blocks|key|597086|text|如果您要添加来自xml的自定义View，也如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|597087|+<com.mypack.MyView
++++++...
++++++/>|code-block|syntax|javascript|597088|你将需要构造函数public+MyView(Context+context,+AttributeSet+attrs)，否则当Android试图膨胀你的View时，你会得到一个Exception。|597089|如果您从xml添加View，并指定android:style属性，如下所示：|597090|+<com.mypack.MyView
++++++style="@styles/MyCustomStyle"
++++++...
++++++/>|597091|第二个构造函数也将被调用，并在应用显式XML属性之前将样式默认为MyCustomStyle。|597092|当您希望应用程序中的所有视图都具有相同的样式时，通常使用第三个构造函数。|597093|entityMap^0|8|3|F|4|0|0|8|1E|23|4|2F|9|0|4|3|9|4|H|D|0|0|W|D|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|1G|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|1H|8|@$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1K|8|@]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1L|8|@]|D|@]|E|$]]]|V|$]]

If you will add your custom <code>View</code> from <code>xml</code> also like :

<pre><code> &lt;com.mypack.MyView
 ...
 /&gt;
</code></pre>

you will need the constructor <code>public MyView(Context context, AttributeSet attrs)</code>, otherwise you will get an <code>Exception</code> when Android tries to inflate your <code>View</code>.

If you add your <code>View</code> from <code>xml</code> and also specify the <code>android:style</code> attribute like :

<pre><code> &lt;com.mypack.MyView
 style="@styles/MyCustomStyle"
 ...
 /&gt;
</code></pre>

the 2nd constructor will also be called and default the style to <code>MyCustomStyle</code> before applying explicit XML attributes.

The third constructor is usually used when you want all of the Views in your application to have the same style.

blocks|key|4634190|text|如果你必须像现在讨论的那样包含三个构造器，你也可以这样做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4634191|public+MyView(Context+context)+{
++this(context,null,0);
}

public+MyView(Context+context,+AttributeSet+attrs)+{
++this(context,attrs,0);
}

public+MyView(Context+context,+AttributeSet+attrs,+int+defStyle)+{
++super(context,+attrs,+defStyle);
++doAdditionalConstructorWork();

}|code-block|syntax|javascript|4634192|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you have to include three constructors like the one under discussion now, you could do this too.

<pre><code>public MyView(Context context) {
 this(context,null,0);
}

public MyView(Context context, AttributeSet attrs) {
 this(context,attrs,0);
}

public MyView(Context context, AttributeSet attrs, int defStyle) {
 super(context, attrs, defStyle);
 doAdditionalConstructorWork();

}
</code></pre>

blocks|key|4634228|text|如果覆盖所有三个构造函数，请不要级联this(...)调用。相反，您应该这样做：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4634229|public+MyView(Context+context)+{
++++super(context);
++++init(context,+null,+0);
}

public+MyView(Context+context,+AttributeSet+attrs)+{
++++super(context,attrs);
++++init(context,+attrs,+0);
}

public+MyView(Context+context,+AttributeSet+attrs,+int+defStyle)+{
++++super(context,+attrs,+defStyle);
++++init(context,+attrs,+defStyle);
}

private+void+init(Context+context,+AttributeSet+attrs,+int+defStyle)+{
++++//+do+additional+work
}|code-block|syntax|javascript|4634230|原因是父类可能在其自己的构造函数中包含默认属性，您可能会意外重写这些属性。例如，这是TextView的构造函数|4634231|public+TextView(Context+context)+{
++++this(context,+null);
}

public+TextView(Context+context,+@Nullable+AttributeSet+attrs)+{
++++this(context,+attrs,+com.android.internal.R.attr.textViewStyle);
}

public+TextView(Context+context,+@Nullable+AttributeSet+attrs,+int+defStyleAttr)+{
++++this(context,+attrs,+defStyleAttr,+0);
}|4634232|如果没有调用super(context)，就不会正确地将R.attr.textViewStyle设置为样式属性。|4634233|entityMap^0|I|9|0|0|16|8|0|0|6|E|S|K|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

If you override all three constructors, please DO NOT CASCADE <code>this(...)</code> CALLS. You should instead be doing this:

