示例
问题:为什么log日志会不打印。
public class MainThreadTestActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main_thread_test);
View view = new View(this);
view.post(new Runnable() {
@Override
public void run() {
Log.i(TAG, "[view.post] >>>> 1 ");
}
});
}
}
源码分析
//View.java
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().post(action);
return true;
}
即:当执行 View.post
方法时,如果 AttachInfo
不为空,则通过 AttachInfo
的 Handler
来执行 Runnable
;否则,将这个 Runnable
抛到 View
的执行队列 HandlerActionQueue
中。
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
mAttachInfo = info;
//...
}
也就是只有当 View
attch 到 Window
后,才会给 AttachInfo
赋值。所以,在 示例 里的代码会直接走入 getRunQueue().post(action)
。
public class HandlerActionQueue {
private HandlerAction[] mActions;
private int mCount;
public void post(Runnable action) {
postDelayed(action, 0);
}
public void postDelayed(Runnable action, long delayMillis) {
final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
synchronized (this) {
if (mActions == null) {
mActions = new HandlerAction[4];
}
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
public void removeCallbacks(Runnable action) {
synchronized (this) {
final int count = mCount;
int j = 0;
final HandlerAction[] actions = mActions;
for (int i = 0; i < count; i++) {
if (actions[i].matches(action)) {
// Remove this action by overwriting it within
// this loop or nulling it out later.
continue;
}
if (j != i) {
// At least one previous entry was removed, so
// this one needs to move to the "new" list.
actions[j] = actions[i];
}
j++;
}
// The "new" list only has j entries.
mCount = j;
// Null out any remaining entries.
for (; j < count; j++) {
actions[j] = null;
}
}
}
public void executeActions(Handler handler) {
synchronized (this) {
final HandlerAction[] actions = mActions;
for (int i = 0, count = mCount; i < count; i++) {
final HandlerAction handlerAction = actions[i];
handler.postDelayed(handlerAction.action, handlerAction.delay);
}
mActions = null;
mCount = 0;
}
}
}
而在 View
中,
//View.java
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
...
// Transfer all pending runnables.
if (mRunQueue != null) {
mRunQueue.executeActions(info.mHandler);
mRunQueue = null;
}
...
}
//ViewRootImpl.java
private void performTraversals() {
...
getRunQueue().executeActions(mAttachInfo.mHandler);
...
}
由于 View view = new View(this);
没有将view attch到window上,所以执行的 View.post
方法将可执行请求都缓存到请求队列里。
示例 中的代码可改为:
View view = new View(this);
rootView.addView(view);
view.post(new Runnable() {