class is not a View subclass
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
final InflateException ie = new InflateException(
attrs.getPositionDescription() + ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
可以看到如果Factory和Factory2创建View失败则会通过反射的方式创建View。
那么Factory和Factory2是在何时设置的呢，其实是在Activity的onCreate()
public class AppCompatActivity extends FragmentActivity implements AppCompatCallback,
TaskStackBuilder.SupportParentable, ActionBarDrawerToggle.DelegateProvider {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
final AppCompatDelegate delegate = getDelegate();
delegate.installViewFactory();//拿到委托对象调用installViewFactory()设置factory
super.onCreate(savedInstanceState);
}
}
最后是调到AppCompatDelegateImplV9的installViewFactory()
@Override
public void installViewFactory() {
LayoutInflater layoutInflater = LayoutInflater.from(mContext);//拿到LayoutInflater
if (layoutInflater.getFactory() == null) {//如果Factory为空
LayoutInflaterCompat.setFactory2(layoutInflater, this);//设置Fractory2
} else {
if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImplV9)) {
Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
+ " so we can not install AppCompat's");
}
}
}
这里有人可能会问明明是setFactory2()为何要判断layoutInflater.getFactory() == null，这里解释下，无论Factory还是Factory2都只是提供给我们一个可以根据xml中标签名字生成View的接口，而Factory2是继承的Factory，只是比Factory多一个方法，支持创建View时添加父布局参数，并且因为Factory2是继承Factory所以无论是setFactory2()或者setFactory()都会给Factory赋值，所以我们只需要判断layoutInflater.getFactory() == null就可以知道是否设置过。并且setFactory()也只能设置一次，多次次设置会报错的。
public interface Factory {
public View onCreateView(String name, Context context, AttributeSet attrs);
}
public interface Factory2 extends Factory {
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);//多一个View parent参数的方法
}
public final Factory getFactory() {
return mFactory;
}
public void setFactory2(Factory2 factory) {
if (mFactorySet) {//判断是否设置过了
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;//设置标记为置为true
if (mFactory == null) {
mFactory = mFactory2 = factory;//给mFactory和mFactory2都赋值
} else {
mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
}
}
public void setFactory(Factory factory) {
if (mFactorySet) {//判断是否设置过了
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;//设置标记为置为true
if (mFactory == null) {
mFactory = factory;//给mFactory赋值
} else {
mFactory = new FactoryMerger(factory, null, mFactory, mFactory2);
}
}
从代码上也印证了上面的结论。接下来回到AppCompatDelegateImplV9的installViewFactory()
@Override
public void installViewFactory() {
LayoutInflater layoutInflater = LayoutInflater.from(mContext);//拿到LayoutInflater
if (layoutInflater.getFactory() == null) {//如果Factory为空
LayoutInflaterCompat.setFactory2(layoutInflater, this);//设置Fractory2
} else {
if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImplV9)) {
Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
+ " so we can not install AppCompat's");
}
}
}
//LayoutInflaterCompat.setFactory2()
public static void setFactory2(
@NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) {
IMPL.setFactory2(inflater, factory);
}
static final LayoutInflaterCompatBaseImpl IMPL;
static {
if (Build.VERSION.SDK_INT >= 21) {
IMPL = new LayoutInflaterCompatApi21Impl();//impl具体实现类
} else {
IMPL = new LayoutInflaterCompatBaseImpl();
}
}
@RequiresApi(21)
static class LayoutInflaterCompatApi21Impl extends LayoutInflaterCompatBaseImpl {
@SuppressWarnings("deprecation")
@Override
public void setFactory(LayoutInflater inflater, LayoutInflaterFactory factory) {
inflater.setFactory2(factory != null ? new Factory2Wrapper(factory) : null);
}
@Override
public void setFactory2(LayoutInflater inflater, LayoutInflater.Factory2 factory) {
inflater.setFactory2(factory);//最终还是调的LayoutInflater.setFactory2()
}
}
可以看到LayoutInflaterCompat.setFactory2(layoutInflater, this)传入的第二个参数Factory2是this，那么我们看下AppCompatDelegateImplV9如何实现的。
@Override
public final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
final View view = callActivityOnCreateView(parent, name, context, attrs);//先调用activity的onCreateView()一般情况下我们都没实现该方法所以返回值为null
if (view != null) {
return view;
}
return createView(parent, name, context, attrs);//所以创建View的方法在此
}
@Override
public View onCreateView(String name, Context context, AttributeSet attrs) {//调用onCreateView(View parent, String name, Context context, AttributeSet attrs)
return onCreateView(null, name, context, attrs);
}
我们在看到createView()方法
@Override
public View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
//根据各种条件创建AppCompatViewInflater
if (mAppCompatViewInflater == null) {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
String viewInflaterClassName =
a.getString(R.styleable.AppCompatTheme_viewInflaterClass);
if ((viewInflaterClassName == null)
|| AppCompatViewInflater.class.getName().equals(viewInflaterClassName)) {
mAppCompatViewInflater = new AppCompatViewInflater();//创建AppCompatViewInflater
