Android源碼學習之組合模式定義及應用

2020-04-11 12:36:43

字體：大中小

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組合模式定義：

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

將對象組合成樹形結構以表示“部分-整體”的層次結構，使得用戶對單個對象和組合對象的使用具有一致性。

如上圖所示（截取自《Head First Design Patterns》一書），主要包括三個部分：

1. Component抽象組件。定義參加組合對象的共有方法和屬性，可以定義一些默認的函數或屬性。

2. Leaf葉子節點。構成組合樹的最小構建單元。

3. Composite樹枝節點組件。它的作用是組合樹枝節點和葉子節點形成一個樹形結構。

高層模塊調用簡單。一棵樹形結構的所有節點都是Component，局部和整體對調用者來說都是一樣的，沒有區別，所以高層模塊不比關心自己處理的是單個對象還是整個組合結構，簡化了高層模塊的代碼。

節點自由擴展增加。使用組合模式，如果想增加一個樹枝節點或者葉子節點都是很簡單的，只要找到它的父節點就可以了，非常容易擴展，符合“開閉原則”。

應用最廣的模式之一。應用在維護和展示部分-整體關系的場景，如樹形菜單、文件夾管理等等。

在Android源碼中，都能找到使用組合模式的例子，其中在《Android源碼學習之觀察者模式應用》介紹到的ViewGroup和View的結構就是一個組合模式，結構圖如下所示：

現在來看看它們是如何利用組合模式組織在一起的，首先在View類定義了有關具體操作，然后在ViewGroup類中繼承View類，并添加相關的增加、刪除和查找孩子View節點，代碼如下：

復制代碼代碼如下:

* @attr ref android.R.styleable#ViewGroup_clipChildren 
* @attr ref android.R.styleable#ViewGroup_clipToPadding 
* @attr ref android.R.styleable#ViewGroup_layoutAnimation 
* @attr ref android.R.styleable#ViewGroup_animationCache 
* @attr ref android.R.styleable#ViewGroup_persistentDrawingCache 
* @attr ref android.R.styleable#ViewGroup_alwaysDrawnWithCache 
* @attr ref android.R.styleable#ViewGroup_addStatesFromChildren 
* @attr ref android.R.styleable#ViewGroup_descendantFocusability 
* @attr ref android.R.styleable#ViewGroup_animateLayoutChanges 
*/ 
public abstract class ViewGroup extends View implements ViewParent, ViewManager {

接著看增加孩子節點函數：

復制代碼代碼如下:

/** 
* Adds a child view. If no layout parameters are already set on the child, the 
* default parameters for this ViewGroup are set on the child. 
* 
* @param child the child view to add 
* 
* @see #generateDefaultLayoutParams() 
*/ 
public void addView(View child) { 
addView(child, -1); 
} 

/** 
* Adds a child view. If no layout parameters are already set on the child, the 
* default parameters for this ViewGroup are set on the child. 
* 
* @param child the child view to add 
* @param index the position at which to add the child 
* 
* @see #generateDefaultLayoutParams() 
*/ 
public void addView(View child, int index) { 
LayoutParams params = child.getLayoutParams(); 
if (params == null) { 
params = generateDefaultLayoutParams(); 
if (params == null) { 
throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null"); 
} 
} 
addView(child, index, params); 
} 

/** 
* Adds a child view with this ViewGroup's default layout parameters and the 
* specified width and height. 
* 
* @param child the child view to add 
*/ 
public void addView(View child, int width, int height) { 
final LayoutParams params = generateDefaultLayoutParams(); 
params.width = width; 
params.height = height; 
addView(child, -1, params); 
} 

/** 
* Adds a child view with the specified layout parameters. 
* 
* @param child the child view to add 
* @param params the layout parameters to set on the child 
*/ 
public void addView(View child, LayoutParams params) { 
addView(child, -1, params); 
} 

/** 
* Adds a child view with the specified layout parameters. 
* 
* @param child the child view to add 
* @param index the position at which to add the child 
* @param params the layout parameters to set on the child 
*/ 
public void addView(View child, int index, LayoutParams params) { 
if (DBG) { 
System.out.println(this + " addView"); 
} 

// addViewInner() will call child.requestLayout() when setting the new LayoutParams 
// therefore, we call requestLayout() on ourselves before, so that the child's request 
// will be blocked at our level 
requestLayout(); 
invalidate(true); 
addViewInner(child, index, params, false); 
}