<pre><code>public MyView(Context context) {
 super(context);
 init(context, null, 0);
}

public MyView(Context context, AttributeSet attrs) {
 super(context,attrs);
 init(context, attrs, 0);
}

public MyView(Context context, AttributeSet attrs, int defStyle) {
 super(context, attrs, defStyle);
 init(context, attrs, defStyle);
}

private void init(Context context, AttributeSet attrs, int defStyle) {
 // do additional work
}
</code></pre>

The reason is that the parent class might include default attributes in its own constructors that you might be accidentally overriding. For example, this is the constructor for <code>TextView</code>:

<pre><code>public TextView(Context context) {
 this(context, null);
}

public TextView(Context context, @Nullable AttributeSet attrs) {
 this(context, attrs, com.android.internal.R.attr.textViewStyle);
}

public TextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
 this(context, attrs, defStyleAttr, 0);
}
</code></pre>

If you did not call <code>super(context)</code>, you would not have properly set <code>R.attr.textViewStyle</code> as the style attr.

blocks|key|531046|text|MyView(上下文上下文)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531047|以编程方式实例化视图时使用。|531048|MyView(上下文上下文，AttributeSet属性)|531049|由LayoutInflater用来应用xml属性。如果其中一个属性名为style，则在布局xml文件中查找显式值之前，将先在样式中查找属性。|offset|length|style|CODE|531050|MyView(Context+context，AttributeSet+attrs，int+defStyleAttr)|531051|假设您希望将默认样式应用于所有小部件，而不必在每个布局文件中指定style。例如，默认情况下将所有复选框设置为粉红色。您可以使用defStyleAttr执行此操作，框架将在您的主题中查找默认样式。|531052|请注意，一段时间前，defStyleAttr被错误地命名为defStyle，关于是否真的需要此构造函数存在一些讨论。请参阅https://code.google.com/p/android/issues/detail?id=12683|531053|MyView(Context+context，AttributeSet+attrs，int+defStyleAttr，int+defStyleRes)|531054|如果你能控制应用程序的基本主题，那么第三个构造函数就能很好地工作。这对谷歌来说是有效的，因为他们将他们的小部件与默认主题一起提供。但是假设您正在编写一个小部件库，并且希望设置一个默认样式，而不需要用户调整他们的主题。现在，您可以使用defStyleRes在前两个构造函数中将其设置为默认值：|531055|public+MyView(Context+context)+{
++super(context,+null,+0,+R.style.MyViewStyle);
++init();
}

public+MyView(Context+context,+AttributeSet+attrs)+{
++super(context,+attrs,+0,+R.style.MyViewStyle);
++init();
}|code-block|syntax|javascript|531056|总而言之|531057|如果你正在实现你自己的视图，那么只需要前两个构造器，并且框架可以调用它们。|531058|如果你希望你的视图是可扩展的，你可以为你的类的子类实现第四个构造器，以便能够使用全局样式。|531059|我没有看到第三个构造函数的实际用例。如果您没有为您的小部件提供默认样式，但仍然希望您的用户能够提供默认样式，那么可以选择一个快捷方式。不应该发生那么多。|531060|entityMap|0|LINK|mutability|MUTABLE|url|https://code.google.com/p/android/issues/detail?id=12683^0|0|0|0|1|E|Z|5|0|0|W|5|0|A|C|T|8|1P|1K|0|0|0|38|B|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1H|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1I|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|1J|8|@$H|1K|I|1L|J|K]|$H|1M|I|1N|J|K]]|9|@]|A|$]]|$1|L|3|M|5|6|7|1O|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1P|8|@$H|1Q|I|1R|J|K]]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1S|8|@$H|1T|I|1U|J|K]|$H|1V|I|1W|J|K]]|9|@$H|1X|I|1Y|1|1Z]]|A|$]]|$1|R|3|S|5|6|7|20|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|21|8|@$H|22|I|23|J|K]]|9|@]|A|$]]|$1|V|3|W|5|X|7|24|8|@]|9|@]|A|$Y|Z]]|$1|10|3|11|5|6|7|25|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|26|8|@]|9|@]|A|$]]|$1|14|3|15|5|6|7|27|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|28|8|@]|9|@]|A|$]]|$1|18|3|-4|5|6|7|29|8|@]|9|@]|A|$]]]|19|$1A|$5|1B|1C|1D|A|$1E|1F]]]]

<h2>MyView(Context context)</h2>

Used when instanciating Views programmatically.

