Android View 四个构造函数详解

在开发android开发过程中，很多人都会遇到自定义view，一般都需要继承自View类，而当你打开View类的源码时，发现会有四个构造函数，那么这四个构造函数是如何使用的呢，怎么合理的利用四个构造函数呢，本文将进行一定探究，希望能够抛砖引玉。

 

一 View类的四个构造函数

先从android源码中把四个构造函数拉出来看看。。

1 第一个构造函数

/**
     * Simple constructor to use when creating a view from code.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     */
    public View(Context context) 

2 第二个构造函数

 /**
     * Constructor that is called when inflating a view from XML. This is called
     * when a view is being constructed from an XML file, supplying attributes
     * that were specified in the XML file. This version uses a default style of
     * 0, so the only attribute values applied are those in the Context's Theme
     * and the given AttributeSet.
     *
     * <p>
     * The method onFinishInflate() will be called after all children have been
     * added.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     * @param attrs The attributes of the XML tag that is inflating the view.
     * @see #View(Context, AttributeSet, int)
     */
    public View(Context context, @Nullable AttributeSet attrs)

3 第三个构造函数

/**
     * Perform inflation from XML and apply a class-specific base style from a
     * theme attribute. This constructor of View allows subclasses to use their
     * own base style when they are inflating. For example, a Button class's
     * constructor would call this version of the super class constructor and
     * supply <code>R.attr.buttonStyle</code> for <var>defStyleAttr</var>; this
     * allows the theme's button style to modify all of the base view attributes
     * (in particular its background) as well as the Button class's attributes.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     * @param attrs The attributes of the XML tag that is inflating the view.
     * @param defStyleAttr An attribute in the current theme that contains a
     *        reference to a style resource that supplies default values for
     *        the view. Can be 0 to not look for defaults.
     * @see #View(Context, AttributeSet)
     */
    public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr)

4 第4个构造函数

/**
     * Perform inflation from XML and apply a class-specific base style from a
     * theme attribute or style resource. This constructor of View allows
     * subclasses to use their own base style when they are inflating.
     * <p>
     * When determining the final value of a particular attribute, there are
     * four inputs that come into play:
     * <ol>
     * <li>Any attribute values in the given AttributeSet.
     * <li>The style resource specified in the AttributeSet (named "style").
     * <li>The default style specified by <var>defStyleAttr</var>.
     * <li>The default style specified by <var>defStyleRes</var>.
     * <li>The base values in this theme.
     * </ol>
     * <p>
     * Each of these inputs is considered in-order, with the first listed taking
     * precedence over the following ones. In other words, if in the
     * AttributeSet you have supplied <code><Button * textColor="#ff000000"></code>
     * , then the button's text will <em>always</em> be black, regardless of
     * what is specified in any of the styles.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     * @param attrs The attributes of the XML tag that is inflating the view.
     * @param defStyleAttr An attribute in the current theme that contains a
     *        reference to a style resource that supplies default values for
     *        the view. Can be 0 to not look for defaults.
     * @param defStyleRes A resource identifier of a style resource that
     *        supplies default values for the view, used only if
     *        defStyleAttr is 0 or can not be found in the theme. Can be 0
     *        to not look for defaults.
     * @see #View(Context, AttributeSet, int)
     */
    public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes)

这四个构造函数在源码中都有注释，为了便于理解，我将注释也贴了出来，有点长，不过利于本文理解。

二 构造函数的调用

【第一个问题】

如果我们在继承了View类实现自定义类时，需要重写哪个构造函数？

回答这个问题，首先需要知道，在定义了View时，我们都调用了哪个构造函数。我这里做了一个简单的实验：

我们声明一个简单的View，继承自TextView，什么都不改，只是在4个constructor中打印几个tag，查看到底哪个构造函数被调用。

【实验】

package com.zhaohui.code.view;

import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.widget.TextView;

import com.example.zhaohui.player.R;

/**
 * Created by zhaohui on 16-8-12.
 */
public class CustomView extends TextView {
    String tag = "customView";

    public CustomView(Context context) {
        super(context);
        Log.d(tag,"First Constructor");
    }

    public CustomView(Context context, AttributeSet attrs) {
        this(context,attrs,android.R.attr.textViewStyle);
        Log.d(tag,"Second Constructor");


        for(int i=0;i<attrs.getAttributeCount();i++){
            Log.d(tag, attrs.getAttributeName(i)+"   :   "+attrs.getAttributeValue(i));
        }

    }

    public CustomView(Context context, AttributeSet attrs, int defStyleAttr) {
        this(context, attrs, defStyleAttr,0);
        Log.d(tag,"Third Constructor");
    }

    public CustomView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr,defStyleRes);
        Log.d(tag,"Fourth Constructor");
    }

}

在布局文件layout中加上这个View

<com.zhaohui.code.view.CustomView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="CustomView"
        android:textSize="30sp"/>

[实验结果]

08-14 05:08:37.847 13687-13687/com.example.zhaohui.player D/customView: Second Constructor

08-14 05:08:37.847 13687