在ViewGroup中我們找到了添加addView()方法，有了增加孩子節點，肯定有相對應刪除孩子節點的方法，接著看：

復制代碼代碼如下:

 

public void removeView(View view) { 
removeViewInternal(view); 
requestLayout(); 
invalidate(true); 
} 

/** 
* Removes a view during layout. This is useful if in your onLayout() method, 
* you need to remove more views. 
* 
* @param view the view to remove from the group 
*/ 
public void removeViewInLayout(View view) { 
removeViewInternal(view); 
} 

/** 
* Removes a range of views during layout. This is useful if in your onLayout() method, 
* you need to remove more views. 
* 
* @param start the index of the first view to remove from the group 
* @param count the number of views to remove from the group 
*/ 
public void removeViewsInLayout(int start, int count) { 
removeViewsInternal(start, count); 
} 

/** 
* Removes the view at the specified position in the group. 
* 
* @param index the position in the group of the view to remove 
*/ 
public void removeViewAt(int index) { 
removeViewInternal(index, getChildAt(index)); 
requestLayout(); 
invalidate(true); 
} 

/** 
* Removes the specified range of views from the group. 
* 
* @param start the first position in the group of the range of views to remove 
* @param count the number of views to remove 
*/ 
public void removeViews(int start, int count) { 
removeViewsInternal(start, count); 
requestLayout(); 
invalidate(true); 
} 

private void removeViewInternal(View view) { 
final int index = indexOfChild(view); 
if (index >= 0) { 
removeViewInternal(index, view); 
} 
} 

private void removeViewInternal(int index, View view) { 

if (mTransition != null) { 
mTransition.removeChild(this, view); 
} 

boolean clearChildFocus = false; 
if (view == mFocused) { 
view.clearFocusForRemoval(); 
clearChildFocus = true; 
} 

if (view.getAnimation() != null || 
(mTransitioningViews != null && mTransitioningViews.contains(view))) { 
addDisappearingView(view); 
} else if (view.mAttachInfo != null) { 
view.dispatchDetachedFromWindow(); 
} 

onViewRemoved(view); 

needGlobalAttributesUpdate(false); 

removeFromArray(index); 

if (clearChildFocus) { 
clearChildFocus(view); 
} 
} 

/** 
* Sets the LayoutTransition object for this ViewGroup. If the LayoutTransition object is 
* not null, changes in layout which occur because of children being added to or removed from 
* the ViewGroup will be animated according to the animations defined in that LayoutTransition 
* object. By default, the transition object is null (so layout changes are not animated). 
* 
* @param transition The LayoutTransition object that will animated changes in layout. A value 
* of <code>null</code> means no transition will run on layout changes. 
* @attr ref android.R.styleable#ViewGroup_animateLayoutChanges 
*/ 
public void setLayoutTransition(LayoutTransition transition) { 
if (mTransition != null) { 
mTransition.removeTransitionListener(mLayoutTransitionListener); 
} 
mTransition = transition; 
if (mTransition != null) { 
mTransition.addTransitionListener(mLayoutTransitionListener); 
} 
} 

/** 
* Gets the LayoutTransition object for this ViewGroup. If the LayoutTransition object is 
* not null, changes in layout which occur because of children being added to or removed from 
* the ViewGroup will be animated according to the animations defined in that LayoutTransition 
* object. By default, the transition object is null (so layout changes are not animated). 
* 
* @return LayoutTranstion The LayoutTransition object that will animated changes in layout. 
* A value of <code>null</code> means no transition will run on layout changes. 
*/ 
public LayoutTransition getLayoutTransition() { 
return mTransition; 
} 

private void removeViewsInternal(int start, int count) { 
final View focused = mFocused; 
final boolean detach = mAttachInfo != null; 
View clearChildFocus = null; 

final View[] children = mChildren; 
final int end = start + count; 

for (int i = start; i < end; i++) { 
final View view = children[i]; 

if (mTransition != null) { 
mTransition.removeChild(this, view); 
} 

if (view == focused) { 
view.clearFocusForRemoval(); 
clearChildFocus = view; 
} 

if (view.getAnimation() != null || 
(mTransitioningViews != null && mTransitioningViews.contains(view))) { 
addDisappearingView(view); 
} else if (detach) { 
view.dispatchDetachedFromWindow(); 
} 

needGlobalAttributesUpdate(false); 

onViewRemoved(view); 
} 

removeFromArray(start, count); 

if (clearChildFocus != null) { 
clearChildFocus(clearChildFocus); 
} 
} 

/** 
* Call this method to remove all child views from the 
* ViewGroup. 
*/ 
public void removeAllViews() { 
removeAllViewsInLayout(); 
requestLayout(); 
invalidate(true); 
} 

