测量控件宽高的四种方案

Android开发过程中，有时候需要测量某一个view的宽高，尤其是初始化的时候，但是在Activity的onCreate,onStart,onResume方法中都不能正确测量宽高。因为view的measure过程和Activity的生命周期不是同步执行的。
那么怎么测量呢，有四种方法，这里记录下来。

onWindowFocusChanged

   /**
     * Called when the current {@link Window} of the activity gains or loses
     * focus.  This is the best indicator of whether this activity is visible
     * to the user.  The default implementation clears the key tracking
     * state, so should always be called.
     *
     * 
Note that this provides information about global focus state, which
     * is managed independently of activity lifecycles.  As such, while focus
     * changes will generally have some relation to lifecycle changes (an
     * activity that is stopped will not generally get window focus), you
     * should not rely on any particular order between the callbacks here and
     * those in the other lifecycle methods such as {@link #onResume}.
     *
     * 
As a general rule, however, a resumed activity will have window
     * focus...  unless it has displayed other dialogs or popups that take
     * input focus, in which case the activity itself will not have focus
     * when the other windows have it.  Likewise, the system may display
     * system-level windows (such as the status bar notification panel or
     * a system alert) which will temporarily take window input focus without
     * pausing the foreground activity.
     *
     * @param hasFocus Whether the window of this activity has focus.
     *
     * @see #hasWindowFocus()
     * @see #onResume
     * @see View#onWindowFocusChanged(boolean)
     */
    public void onWindowFocusChanged(boolean hasFocus) {
    }

onWindowFocusChanged这个方法的含义是，view已经初始化完毕了,宽高已经准备好了，这时候去测量宽高是没有问题的。需要注意的是，onWindowFocusChanged会被调用很多次，当Activity的窗口得到焦点和失去焦点的时候均会被调用一次，具体来说，Activity继续执行和暂停执行的时候，onWindowFocusChanged均会被调用。如果频繁的onResume和onPause，那么onWindowFocusChanged也会被频繁调用

    override fun onWindowFocusChanged(hasFocus: Boolean) {
        super.onWindowFocusChanged(hasFocus)
        if (hasFocus) {
            val measuredWidth = tv_standard.measuredWidth
            val measuredHeight = tv_standard.measuredHeight
            Log.i("MainActivity", "windowFocusChanged===>width:$measuredWidth,height:$measuredHeight")

        }
    }

view.post(runnable)

通过post可以将一个runnable投递到消息队列的尾部，然后等待Looper调用此runnable的时候，view已经初始化好了

    override fun onStart() {
        super.onStart()
        tv_standard.post {
            val measuredWidth = tv_standard.measuredWidth
            val measuredHeight = tv_standard.measuredHeight
            Log.i("MainActivity", "post===>width:$measuredWidth,height:$measuredHeight")
        }
    }

ViewTreeObserver

使用ViewTreeObserver的众多回调可以完成这个功能，比如使用OnGlobalLayoutListener这个接口，当View树的状态发送改变或者View树内部的View可见性改变时，onGlobalLayout方法将会被回调，因此这是获取View的宽高一个很好的时机。需要注意的，伴随着View树的状态改变，onGlobalLayout会被调用多次。

     val viewTreeObserver = tv_standard.viewTreeObserver
        viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
            override fun onGlobalLayout() {
                tv_standard.viewTreeObserver.removeOnGlobalLayoutListener(this)
                val measuredWidth = tv_standard.measuredWidth
                val measuredHeight = tv_standard.measuredHeight
                printD("viewTreeObserver===>width:$measuredWidth,height:$measuredHeight")
            }
        })

view.measure(int widthMeasureSpec,int heightMeasureSpec)

通过手动对View进行测量，得到view的宽高。这种方法比较复杂，根据view的LayoutParams来区分

• match_parent
  直接放弃，无法measure出具体的宽高，原因很简单，view的测量过程，构造此种MeasureSpec需要知道parentSzie，即父容器的剩余空间，二这个号死后无法知道parentSize的大小，所以不能测量出view的大小

  val widthSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY)
  val heightSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY)
  tvStandard.measure(widthSpec, heightSpec)

• wrap_content

  val widthSpec = View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.EXACTLY)
  val heightSpec = View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.EXACTLY)
  tvStandard.measure(widthSpec, heightSpec)

可以看到(1<<30)-1 ，通过分析Measure的实现可以知道，View的尺寸使用30位二进制表示，也就是说最大是30个1（即2^30-1），在最大模式下，用View理论上能支持的最大值去构造MeasureSpec是合理的。

注：本文章知识点来自学习《Android开发艺术探索》一书