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关于java:Android中的AppCompatActivity的偷梁换柱之UI偷换

PS:本文系转载文章,浏览原文可读性会更好,文章开端有原文链接

ps:源码是基于 android api 27 来剖析的

应用 Android API 21 的时候,google 举荐咱们用的 Activity 是 ActionBarActivity;在 Android Level 21 之后,Android 引入了 Material Design 的设计,为了反对 Material,Color、调色版、Toolbar 等各种新个性,AppCompatActivity 就利用而生;AppCompatActivity 为了兼容以前的货色,就做了移花接木,本篇文章咱们就写移花接木中的 UI 偷换。

为了不便剖析过程,咱们写一个 demo,demo 的 AppCompatActivity 的代码如下所示:

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {

    //1、super.onCreate(savedInstanceState);

    //2、setContentView(R.layout.activity_main);

    Button btn = findViewById(R.id.btn);
}

}

咱们来看正文 1 中的代码,也就是 AppCompatActivity 的 onCreate 办法;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    //3、final AppCompatDelegate delegate = getDelegate();
    
    //4、delegate.installViewFactory();
    ......
}

正文 3 中的 getDelegate 办法最终会调用到 AppCompatDelegate 的 create(Context context, Window window,AppCompatCallback callback) 办法;

private static AppCompatDelegate create(Context context, Window window,

        AppCompatCallback callback) {if (BuildCompat.isAtLeastO()) {return new AppCompatDelegateImplO(context, window, callback);
    } else 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 if (Build.VERSION.SDK_INT >= 14) {return new AppCompatDelegateImplV14(context, window, callback);
    } else if (Build.VERSION.SDK_INT >= 11) {return new AppCompatDelegateImplV11(context, window, callback);
    } else {return new AppCompatDelegateImplV9(context, window, callback);
    }

}

create(Context context, Window window,AppCompatCallback callback) 办法会依据 Android API 来初始化 AppCompatDelegate 相应的子类对象,咱们就拿 AppCompatDelegateImplV9 来进行剖析;咱们回到下面的 AppCompatActivity 的 onCreate 办法中的正文 4,这时候的 delegate 实质就是 AppCompatDelegateImplV9 类型的对象,那么正文 4 的具体方法如下所示;

@Override
public void installViewFactory() {LayoutInflater layoutInflater = LayoutInflater.from(mContext);
    if (layoutInflater.getFactory() == null) {
        
        //5、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");
        }
    }
}

这里的 layoutInflater.getFactory() == null 条件为 true,所以咱们看正文 5 的代码,也就是 LayoutInflaterCompat 的 setFactory2(@NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) 办法;

public static void setFactory2(

        @NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) {IMPL.setFactory2(inflater, factory);

}

这里的 IMPL 是 LayoutInflaterCompatBaseImpl 类型的对象,然而 LayoutInflaterCompatBaseImpl 的实现类有 2 个,咱们先看 IMPL 的实例化;

static {

    if (Build.VERSION.SDK_INT >= 21) {IMPL = new LayoutInflaterCompatApi21Impl();
    } else {IMPL = new LayoutInflaterCompatBaseImpl();
    }

}

它是依据 Android API 是否大于等于 21 来决定用哪个子类来实例化的,咱们就选 LayoutInflaterCompat 的动态外部类 LayoutInflaterCompatBaseImpl 来剖析,咱们看看 LayoutInflaterCompatBaseImpl 的 setFactory2(LayoutInflater inflater, LayoutInflater.Factory2 factory) 办法;

public void setFactory2(LayoutInflater inflater, LayoutInflater.Factory2 factory) {

    inflater.setFactory2(factory);
    ......

}

咱们再往下看 LayoutInflater 的 setFactory2(LayoutInflater.Factory2 factory) 办法;

public void setFactory2(LayoutInflater.Factory2 factory) {

    ......
    //6、if (mFactory == null) {mFactory = mFactory2 = factory;} else {mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
    }

}

在下面所讲的 AppCompatDelegateImplV9 的 installViewFactory 办法中的 layoutInflater.getFactory() == null 条件为 true,下面的 layoutInflater.getFactory() 拿到的其实是 LayoutInflater 的 mFactory,所以这里的 mFactory == null 也为 true;在下面所说的,咱们拿的是 AppCompatDelegate 的子类 AppCompatDelegateImplV9 来做剖析的,所以 LayoutInflater 的 setFactory2(LayoutInflater.Factory2 factory) 办法中的 factory、mFactory2 和 mFactory 实质上是 AppCompatDelegateImplV9 对象,这里的 LayoutInflater 的 setFactory2(LayoutInflater.Factory2 factory) 办法只是保留了 AppCompatDelegateImplV9 对象。

