RelativeLayout measure方法报NullPoi

作者: 三十二蝉 | 来源:发表于2017-03-01 15:02 被阅读946次

    问题描述

    之前开发中碰到这样一个bug,部分机型会有崩溃现象,错误内容如下:

    - E/AndroidRuntime( 3579): FATAL EXCEPTION: main
    - E/AndroidRuntime( 3579): java.lang.NullPointerException
    - E/AndroidRuntime( 3579):    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:431)
    - E/AndroidRuntime( 3579):    at android.view.View.measure(View.java:8462)
    

    很奇怪,明明没有什么特别的地方,但是在Android4.3的机器上就是会有崩溃~~~

    问题分析

    在stackoverflow上发现有人碰到过相同的问题。

    • 问题原因
      RelativeLayout的onMeasure方法中有这样一段代码:
    if (isWrapContentWidth) {
                // Width already has left padding in it since it was calculated by looking at
                // the right of each child view
                width += mPaddingRight;
    
                if (mLayoutParams != null && mLayoutParams.width >= 0) {
                    width = Math.max(width, mLayoutParams.width);
                }
    
                width = Math.max(width, getSuggestedMinimumWidth());
                width = resolveSize(width, widthMeasureSpec);
    

    看起来没有问题,但是对比Android 4.3源码,

    if (mLayoutParams.width >= 0) {
          width = Math.max(width, mLayoutParams.width);
    }
    

    这也就是为什么会在Android4.3手机上出现崩溃的原因之一。
    再深入探究一下mLayoutParams这个参数为什么会是null。首先看代码的使用方法:

    contentView = (ViewGroup) LayoutInflater.
                   rom(context).inflate(R.layout.select_text_oprate_window, null);
    

    这句代码的意思是,从xml中实例化View视图,但是父视图为空,所以从xml实例化的View视图没有办法attach到View层次树中。那么这个和mLayoutParams有什么关系??~~~
    首先看看mLayoutParams这个参数表示什么意思:

    /**
         * The layout parameters associated with this view and used by the parent
         * {@link android.view.ViewGroup} to determine how this view should be
         * laid out.
         * {@hide}
         */
        protected ViewGroup.LayoutParams mLayoutParams;
    

    从注释中可以看出,这个参数表示子View布局的期望值,实际值由父View,或者说整个View树决定。

     /**
         * Get the LayoutParams associated with this view. All views should have
         * layout parameters. These supply parameters to the <i>parent</i> of this
         * view specifying how it should be arranged. There are many subclasses of
         * ViewGroup.LayoutParams, and these correspond to the different subclasses
         * of ViewGroup that are responsible for arranging their children.
         *
         * This method may return null if this View is not attached to a parent
         * ViewGroup or {@link #setLayoutParams(android.view.ViewGroup.LayoutParams)}
         * was not invoked successfully. When a View is attached to a parent
         * ViewGroup, this method must not return null.
         *
         * @return The LayoutParams associated with this view, or null if no
         *         parameters have been set yet
         */
        @ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
        public ViewGroup.LayoutParams getLayoutParams() {
            return mLayoutParams;
        }
    

    在来看这个函数的get方法。注释里面写了两条:1、mLayoutParams这个参数必须设置;2、当View视图attach到父视图(View层次树),mLayoutParams这个参数肯定不会为null。

    问题解决

    回到项目代码,

    contentView = (ViewGroup) LayoutInflater.
                 rom(context).inflate(R.layout.select_text_oprate_window, null);
    

    首先,这种inflate方法没有父布局,所以也没有attach这一说~~~
    再者,项目代码中没有显示调用contentView.setLayoutParams(params)方法。按照源码中的说法,这个参数可能报NullPointerException。(为什么是可能??!!)
    解决方法:

    • 采用一般建议的inflate方法
    contentView = (ViewGroup) LayoutInflater.
                 rom(context).inflate(R.layout.select_text_oprate_window, parent,false);
    
    • 显示setLayoutParams
    contentView = (ViewGroup) LayoutInflater.
                    from(context).inflate(R.layout.select_text_oprate_window, null);
    
     ViewGroup.LayoutParams params = new ViewGroup.LayoutParams(
                    ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
            contentView.setLayoutParams(params);
    

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