在Activity中获取View的高度(Android开发艺术探索学习笔记）

在onCreate里面，你是无法获得长宽值的，始终为0，为什么会这样呢？因为View的measure过程和Activity的生命周期不是同步的。所以下面来讨论一下在Activity中获取View的高度几种方法。

1.onWindowFocusChanged()
这个方法在onResume()和onPause()也会被调用，如果onResume()和onPause()被频繁调用，那么onWindowFocusChanged()也会被频繁调用。

    /**     * 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.     * ……     */    public void onWindowFocusChanged(boolean hasFocus) {    }

2.View.post()
通过post将一个Runnable投递到消息队列尾部。等Runnable被调用的时候，View已经初始化好了。

    /**     * <p>Causes the Runnable to be added to the message queue.     * The runnable will be run on the user interface thread.</p>     */    public boolean post(Runnable action) {        final AttachInfo attachInfo = mAttachInfo;        if (attachInfo != null) {            return attachInfo.mHandler.post(action);        }        // Assume that post will succeed later        ViewRootImpl.getRunQueue().post(action);        return true;    }

3.ViewTreeObserver
使用OnGlobalLayoutListener()，监听布局完成，但是会多次调用，所以要及时remove掉Listener。

        final ViewTreeObserver viewTreeObserver = mView.getViewTreeObserver();        viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {            @Override            public void onGlobalLayout() {                viewTreeObserver .removeOnLayoutChangeListener(this);            }        });

类似的还有使用OnDrawListener()和OnPreDrawListener。

4.直接使用measure()

        int w = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);        int h = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);        mView.measure(w, h);        int height = mView.getMeasuredHeight();        int width = mView.getMeasuredWidth();        System.out.println("measure width=" + width + " height=" + height);