AppCompatActivity的魔术——如何做到适配新控件

大家都知道google要求使用app的模板类继承AppCompatActivity

这是一个继承FragmentAcvitivy的类，他是怎么做到让过时控件去使用酷炫的新特性的呢？

来看源码

核心变量 private AppCompatDelegate mDelegate;

这是兼容的老套路，便于在版本迭代的时候统一维护升级与兼容

/**
* @return The {@link AppCompatDelegate} being used by this Activity.
*/
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}

调用静态方法create 其中第二个参数是第一个参数activity的getWindow()即phoneWindow

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);
}
}

这些不同版本的Impl类之间是继承关系 也就是说基类实现类其实是ImplV9



回来看AppCompatActivity的onCreate方法

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
final AppCompatDelegate delegate = getDelegate();
delegate.installViewFactory(); //在onCreate的顶部调用 奥秘就在这里
delegate.onCreate(savedInstanceState);
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方法仅在ImplV9中有实现

@Override
public void installViewFactory() {
LayoutInflater layoutInflater = LayoutInflater.from(mContext);
if (layoutInflater.getFactory() == null) {
LayoutInflaterCompat.setFactory(layoutInflater, this); //在这里做替换
} else { //如果被抢注册了 那么遵从之前设置 并打印失败替换日志
if (!(LayoutInflaterCompat.getFactory(layoutInflater)
instanceof AppCompatDelegateImplV9)) {
Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
+ " so we can not install AppCompat's");
}
}
}

那么就来看这个LayoutInflaterCompat的静态方法setFactory

/**
* Attach a custom Factory interface for creating views while using
* this LayoutInflater. This must not be null, and can only be set once;
* after setting, you can not change the factory.
*
* @see LayoutInflater#setFactory(android.view.LayoutInflater.Factory)
*/
public static void setFactory(LayoutInflater inflater, LayoutInflaterFactory factory) {
IMPL.setFactory(inflater, factory);
}

又是个代理模式的兼容实现IMPL

static final LayoutInflaterCompatImpl IMPL;
static {
final int version = Build.VERSION.SDK_INT;
if (version >= 21) {
IMPL = new LayoutInflaterCompatImplV21();
} else if (version >= 11) {
IMPL = new LayoutInflaterCompatImplV11();
} else {
IMPL = new LayoutInflaterCompatImplBase();
}
}

先来看一下v21 5.0以上的实现

class LayoutInflaterCompatHC {
private static final String TAG = "LayoutInflaterCompatHC";

private static Field sLayoutInflaterFactory2Field;
private static boolean sCheckedField;

static class FactoryWrapperHC extends LayoutInflaterCompatBase.FactoryWrapper
implements LayoutInflater.Factory2 {

FactoryWrapperHC(LayoutInflaterFactory delegateFactory) {
super(delegateFactory);
}

@Override
public View onCreateView(View parent, String name, Context context,
AttributeSet attributeSet) {
return mDelegateFactory.onCreateView(parent, name, context, attributeSet);
}
}

static void setFactory(LayoutInflater inflater, LayoutInflaterFactory factory) {
final LayoutInflater.Factory2 factory2 = factory != null
? new FactoryWrapperHC(factory) : null;
inflater.setFactory2(factory2);

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, factory2);
}
}

/**
* For APIs >= 11 && < 21, there was a framework bug that prevented a LayoutInflater's
* Factory2 from being merged properly if set after a cloneInContext from a LayoutInflater
* that already had a Factory2 registered. We work around that bug here. If we can't we
* log an error.
*/
static void forceSetFactory2(LayoutInflater inflater, LayoutInflater.Factory2 factory) {
if (!sCheckedField) {
try {
sLayoutInflaterFactory2Field = LayoutInflater.class.getDeclaredField("mFactory2");
sLayoutInflaterFactory2Field.setAccessible(true);
} catch (NoSuchFieldException e) {
Log.e(TAG, "forceSetFactory2 Could not find field 'mFactory2' on class "
+ LayoutInflater.class.getName()
+ "; inflation may have unexpected results.", e);
}
sCheckedField = true;
}
if (sLayoutInflaterFactory2Field != null) {
try {
sLayoutInflaterFactory2Field.set(inflater, factory);
} catch (IllegalAccessException e) {
Log.e(TAG, "forceSetFactory2 could not set the Factory2 on LayoutInflater "
+ inflater + "; inflation may have unexpected results.", e);
}
}
}
}

仔细来看一下这个setFactory方法
LayoutInflater的setFactory方法要的参数是一个LayoutInflater.Factory

而入参却是一个android.support.v4.view.LayoutInflaterFactory

/**
* Used with {@code LayoutInflaterCompat.setFactory()}. Offers the same API as
* {@code LayoutInflater.Factory2}.
*/
public interface LayoutInflaterFactory {

/**
* Hook you can supply that is called when inflating from a LayoutInflater.
* You can use this to customize the tag names available in your XML
* layout files.
*
* @param parent The parent that the created view will be placed
* in; <em>note that this may be null</em>.
* @param name Tag name to be inflated.
* @param context The context the view is being created in.
* @param attrs Inflation attributes as specified in XML file.
*
* @return View Newly created view. Return null for the default
*         behavior.
*/
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);

适配器模式，用代理包装类FactoryWrapperHC extends LayoutInflaterCompatBase.FactoryWrapper implements LayoutInflater.Factory2实现转化

回来看看ImplV9到底塞了一个什么Factory进去

LayoutInflaterCompat.setFactory(layoutInflater, this);

也就是v9自己实现的LayoutInflaterFactory接口onCreateView方法

/**
* From {@link android.support.v4.view.LayoutInflaterFactory}
*/
@Override
public final View onCreateView(View parent, String name,
Context context, AttributeSet attrs) {
// First let the Activity's Factory try and inflate the view
final View view = callActivityOnCreateView(parent, name, context, attrs);
//常用流程里 并不会返回view 但是有个大大的疑问？
if (view != null) {
return view;
}

// If the Factory didn't handle it, let our createView() method try
return createView(parent, name, context, attrs); //重点来了
}

createView方法

@Override
public View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
final boolean isPre21 = Build.VERSION.SDK_INT < 21;

if (mAppCompatViewInflater == null) {
mAppCompatViewInflater = new AppCompatViewInflater();
}

// We only want the View to inherit its context if we're running pre-v21
final boolean inheritContext = isPre21 && shouldInheritContext((ViewParent) parent);
//原来createView被替换成了这个AppCompatViewInflater
//shouldInheritContext方法：是否继承Context
return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
isPre21, /* 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 */
);
}

就是他 这个AppCompatViewInflater 干的！

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 it's android:onClick
checkOnClickListener(view, attrs);
}

return view;
}

待解决疑问：

AppCompatDelegateImplV9类里的onCreateView方法先交给Activity的Factory处理

View callActivityOnCreateView(View parent, String name, Context context, AttributeSet attrs) {
// Let the Activity's LayoutInflater.Factory try and handle it
if (mOriginalWindowCallback instanceof LayoutInflater.Factory) {
final View result = ((LayoutInflater.Factory) mOriginalWindowCallback)
.onCreateView(name, context, attrs);
if (result != null) {
return result;
}
}
return null;
}

mOriginalWindowCallback这个CallBack是什么时候被set成了一个LayoutInflater.Factory？？？

全局能找出setCallBack的 只有setActionBar和DelegateImplBase的构造 后者也只是把原Window的Callback用装饰模式套了个AppCompatWindowCallbackBase 真的是看得一脸懵逼~~~ 完全不知道这强转代码意义何在

源码中无从得知 待探究framework源码寻找答案