/** 
* Called by a ViewGroup subclass to remove child views from itself, 
* when it must first know its size on screen before it can calculate how many 
* child views it will render. An example is a Gallery or a ListView, which 
* may "have" 50 children, but actually only render the number of children 
* that can currently fit inside the object on screen. Do not call 
* this method unless you are extending ViewGroup and understand the 
* view measuring and layout pipeline. 
*/ 
public void removeAllViewsInLayout() { 
final int count = mChildrenCount; 
if (count <= 0) { 
return; 
} 

final View[] children = mChildren; 
mChildrenCount = 0; 

final View focused = mFocused; 
final boolean detach = mAttachInfo != null; 
View clearChildFocus = null; 
needGlobalAttributesUpdate(false); 

for (int i = count - 1; i >= 0; i--) { 
final View view = children[i]; 

if (mTransition != null) { 
mTransition.removeChild(this, view); 
} 

if (view == focused) { 
view.clearFocusForRemoval(); 
clearChildFocus = view; 
} 

if (view.getAnimation() != null || 
(mTransitioningViews != null && mTransitioningViews.contains(view))) { 
addDisappearingView(view); 
} else if (detach) { 
view.dispatchDetachedFromWindow(); 
} 

onViewRemoved(view); 

view.mParent = null; 
children[i] = null; 
} 

if (clearChildFocus != null) { 
clearChildFocus(clearChildFocus); 
} 
} 

/** 
* Finishes the removal of a detached view. This method will dispatch the detached from 
* window event and notify the hierarchy change listener. 
* 
* @param child the child to be definitely removed from the view hierarchy 
* @param animate if true and the view has an animation, the view is placed in the 
* disappearing views list, otherwise, it is detached from the window 
* 
* @see #attachViewToParent(View, int, android.view.ViewGroup.LayoutParams) 
* @see #detachAllViewsFromParent() 
* @see #detachViewFromParent(View) 
* @see #detachViewFromParent(int) 
*/ 
protected void removeDetachedView(View child, boolean animate) { 
if (mTransition != null) { 
mTransition.removeChild(this, child); 
} 

if (child == mFocused) { 
child.clearFocus(); 
} 

if ((animate && child.getAnimation() != null) || 
(mTransitioningViews != null && mTransitioningViews.contains(child))) { 
addDisappearingView(child); 
} else if (child.mAttachInfo != null) { 
child.dispatchDetachedFromWindow(); 
} 

onViewRemoved(child); 
}

同樣的，也有查找獲得孩子節點的函數：

復制代碼代碼如下:

/** 
* Returns the view at the specified position in the group. 
* 
* @param index the position at which to get the view from 
* @return the view at the specified position or null if the position 
* does not exist within the group 
*/ 
public View getChildAt(int index) { 
if (index < 0 || index >= mChildrenCount) { 
return null; 
} 
return mChildren[index]; 
}

注：其中具體葉子節點，如Button，它是繼承TextView的，TextView是繼承View的，代碼如下：

復制代碼代碼如下:

public class TextView extends View implements ViewTreeObserver.OnPreDrawListener { 
。。。 
}

注：其中使用（繼承）到ViewGroup類的有我們常用的容器類（包裝和容納各種View），如LinearLayout、FrameLayout等，代碼如下：

復制代碼代碼如下:

public class LinearLayout extends ViewGroup { 
public static final int HORIZONTAL = 0; 
public static final int VERTICAL = 1; 
。。。 
} 

public class FrameLayout extends ViewGroup { 
... 
} 

public class RelativeLayout extends ViewGroup { 
private static final String LOG_TAG = "RelativeLayout"; 

private static final boolean DEBUG_GRAPH = false; 
... 
} 

public class AbsoluteLayout extends ViewGroup { 
public AbsoluteLayout(Context context) { 
super(context); 
} 
} 
...