<h2>MyView(Context context, AttributeSet attrs)</h2>

Used by the <code>LayoutInflater</code> to apply xml attributes. If one of this attribute is named <code>style</code>, attributes will be looked up the the style before looking for explicit values in the layout xml file.

<h2>MyView(Context context, AttributeSet attrs, int defStyleAttr)</h2>

Suppose you want to apply a default style to all widgets without having to specify <code>style</code> in each layout file. For an example make all checkboxes pink by default. You can do this with defStyleAttr and the framework will lookup the default style in your theme.

Note that <code>defStyleAttr</code> was incorrectly named <code>defStyle</code> some time ago and there is some discussion about whether this constructor is really needed or not. See <a href="https://code.google.com/p/android/issues/detail?id=12683" rel="noreferrer">https://code.google.com/p/android/issues/detail?id=12683</a>

<h2>MyView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)</h2>

The 3rd constructor works well if you have control over the base theme of the applications. That is working for google because they ship their widgets along side the default Themes. But suppose you're writing a widget library and you want a default style to be set without your users needing to tweak their theme. You can now do this using <code>defStyleRes</code> by setting it to the default value in the 2 first constructors:

<pre><code>public MyView(Context context) {
 super(context, null, 0, R.style.MyViewStyle);
 init();
}

public MyView(Context context, AttributeSet attrs) {
 super(context, attrs, 0, R.style.MyViewStyle);
 init();
}
</code></pre>

<h2>All in all</h2>

If you're implementing your own views, only the 2 first constructors should be needed and can be called by the framework. 

If you want your Views to be extensible, you might implement the 4th constructor for children of your class to be able to use global styling. 

I don't see a real use case for the 3rd constructor. Maybe a shortcut if you don't provide a default style for your widget but still want your users to be able to do so. Shouldn't happen that much.

blocks|key|597264|text|第三个构造器更像是complicated.Let我举了一个例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|597265|Support-v7+SwitchCompact包从24版本开始支持thumbTint和trackTint属性，而23版本不支持them.Now。你想在23版本中支持它们，你将如何实现？|offset|length|style|CODE|597266|我们假设使用自定义视图扩展SwitchCompact的SupportedSwitchCompact。|597267|public+SupportedSwitchCompat(Context+context)+{
++++this(context,+null);
}

public+SupportedSwitchCompat(Context+context,+AttributeSet+attrs)+{
++++this(context,+attrs,+0);
}

public+SupportedSwitchCompat(Context+context,+AttributeSet+attrs,+int+defStyleAttr)+{
++++super(context,+attrs,+defStyleAttr);
++++init();
}

private+void+init(){
++++mThumbDrawable+=+getThumbDrawable();
++++mTrackDrawable+=+getTrackDrawable();
++++applyTint();
}|code-block|syntax|javascript|597268|这是一种传统的代码风格。注意，在这里，我们将0传递给第三个参数。当您运行代码时，您会发现getThumbDrawable()总是返回null，这是多么奇怪，因为方法getThumbDrawable()是它的超类SwitchCompact的方法。|BOLD|597269|如果你把R.attr.switchStyle传递给第三个参数，所有的东西都变成well.So了，为什么？|597270|第三个参数是一个简单的属性。该属性指向上述大小写的样式，系统会在当前主题中找到switchStyle+resource.In属性，幸运的是系统会找到它。|597271|在frameworks/base/core/res/res/values/themes.xml中，您将看到：|597272|<style+name="Theme">
++++<item+name="switchStyle">@style/Widget.CompoundButton.Switch</item>
</style>|597273|entityMap^0|0|B|D|Y|9|18|9|0|D|D|R|M|0|0|V|2J|18|I|2A|I|2X|D|0|0|1G|4|I|0|0|24|13|B|0|0|1I|1|1A|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@$D|13|E|14|F|G]|$D|15|E|16|F|G]|$D|17|E|18|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|19|8|@$D|1A|E|1B|F|G]|$D|1C|E|1D|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|1E|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1F|8|@$D|1G|E|1H|F|Q]|$D|1I|E|1J|F|G]|$D|1K|E|1L|F|G]|$D|1M|E|1N|F|G]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1O|8|@$D|1P|E|1Q|F|Q]|$D|1R|E|1S|F|G]]|9|@]|A|$]]|$1|T|3|U|5|6|7|1T|8|@$D|1U|E|1V|F|Q]|$D|1W|E|1X|F|G]]|9|@]|A|$]]|$1|V|3|W|5|6|7|1Y|8|@$D|1Z|E|20|F|Q]|$D|21|E|22|F|G]]|9|@]|A|$]]|$1|X|3|Y|5|L|7|23|8|@]|9|@]|A|$M|N]]|$1|Z|3|-4|5|6|7|24|8|@]|9|@]|A|$]]]|10|$]]

The third constructor is much more complicated.Let me hold an example.

