首页 > 代码库 > 【转】LayoutInflater——80%的Android程序员对它并不了解甚至错误使用

【转】LayoutInflater——80%的Android程序员对它并不了解甚至错误使用

2024-07-16 23:14:45   222人阅读

这个标题起的有点夸张哈，但是LayoutInflater这个类的一些用法，在Android开发者使用的过程中，确实存在着一些很普遍的误区，最起码我研究的这么多小项目的源代码，基本上都在错误的使用这个类。今天，看到了一篇文章讲LayoutInflater的用法，瞬间感觉自己对这个类确实不够了解，于是简单的看了下LayoutInflater类的源代码，对这个类有了新的认识。

首先，LayoutInflater这个类是用来干嘛的呢？

我们最常用的便是LayoutInflater的inflate方法，这个方法重载了四种调用方式，分别为：

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)

 

这四种使用方式中，我们最常用的是第一种方式，inflate方法的主要作用就是将xml转换成一个View对象，用于动态的创建布局。虽然重载了四个方法，但是这四种方法最终调用的，还是第四种方式。第四种方式也很好理解，内部实现原理就是利用Pull解析器，对Xml文件进行解析，然后返回View对象。

我们以我们经常使用的第一种形式为例，你在重写BaseAdapter的getView方法的时候是否这样做过

 

?
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;
}

inflate方法有三个参数，分别是

 

1.resource 布局的资源id

2.root 填充的根视图

3.attachToRoot 是否将载入的视图绑定到根视图中

在这个例子中，我们将root参数设为空，功能确实实现了，但是这里还隐藏着一个隐患，这种方式并不是inflate正确的使用姿势，下面我们通过一个Demo，来说一下这样使用造成的弊端。

 

首先，我们建立一个这样的项目

\

<喎?"http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+1eLA78j9uPa958Pmo6zSu7j21ve958Pmo6zBvbj2suLK1L3nw+ajrLK8vtbOxLz+1tCjrNb3vefD5ta7uLrU8L3nw+bM+Neqo6zBvbj2suLK1L3nw+a2vMrH0ru49rzytaW1xExpc3R2aWV3o6xpdGVtsry+1s/Uyr7Qp7n7yOfPwjwvcD4KPHA+PGltZyBzcmM9"http://www.2cto.com/uploadfile/Collfiles/20140701/20140701085327132.jpg" alt="\">

 

对应的布局文件如下

 

?
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的代码如下

 

 

?
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的代码如下

 

 

?
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;
        }
 
    }
 
}


两个文件最关键的区别就一句话，

 

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);

 

我们先看一下显示效果，再说两者的区别

OneActivity效果

\

 

TwoActivity的显示效果

\

 

我们可以很明显的看出来，使用第一种方式，根布局的高度设置60dp没有起作用，系统还是按照包裹内容的方式加载的，为什么会产生这种效果呢？我们从需要inflate方法的源代码中找一下答案。

 

首先，方式一的源代码实现

 

?
1
2
3
public View inflate(XmlPullParser parser, ViewGroup root) {
        return inflate(parser, root, root != null);
    }


当我们使用方式一，并且第二个参数传入null的时候，默认调用的是下面的方法，并且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();
        }
    }


在这一个方法中，pull解析器将资源id转化成XmlResourceParser对象，又传给了第四种方式，所以我们需要重点看的还是第四种方式是如何实现的

 

 

?
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>


代码比较长，我们重点关注下面的代码

 

 

?
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);
                        }
                    }


这些代码的意思就是，当我们传进来的root参数不是空的时候，并且attachToRoot是false的时候，也就是上面的TwoActivity的实现方式的时候，会给temp设置一个LayoutParams参数。那么这个temp又是干嘛的呢？

 

 

?
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>
现在应该明白了吧，当我们传进来的root不是null，并且第三个参数是false的时候，这个temp就被加入到了root中，并且把root当作最终的返回值返回了。而当我们设置root为空的时候，没有设置LayoutParams参数的temp对象，作为返回值返回了。
<p></p>
<p>因此，我们可以得出下面的结论：</p>
<p>1.若我们采用convertView = inflater.inflate(R.layout.item_list,
 null);方式填充视图，item布局中的根视图的layout_XX属性会被忽略掉，然后设置成默认的包裹内容方式</p>
<p>2.如果我们想保证item的视图中的参数不被改变，我们需要使用convertView
 = inflater.inflate(R.layout.item_list, parent,false);这种方式进行视图的填充</p>
<p>3.除了使用这种方式，我们还可以设置item布局的根视图为包裹内容，然后设置内部控件的高度等属性，这样就不会修改显示方式了。</p>
<p><br>
</p>
<p>最后，给出那篇文章的链接http://blog.jobbole.com/72156/
大家可以去看看</p>                    


联系
我们