LayoutInflater這個類的一些用法,在Android開發者使用的過程中,確實存在着一些很普遍的誤區,最起碼我研究的這麼多小項目的源代碼,基本上都在錯誤的使用這個類。今天,看到了一篇文章講LayoutInflater的用法,瞬間感覺自己對這個類確實不夠了解,於是簡單的看了下LayoutInflater類的源代碼,對這個類有了新的認識。
首先,LayoutInflater這個類是用來幹嘛的呢?
我們最常用的便是LayoutInflater的inflate方法,這個方法重載了四種調用方式,分別爲:
1. public View inflate(int resource, ViewGroup root)
2. public View inflate(int resource, ViewGroup root, boolean attachToRoot)
3.public View inflate(XmlPullParser parser, ViewGroup root)
4.public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)
這四種使用方式中,我們最常用的是第一種方式,inflate方法的主要作用就是將xml轉換成一個View對象,用於動態的創建佈局。雖然重載了四個方法,但是這四種方法最終調用的,還是第四種方式。第四種方式也很好理解,內部實現原理就是利用Pull解析器,對Xml文件進行解析,然後返回View對象。
我們以我們經常使用的第一種形式爲例,你在重寫BaseAdapter的getView方法的時候是否這樣做過
1
2
3
4
5
6
|
public
View getView( int
position, View convertView, ViewGroup parent) { if
(convertView == null )
{ convertView
= inflate(R.layout.item_row, null ); } return
convertView; } |
1.resource 佈局的資源id
2.root 填充的根視圖
3.attachToRoot 是否將載入的視圖綁定到根視圖中
在這個例子中,我們將root參數設爲空,功能確實實現了,但是這裏還隱藏着一個隱患,這種方式並不是inflate正確的使用姿勢,下面我們通過一個Demo,來說一下這樣使用造成的弊端。
首先,我們建立一個這樣的項目
對應的佈局文件如下
1
2
3
4
5
6
|
<!--?xml
version= "1.0"
encoding= "utf-8" ?--> <linearlayout
xmlns:android= "http://schemas.android.com/apk/res/android"
android:layout_width= "match_parent"
android:layout_height= "60dp"
android:background= "@android:color/holo_orange_light"
android:gravity= "center"
android:orientation= "vertical" > <textview
android:id= "@+id/tv"
android:layout_width= "wrap_content"
android:layout_height= "wrap_content"
android:text= "11"
android:textcolor= "@android:color/black"
android:textsize= "22sp" > </textview></linearlayout> |
OneActivity的代碼如下
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
|
public
class
OneActivity extends
Activity { private
ListView list1; @Override protected
void
onCreate(Bundle savedInstanceState) { super .onCreate(savedInstanceState); setContentView(R.layout.activity_one); list1
= (ListView) findViewById(R.id.list1); list1.setAdapter( new
MyAdapter( this )); } private
class
MyAdapter extends
BaseAdapter { private
LayoutInflater inflater; MyAdapter(Context
context) { inflater
= LayoutInflater.from(context); } @Override public
int
getCount() { return
20 ; } @Override public
Object getItem( int
position) { return
position; } @Override public
long
getItemId( int
position) { return
position; } @Override public
View getView( int
position, View convertView, ViewGroup parent) { if
(convertView == null )
{ convertView
= inflater.inflate(R.layout.item_list, null ); } TextView
tv = (TextView) convertView.findViewById(R.id.tv); tv.setText(position+ "" ); return
convertView; } } } |
TwoActivity的代碼如下
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
|
public
class
TwoActivity extends
Activity { private
ListView list2; @Override protected
void
onCreate(Bundle savedInstanceState) { super .onCreate(savedInstanceState); setContentView(R.layout.activity_two); list2
= (ListView) findViewById(R.id.list2); list2.setAdapter( new
MyAdapter( this )); } private
class
MyAdapter extends
BaseAdapter { private
LayoutInflater inflater; MyAdapter(Context
context) { inflater
= LayoutInflater.from(context); } @Override public
int
getCount() { return
20 ; } @Override public
Object getItem( int
position) { return
position; } @Override public
long
getItemId( int
position) { return
position; } @Override public
View getView( int
position, View convertView, ViewGroup parent) { if
(convertView == null )
{ convertView
= inflater.inflate(R.layout.item_list, parent, false ); } TextView
tv = (TextView) convertView.findViewById(R.id.tv); tv.setText(position
+ "" ); return
convertView; } } } |
兩個文件最關鍵的區別就一句話,
在getView方法中,OneActivity是
convertView = inflater.inflate(R.layout.item_list, null);
在getView方法中,TwoActivity是
convertView = inflater.inflate(R.layout.item_list, parent,false);
我們先看一下顯示效果,再說兩者的區別
OneActivity效果
TwoActivity的顯示效果
我們可以很明顯的看出來,使用第一種方式,根佈局的高度設置60dp沒有起作用,系統還是按照包裹內容的方式加載的,爲什麼會產生這種效果呢?我們從需要inflate方法的源代碼中找一下答案。
首先,方式一的源代碼實現
1
2
3
|
public
View inflate(XmlPullParser parser, ViewGroup root) { return
inflate(parser, root, root != null ); } |
當我們使用方式一,並且第二個參數傳入null的時候,默認調用的是下面的方法,並且attachToRoot是false
1
2
3
4
5
6
7
8
9
|
public
View inflate( int
resource, ViewGroup root, boolean
attachToRoot) { if
(DEBUG) System.out.println( "INFLATING
from resource: "
+ resource); XmlResourceParser