} else {
try {
Class viewInflaterClass = Class.forName(viewInflaterClassName);
mAppCompatViewInflater =
(AppCompatViewInflater) viewInflaterClass.getDeclaredConstructor()
.newInstance();//创建AppCompatViewInflater
} catch (Throwable t) {
Log.i(TAG, "Failed to instantiate custom view inflater "
+ viewInflaterClassName + ". Falling back to default.", t);
mAppCompatViewInflater = new AppCompatViewInflater();//创建AppCompatViewInflater
}
}
}
return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */
true, /* Read read app:theme as a fallback at all times for legacy reasons */
VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */
);//最终调用mAppCompatViewInflater.createView()创建View
}
最终创建View是通过AppCompatViewInflater的createView()方法
final View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs, boolean inheritContext,
boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
final Context originalContext = context;
//拿到context
if (inheritContext && parent != null) {
context = parent.getContext();
}
if (readAndroidTheme || readAppTheme) {
// We then apply the theme on the context, if specified
context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
}
if (wrapContext) {
context = TintContextWrapper.wrap(context);
}
View view = null;
// We need to 'inject' our tint aware Views in place of the standard framework versions
switch (name) {//根据标签名字创建对应的View
case "TextView":
view = createTextView(context, attrs);
verifyNotNull(view, name);
break;
case "ImageView":
view = createImageView(context, attrs);
verifyNotNull(view, name);
break;
case "Button":
view = createButton(context, attrs);
verifyNotNull(view, name);
break;
case "EditText":
view = createEditText(context, attrs);
verifyNotNull(view, name);
break;
case "Spinner":
view = createSpinner(context, attrs);
verifyNotNull(view, name);
break;
case "ImageButton":
view = createImageButton(context, attrs);
verifyNotNull(view, name);
break;
case "CheckBox":
view = createCheckBox(context, attrs);
verifyNotNull(view, name);
break;
case "RadioButton":
view = createRadioButton(context, attrs);
verifyNotNull(view, name);
break;
case "CheckedTextView":
view = createCheckedTextView(context, attrs);
verifyNotNull(view, name);
break;
case "AutoCompleteTextView":
view = createAutoCompleteTextView(context, attrs);
verifyNotNull(view, name);
break;
case "MultiAutoCompleteTextView":
view = createMultiAutoCompleteTextView(context, attrs);
verifyNotNull(view, name);
break;
case "RatingBar":
view = createRatingBar(context, attrs);
verifyNotNull(view, name);
break;
case "SeekBar":
view = createSeekBar(context, attrs);
verifyNotNull(view, name);
break;
default:
// The fallback that allows extending class to take over view inflation
// for other tags. Note that we don't check that the result is not-null.
// That allows the custom inflater path to fall back on the default one
// later in this method.
view = createView(context, name, attrs);
}
if (view == null && originalContext != context) {
// If the original context does not equal our themed context, then we need to manually
// inflate it using the name so that android:theme takes effect.
view = createViewFromTag(context, name, attrs);
}
if (view != null) {
// If we have created a view, check its android:onClick
checkOnClickListener(view, attrs);
}
return view;
}
接下来我们看下createTextView()方法
@NonNull
protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {
return new AppCompatTextView(context, attrs);
}
看发现了什么，对于xml中的TextView实际创建的是AppCompatTextView，对没错这就是继承AppCompatActivity后他对我们xml中部分布局做了兼容。
到这里View的创建其实就说完了，这里可以回答第一个问题了。
如何根据xml创建View的？
View的创建是在
LayoutInflater.createViewFromTag()方法，依次先用Factory2和Factory创建，如果创建失败在通过mPrivateFactory创建，如果还是失败则通过onCreateView()或者createView()方法创建。
这里我们可以试着自己实现的一个Factory2然后在super.onCreate()之前设置给LayoutInflater将TextView标签解析为一个Button
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
if (LayoutInflater.from(this).getFactory() == null) {
LayoutInflater.from(this).setFactory2(new LayoutInflater.Factory2() {
@Override
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
View view = null;
switch (name) {
case "TextView":
view = new Button(context, attrs);
break;
}
return view;
}
@Override
public View onCreateView(String name, Context context, AttributeSet attrs) {
return null;
}
});
}
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</FrameLayout>
可以看到xml中TextView变成了一个Button。
总结
整体流程用一句话来说就是setContentView完成DecorView相关布局初始化并将我们的布局通过LayoutInflater.inflate()方法添加到id为Content的ViewGroup上。
具体细节的话就是我们前面那个三个问题
-
创建的View添加到了哪？
答：添加到了id为
android.R.id.content的view上。 -
如何读取xml中View相关属性的？
通过
root.generateLayoutParams(attrs)根据root不同创建不同的LayoutParams读取xml中相关属性 -
如何根据xml创建View的？
View的创建是在
LayoutInflater.createViewFromTag()方法，依次先用Factory2和Factory创建，如果创建失败在通过mPrivateFactory创建，如果还是失败则通过onCreateView()或者createView()方法创建。