可能你曾经发现过，当你的activity继承的是AppCompatActivity时，如果在布局中设置一个TextView，这时候我们去打印这个TextView，打出来的是AppCompatTextView，那么为什么会这样？找到AppCompatActivity，看一下它时如何设置布局，就能找到答案
首先我们先看onCreate方法，这里有一个重要的伏笔
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
final AppCompatDelegate delegate = getDelegate();
//这个方法是最关键的
delegate.installViewFactory();
delegate.onCreate(savedInstanceState);
//可以看到这是系统切换白天夜晚主题的方式，在api23之后可用
if (delegate.applyDayNight() && mThemeId != 0) {
// If DayNight has been applied, we need to re-apply the theme for
// the changes to take effect. On API 23+, we should bypass
// setTheme(), which will no-op if the theme ID is identical to the
// current theme ID.
if (Build.VERSION.SDK_INT >= 23) {
onApplyThemeResource(getTheme(), mThemeId, false);
} else {
setTheme(mThemeId);
}
}
super.onCreate(savedInstanceState);
}
installViewFactory，这是一个抽象方法,最终的实现是在AppCompatDelegateImplV9中实现，这里先放着
public abstract void installViewFactory();
我们先看AppCompatActivity的setContentView
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
已经知道setContentView是个抽象方法，所以看getDelegate得到的是什么，不断的点进去可以找到这里
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {
final int sdk = Build.VERSION.SDK_INT;
if (BuildCompat.isAtLeastN()) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (sdk >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (sdk >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else if (sdk >= 11) {
return new AppCompatDelegateImplV11(context, window, callback);
} else {
return new AppCompatDelegateImplV9(context, window, callback);
}
}
这里的源码其实已经看过了，我们直接找到AppCompatDelegateImplV9中去看setContentView
@Override
public void setContentView(int resId) {
//初始化DecorView等一系列动作，这个就不看了，主要看下边
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
LayoutInflater.from(mContext).inflate(resId, contentParent);是如何inflate view的
看到这里了，顺便看看LayoutInflater系统服务如何创建
进入LayoutInflator中
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;
}
Context中的getSystemService
public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name);
进入ContextImpl中
@Override
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}
进入SystemServiceRegistry,故名思意，系统服务注册功能
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
可见是从一个静态集合中取出来的,那么可以猜测在应用启动的时候，这些服务已经被注册了
private static final HashMap> SYSTEM_SERVICE_FETCHERS =
new HashMap>();
//静态代码块注册了所有的系统服务，并且只会注册一次
static {
......
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}});
.....
}
然后我们该回归正题去看inflate了，一直往里点，可以找到这个方法
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
......
//通过xml解析出来的tag创建view
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
......
}
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
.......
//看mFactory2 有没有设置，没有就走默认的下边的代码，这个
//mFactory2 是关键
try {
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
//默认情况下也就是没有设置Factory，会走到这里
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
//name中没有点，说明是创建系统View，View的创建是反射得到的
view = onCreateView(parent, name, attrs);
} else {
//创建自定义View
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
}
看到这里很想知道mFactory2 或者mFactory或者mPrivateFactory是何时设置的，这时我们再回过头来看AppCompatActivity onCreate方法中那段代码
delegate.installViewFactory();
最终可以找到AppCompatDelegateImplV9中的installViewFactory
@Override
public void installViewFactory() {
LayoutInflater layoutInflater = LayoutInflater.from(mContext);
if (layoutInflater.getFactory() == null) {
LayoutInflaterCompat.setFactory2(layoutInflater, this);
} 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(layoutInflater, this);通过LayoutInflaterCompat将factory设置给LayoutInflater
public void setFactory2(LayoutInflater inflater, LayoutInflater.Factory2 factory) {
inflater.setFactory2(factory);
final LayoutInflater.Factory f = inflater.getFactory();
if (f instanceof LayoutInflater.Factory2) {
// The merged factory is now set to getFactory(), but not getFactory2() (pre-v21).
// We will now try and force set the merged factory to mFactory2
forceSetFactory2(inflater, (LayoutInflater.Factory2) f);
} else {
// Else, we will force set the original wrapped Factory2
forceSetFactory2(inflater, factory);
}
}
这时候Factory2有值了，那么上边的那行判断就生效了
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
Factory是个接口，最终会进入实现类AppCompatDelegateImplV9中
@Override
public View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
if (mAppCompatViewInflater == null) {
mAppCompatViewInflater = new AppCompatViewInflater();
}
boolean inheritContext = false;
if (IS_PRE_LOLLIPOP) {
inheritContext = (attrs instanceof XmlPullParser)
// If we have a XmlPullParser, we can detect where we are in the layout
? ((XmlPullParser) attrs).getDepth() > 1
// Otherwise we have to use the old heuristic
: shouldInheritContext((ViewParent) parent);
}
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 */
);
}
在AppCompatViewInflator中
public 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;
// We can emulate Lollipop's android:theme attribute propagating down the view hierarchy
// by using the parent's 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) {
case "TextView":
view = new AppCompatTextView(context, attrs);
break;
case "ImageView":
view = new AppCompatImageView(context, attrs);
break;
case "Button":
view = new AppCompatButton(context, attrs);
break;
case "EditText":
view = new AppCompatEditText(context, attrs);
break;
case "Spinner":
view = new AppCompatSpinner(context, attrs);
break;
case "ImageButton":
view = new AppCompatImageButton(context, attrs);
break;
case "CheckBox":
view = new AppCompatCheckBox(context, attrs);
break;
case "RadioButton":
view = new AppCompatRadioButton(context, attrs);
break;
case "CheckedTextView":
view = new AppCompatCheckedTextView(context, attrs);
break;
case "AutoCompleteTextView":
view = new AppCompatAutoCompleteTextView(context, attrs);
break;
case "MultiAutoCompleteTextView":
view = new AppCompatMultiAutoCompleteTextView(context, attrs);
break;
case "RatingBar":
view = new AppCompatRatingBar(context, attrs);
break;
case "SeekBar":
view = new AppCompatSeekBar(context, attrs);
break;
}
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;
}
可以看到，TextView ImageView等等就是这样被替换为AppCompatTextView和AppCompatImageView的
此时总算知道了，为什么继承自AppCompatActivity时，如果在布局中设置一个TextView，这时候我们去打印这个TextView，打出来的是AppCompatTextView