invalidate
這裏我們從源碼中去簡單扒一下View的invalidate()的方法：
1 /**
2 * Invalidate the whole view. If the view is visible,
3 * {@link #onDraw(android.graphics.Canvas)} will be called at some point in
4 * the future.
5 * <p>
6 * This must be called from a UI thread. To call from a non-UI thread, call
7 * {@link #postInvalidate()}.
8 */
9 public void invalidate() {
10 invalidate(true);
11 }
從方法中的註釋中看，我們知道invalidate方法會刷新整個View，並且當這個View的可見性爲VISIBLE的時候，View的onDraw()方法將會被調用。另外注意的是這個方法只能在UI線程中去調用。
上面就能夠基本知道invalidate方法是幹什麼的了。我們往下接着看源碼：
1 /**
2 * This is where the invalidate() work actually happens. A full invalidate()
3 * causes the drawing cache to be invalidated, but this function can be
4 * called with invalidateCache set to false to skip that invalidation step
5 * for cases that do not need it (for example, a component that remains at
6 * the same dimensions with the same content).
7 *
8 * @param invalidateCache Whether the drawing cache for this view should be
9 * invalidated as well. This is usually true for a full
10 * invalidate, but may be set to false if the View's contents or
11 * dimensions have not changed.
12 * @hide
13 */
14 public void invalidate(boolean invalidateCache) {
15 invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
16 }
我們看到，invalidate()方法中是調用invalidate(true)，參數true的意思是需要整體刷新，當View的內容和大小沒有任何變化時我們可以傳入false。刷新的細節這裏不做詳細介紹。
postInvalidate
接下來看下postInvalidate()的實現：
1 /**
2 * <p>Cause an invalidate to happen on a subsequent cycle through the event loop.
3 * Use this to invalidate the View from a non-UI thread.</p>
4 *
5 * <p>This method can be invoked from outside of the UI thread
6 * only when this View is attached to a window.</p>
7 *
8 * @see #invalidate()
9 * @see #postInvalidateDelayed(long)
10 */
11 public void postInvalidate() {
12 postInvalidateDelayed(0);
13 }
14
15 public void postInvalidateDelayed(long delayMilliseconds) {
16 // We try only with the AttachInfo because there's no point in invalidating
17 // if we are not attached to our window
18 final AttachInfo attachInfo = mAttachInfo;
19 if (attachInfo != null) {
20 attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);
21 }
22 }
23
24 public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {
25 Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);
26 mHandler.sendMessageDelayed(msg, delayMilliseconds);
27 }
從上面的方法註釋中可以知道，postInvalidate是可以在非UI線程中去調用刷新UI的，那是如何做到的呢？從上面的方法調用棧中可以看出來，調用postInvalidate方法最後會調用View中的mHander發送一個MSG_INVALIDATE的消息。mHandler是ViewRootHandler的一個實例，從ViewRootHandler的handleMessage()方法中一探究竟(方法較長，只截取部分)：
1 @Override
2 public void handleMessage(Message msg) {
3 switch (msg.what) {
4 case MSG_INVALIDATE:
5 ((View) msg.obj).invalidate();
6 break;
7 case MSG_INVALIDATE_RECT:
8 final View.AttachInfo.InvalidateInfo info = (View.AttachInfo.InvalidateInfo) msg.obj;
9 info.target.invalidate(info.left, info.top, info.right, info.bottom);
10 info.recycle();
11 break;
12 case MSG_PROCESS_INPUT_EVENTS:
13 mProcessInputEventsScheduled = false;
14 doProcessInputEvents();
15 break;
在Handler中最後還是會調用View的invalidate()方法去刷新，只不過postInvalidate()方法是通過Handler將刷新事件通知發到Handler的handlerMessage中去執行invalidate的。
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