parser = getContext().getResources().getLayout(resource); try
{ return
inflate(parser, root, attachToRoot); }
finally
{ parser.close(); } } |
在這一個方法中,pull解析器將資源id轉化成XmlResourceParser對象,又傳給了第四種方式,所以我們需要重點看的還是第四種方式是如何實現的
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
|
public
View inflate(XmlPullParser parser, ViewGroup root, boolean
attachToRoot) { synchronized
(mConstructorArgs) { Trace.traceBegin(Trace.TRACE_TAG_VIEW,
"inflate" ); final
AttributeSet attrs = Xml.asAttributeSet(parser); Context
lastContext = (Context)mConstructorArgs[ 0 ]; mConstructorArgs[ 0 ]
= mContext; View
result = root; try
{ //
Look for the root node. int
type; while
((type = parser.next()) != XmlPullParser.START_TAG && type
!= XmlPullParser.END_DOCUMENT) { //
Empty } if
(type != XmlPullParser.START_TAG) { throw
new
InflateException(parser.getPositionDescription() +
":
No start tag found!" ); } final
String name = parser.getName(); if
(DEBUG) { System.out.println( "**************************" ); System.out.println( "Creating
root view: " +
name); System.out.println( "**************************" ); } if
(TAG_MERGE.equals(name)) { if
(root == null
|| !attachToRoot) { throw
new
InflateException( "<merge>
can be used only with a valid " +
"ViewGroup
root and attachToRoot=true" ); } rInflate(parser,
root, attrs, false ); }
else
{ //
Temp is the root view that was found in the xml View
temp; if
(TAG_1995.equals(name)) { temp
= new
BlinkLayout(mContext, attrs); }
else
{ temp
= createViewFromTag(root, name, attrs); } ViewGroup.LayoutParams
params = null ; if
(root != null )
{ if
(DEBUG) { System.out.println( "Creating
params from root: "
+ root); } //
Create layout params that match root, if supplied params
= root.generateLayoutParams(attrs); if
(!attachToRoot) { //
Set the layout params for temp if we are not //
attaching. (If we are, we use addView, below) temp.setLayoutParams(params); } } if
(DEBUG) { System.out.println( "----->
start inflating children" ); } //
Inflate all children under temp rInflate(parser,
temp, attrs, true ); if
(DEBUG) { System.out.println( "----->
done inflating children" ); } //
We are supposed to attach all the views we found (int temp) //
to root. Do that now. if
(root != null
&& attachToRoot) { root.addView(temp,
params); } //
Decide whether to return the root that was passed in or the //
top view found in xml. if
(root == null
|| !attachToRoot) { result
= temp; } } }
catch
(XmlPullParserException e) { InflateException
ex = new
InflateException(e.getMessage()); ex.initCause(e); throw
ex; }
catch
(IOException e) { InflateException
ex = new
InflateException( parser.getPositionDescription() +
":
"
+ e.getMessage()); ex.initCause(e); throw
ex; }
finally
{ //
Don't retain static reference on context. mConstructorArgs[ 0 ]
= lastContext; mConstructorArgs[ 1 ]
= null ; } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return
result; } }</merge> |
代碼比較長,我們重點關注下面的代碼
1
2
3
4
5
6
7
8
9
10
11
12
13
|
if
(root != null )
{ if
(DEBUG) { System.out.println( "Creating
params from root: "
+ root); } //
Create layout params that match root, if supplied params
= root.generateLayoutParams(attrs); if
(!attachToRoot) { //
Set the layout params for temp if we are not //
attaching. (If we are, we use addView, below) temp.setLayoutParams(params); } } |
這些代碼的意思就是,當我們傳進來的root參數不是空的時候,並且attachToRoot是false的時候,也就是上面的TwoActivity的實現方式的時候,會給temp設置一個LayoutParams參數。那麼這個temp又是幹嘛的呢?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
|
<pre
name= "code"
class = "java" > //
We are supposed to attach all the views we found (int temp) //
to root. Do that now. if
(root != null
&& attachToRoot) { root.addView(temp,
params); } //
Decide whether to return the root that was passed in or the //
top view found in xml. if
(root == null
|| !attachToRoot) { result
= temp; }</pre><br> <br> <p></p> <pre
class = "brush:java;" ></pre> <br> 現在應該明白了吧,當我們傳進來的root不是 null ,並且第三個參數是 false 的時候,這個temp就被加入到了root中,並且把root當作最終的返回值返回了。而當我們設置root爲空的時候,沒有設置LayoutParams參數的temp對象,作爲返回值返回了。 <p></p> <p>因此,我們可以得出下面的結論:</p> <p> 1 .若我們採用convertView
= inflater.inflate(R.layout.item_list, null );方式填充視圖,item佈局中的根視圖的layout_XX屬性會被忽略掉,然後設置成默認的包裹內容方式</p> <p> 2 .如果我們想保證item的視圖中的參數不被改變,我們需要使用convertView =
inflater.inflate(R.layout.item_list, parent, false );這種方式進行視圖的填充</p> <p> 3 .除了使用這種方式,我們還可以設置item佈局的根視圖爲包裹內容,然後設置內部控件的高度等屬性,這樣就不會修改顯示方式了。</p> <p><br> </p> |