一、概述
在Activity
当中,我们一般都会调用setContentView
方法来初始化布局。
二、与ContentView
相关的方法
在Activity
当中,与ContentView
相关的函数有下面这几个,我们先看一下它们的注释说明:
/**
* Set the activity content from a layout resource. The resource will be
* inflated, adding all top-level views to the activity.
*
* @param layoutResID Resource ID to be inflated.
*
* @see #setContentView(android.view.View)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
/**
* Set the activity content to an explicit view. This view is placed
* directly into the activity's view hierarchy. It can itself be a complex
* view hierarchy. When calling this method, the layout parameters of the
* specified view are ignored. Both the width and the height of the view are
* set by default to {@link ViewGroup.LayoutParams#MATCH_PARENT}. To use
* your own layout parameters, invoke
* {@link #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)}
* instead.
*
* @param view The desired content to display.
*
* @see #setContentView(int)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(View view) {
getWindow().setContentView(view);
initWindowDecorActionBar();
}
/**
* Set the activity content to an explicit view. This view is placed
* directly into the activity's view hierarchy. It can itself be a complex
* view hierarchy.
*
* @param view The desired content to display.
* @param params Layout parameters for the view.
*
* @see #setContentView(android.view.View)
* @see #setContentView(int)
*/
public void setContentView(View view, ViewGroup.LayoutParams params) {
getWindow().setContentView(view, params);
initWindowDecorActionBar();
}
/**
* Add an additional content view to the activity. Added after any existing
* ones in the activity -- existing views are NOT removed.
*
* @param view The desired content to display.
* @param params Layout parameters for the view.
*/
public void addContentView(View view, ViewGroup.LayoutParams params) {
getWindow().addContentView(view, params);
initWindowDecorActionBar();
}
通过上面的注释,可以看到这4个方法的用途:
- 第一种:渲染
layouResId
对应的布局,并将它添加到activity
的顶级View
中。 - 第二种:将
View
添加到activity
的布局当中,它的默认宽高都是ViewGroup.LayoutParams#MATCH_PARENT
。 - 第三种:和上面相同,但是指定了
LayoutParams
。 - 第四种:将内容添加进去,并且必须指定
LayoutParams
,已经存在的View
不会被移除。
这四种方法其实都是调用了PhoneWindow.java
中的方法,通过源码我们可以发现setContentView(View view, ViewGroup.LayoutParams params)
和setContentView(@LayoutRes int layoutResID)
的步骤基本上是一样的,只不过是在添加到布局的时候,前者因为已经获得了View
的实例,因此用的是addView
的方法,而后者因为需要先inflate
,所以,使用的是LayoutInflater
。
三、setContentView
方法
下面我们以setContentView(@LayoutRes int layoutResID)
为例,看一下具体的实现步骤:
3.1 setContentView
@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);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
首先,我们会判断mContentParent
是否为空,通过添加的代码我们可以知道,这个mContentParent
其实就是layoutResId
最后渲染出的布局所对应的父容器,当这个ContentParent
为空时,调用了installDecor
,mContentParent
就是在里面初始化的。
3.2 installDecor()
private void installDecor() {
//如果DecorView不存在,那么先生成它,它其实是一个FrameLayout。
if (mDecor == null) {
mDecor = generateDecor();
}
//如果`ContentParent`不存在,那么也生成它,此时传入了前面的`DecorView`
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(R.id.decor_content_parent);
if (decorContentParent != null) {
mDecorContentParent = decorContentParent;
}
}
}
我们可以看到,mDecor
是一个FrameLayout
,它和mContentParent
的关系是通过mContentParent = generateLayout(mDecor)
产生。
3.3 generateLayout(DecorView decor)
protected ViewGroup generateLayout(DecorView decor) {
//...首先根据不同的情况,给`layoutResource`赋予不同的值.
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;
ViewGroup contentParent = (ViewGroup) findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}
//...
return contentParent;
}
在上面赋值的过程中,我们主要关注以下几个变量,mContentRoot/mContentParent/mDecorContent
:
-
mContentRoot
一定是mDecor
的下一级子容器。 -
mContentParent
是mDecor
当中id
为R.id.content
的ViewGroup
,但是它和mDecor
的具体层级关系不确定,这依赖于mContentRoot
是通过哪个xml
渲染出来。 -
mContentParent
一定是传入的layoutResId
进行inflate
完成之后的父容器,它一定不会为空,否则会抛出异常,我们setContentView(xxx)
方法传入的布局,就是它的子View
。
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
// This is the view in which the window contents are placed. It is either
// mDecor itself, or a child of mDecor where the contents go.
private ViewGroup mContentParent;
-
mDecorContent
则是mDecor
当中id
为decor_content_parent
的ViewGroup
,但是也有可能mDecor
当中没有这个id
的View
,这需要依赖与我们的mContentRoot
是使用了哪个xml
来inflate
的。
再回到前面setContentView
的地方,继续往下看,当mContentParent
不为空的时候,那么会移除它底下的所有子View
。
之后会调用mLayoutInflater.inflate(layoutResID, mContentParent);
方法,把传入的View
添加到mContentParent
当中,最后回调一个监听。
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
3.4 mContentParentExplicitlySet
标志位
在setContentView
的最后,将mContentParentExplicitlySet
这个变量设置为true
,这个变量其实是用在requestFeature
当中,也就是说,我们必须在调用setContentView
之前,调用requestFeature
,否则就会抛出下面的异常:
@Override
public boolean requestFeature(int featureId) {
if (mContentParentExplicitlySet) {
throw new AndroidRuntimeException("requestFeature() must be called before adding content");
}
return super.requestFeature(featureId);
}
因此:requestFeature(xxx)
必须要在调用setContentView(xxx)
之前。
三、addContentView(View view, ViewGroup.LayoutParams params)
下面我们再来看一下,addContentView
方法:
@Override
public void addContentView(View view, ViewGroup.LayoutParams params) {
if (mContentParent == null) {
installDecor();
}
mContentParent.addView(view, params);
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
可以看到,它和set
方法的区别就是,它在添加到mContentParent
之前,并没有把mContentParent
的所有子View
都移除,而是将它直接添加进去,通过布局分析软件,可以看到mContentParent
的类型为ContentFrameLayout
,它其实是一个FrameLayout
,因此,它会覆盖在mContentParent
已有子View
之上。
四、将添加的布局和Activity
的Window
关联起来
在上面的分析当中,我们仅仅是初始化了一个DecorView
,并根据设置的Style
属性,传入的ContentView
来初始化它的子布局,但是这时候它还有真正和Activity
的Window
关联起来,关联的地方在ActivityThread.java
中:
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
r = performResumeActivity(token, clearHide, reason);
if (r != null) {
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (a.mVisibleFromClient && !a.mWindowAdded) {
a.mWindowAdded = true;
wm.addView(decor, l);
}
} else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
} else {
}
}
从源码中可以看到,如果在执行handleResumeActivity
时,之前DecorView
没有被添加到WindowManager
当中时,那么它的第一次添加是在onResume()
方法执行完之后添加的。