Support-v7 <code>SwitchCompact</code> package supports <code>thumbTint</code> and <code>trackTint</code> attribute since 24 version while 23 version does not support them.Now you want to support them in 23 version and how will you do to achieve this?

We assume to use custom View <code>SupportedSwitchCompact</code> extends <code>SwitchCompact</code>.

<pre><code>public SupportedSwitchCompat(Context context) {
 this(context, null);
}

public SupportedSwitchCompat(Context context, AttributeSet attrs) {
 this(context, attrs, 0);
}

public SupportedSwitchCompat(Context context, AttributeSet attrs, int defStyleAttr) {
 super(context, attrs, defStyleAttr);
 init();
}

private void init(){
 mThumbDrawable = getThumbDrawable();
 mTrackDrawable = getTrackDrawable();
 applyTint();
}
</code></pre>

It's a traditional code style.Note we pass 0 to the third param here. When you run the code, you will find <code>getThumbDrawable()</code> always return null how strange it is because the method <code>getThumbDrawable()</code> is its super class <code>SwitchCompact</code>'s method.

If you pass <code>R.attr.switchStyle</code> to the third param, everything goes well.So why?

The third param is a simple attribute. The attribute points to a style resource.In above case, the system will find <code>switchStyle</code> attribute in current theme fortunately system finds it.

In <code>frameworks/base/core/res/res/values/themes.xml</code>, you will see:

<pre><code>&lt;style name="Theme"&gt;
 &lt;item name="switchStyle"&gt;@style/Widget.CompoundButton.Switch&lt;/item&gt;
&lt;/style&gt;
</code></pre>

blocks|key|530243|text|Kotlin似乎消除了很多这种痛苦：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|530244|class+MyView
@JvmOverloads+constructor(context:+Context,+attrs:+AttributeSet?+=+null,+defStyle:+Int+=+0)
++++:+View(context,+attrs,+defStyle)|code-block|syntax|javascript|530245|@JvmOverload将生成所有必需的构造函数(参见该注释的documentation)，每个构造函数都可能调用super()。然后，只需将初始化方法替换为Kotlin+init+{}块。样板代码消失了！|offset|length|530246|entityMap|0|LINK|mutability|MUTABLE|url|https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-overloads/^0|0|0|V|D|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

Kotlin seems to take away a lot of this pain:

<pre><code>class MyView
@JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyle: Int = 0)
 : View(context, attrs, defStyle)
</code></pre>

@JvmOverloads will generate all required constructors (see that annotation's <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-overloads/" rel="noreferrer">documentation</a>), each of which presumably calls super(). Then, simply replace your initialization method with a Kotlin init {} block. Boilerplate code gone!

When creating a custom view, I have noticed that many people seem to do it like this:

<pre><code>public MyView(Context context) {
 super(context);
 // this constructor used when programmatically creating view
 doAdditionalConstructorWork();
}

public MyView(Context context, AttributeSet attrs) {
 super(context, attrs);
 // this constructor used when creating view through XML
 doAdditionalConstructorWork();
}

private void doAdditionalConstructorWork() {

 // init variables etc.
}
</code></pre>

My first question is, what about the constructor <code>MyView(Context context, AttributeSet attrs, int defStyle)</code>? I'm not sure where it is used, but I see it in the super class. Do I need it, and where is it used?

There's <a href="https://stackoverflow.com/questions/9195785/should-i-call-super-or-call-this-for-android-custom-view-constructors">another part to this question</a>.

Do I need all three constructors for an Android custom view?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在创建自定义视图时，我注意到很多人似乎是这样做的：public MyView(Context context) {  super(context);  // this constructor used when programmatically creating view  doAdditionalConstructo...

问Android自定义视图是否需要所有这三个构造函数？
EN

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问Android自定义视图是否需要所有这三个构造函数？EN