咱们回到 demo 中正文 2 的代码,也就是 AppCompatActivity 的 setContentView(@LayoutRes int layoutResID) 办法;

@Override
public void setContentView(@LayoutRes int layoutResID) {getDelegate().setContentView(layoutResID);
}

下面说过 getDelegate 办法拿到的是 AppCompatDelegate 类型的对象,咱们就拿 AppCompatDelegate 的子类 AppCompatDelegateImplV9 来剖析,所以看 AppCompatDelegateImplV9 的 setContentView(int resId) 办法;

@Override
public void setContentView(int resId) {
    ......
    LayoutInflater.from(mContext).inflate(resId, contentParent);
    ......
}

这里通过 LayoutInflater 的 inflate(@LayoutRes int resource, @Nullable ViewGroup root) 办法从 xml 文件解析元素生成对应的 View,咱们看 inflate(@LayoutRes int resource, @Nullable ViewGroup root) 办法;

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {

    return inflate(resource, root, root != null);

}

inflate(@LayoutRes int resource, @Nullable ViewGroup root) 办法调用了 LayoutInflater 的 inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) 办法;

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {

    final Resources res = getContext().getResources();
    if (DEBUG) {Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("+ Integer.toHexString(resource) +")");
    }

    final XmlResourceParser parser = res.getLayout(resource);
    try {return inflate(parser, root, attachToRoot);
    } finally {parser.close();
    }

}

inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) 办法又调用了 LayoutInflater 的 inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) 办法;

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {

    synchronized (mConstructorArgs) {
        ......
        try {
            ......
            if (TAG_MERGE.equals(name)) {......} else {
                // Temp is the root view that was found in the xml
                //7、final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                ......
            }

        } catch (XmlPullParserException e) {......} catch (Exception e) {......} finally {......}

        return result;
    }

}

咱们看看正文 7 中的办法,也就是 LayoutInflater 的 createViewFromTag(View parent, String name, Context context, AttributeSet attrs) 办法,它的作用是创立 View;

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {

    return createViewFromTag(parent, name, context, attrs, false);

}

createViewFromTag(View parent, String name, Context context, AttributeSet attrs) 办法又调用了 LayoutInflater 的 createViewFromTag(View parent, String name, Context context, AttributeSet attrs,boolean ignoreThemeAttr) 办法;

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,

                       boolean ignoreThemeAttr) {
    ......
    try {
        View view;
        if (mFactory2 != null) {
            //8、view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {view = mFactory.onCreateView(name, context, attrs);
        } else {view = null;}
        ......
        return view;
    } catch (InflateException e) {......} catch (ClassNotFoundException e) {......} catch (Exception e) {......}

}

这里看一下正文 8 中的 mFactory2,下面咱们对 mFactory2 进行了赋值,它的值就是咱们下面所说的拿来做剖析的 AppCompatDelegateImplV9,咱们来看 AppCompatDelegateImplV9 的 onCreateView(View parent, String name, Context context, AttributeSet attrs) 办法;

@Override
public final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
     ......
    // If the Factory didn't handle it, let our createView() method try
    return createView(parent, name, context, attrs);
}

onCreateView(View parent, String name, Context context, AttributeSet attrs) 办法调用了 AppCompatDelegateImplV9 的 createView(View parent, final String name, @NonNull Context context,@NonNull AttributeSet attrs) 办法;

@Override
public View createView(View parent, final String name, @NonNull Context context,
                       @NonNull AttributeSet attrs) {
    ......
    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 */);
}

mAppCompatViewInflater 是 AppCompatViewInflater 类型的对象,咱们看 AppCompatViewInflater 的 createView(View parent, final String name, @NonNull Context context,@NonNull AttributeSet attrs,boolean inheritContext,boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) 办法;

public final View createView(View parent, final String name, @NonNull Context context,

                             @NonNull AttributeSet attrs, boolean inheritContext,
                             boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
    ......
    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;
    }
    ......
    return view;

}

看到没,如果咱们的类持续的是 AppCompatActivity,而不是 Activity,那么在 xml 文件中的 View 元素被创立之前会被拦挡,在 xml 文件中的 View 元素是 XXX,那么真正创立进去的是 AppCompatXXX,AppCompatActivity 把咱们 xml 文件中实在的 View 元素 XXX 换成了 AppCompatXXX。

咱们验证一下,把 demo 里的 Button 输入一下;

Button btn = findViewById(R.id.btn);
Log.d(“MainActivity”,”—Button = ” + btn);

日志打印如下所示;

D/MainActivity: —Button = android.support.v7.widget.AppCompatButton{adc47a3 VFED..C.. ……I. 0,0-0,0 #7f070021 app:id/btn}

看见没,咱们 demo 中的 Button 被替换成了 AppCompatButton,Button 实质上是 AppCompatButton 了。

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