Android 設計模式 - 模版方法模式
摘要：
使用模版方法模式模擬不同計算機的啟動流程.
public abstract class AbstractComputer {
protected void powerOn(){
System.out.println("開啟電源");
}
...
使用模版方法模式模擬不同計算機的啟動流程.
public abstract class AbstractComputer {
protected void powerOn(){
System.out.println("開啟電源");
}
protected void checkHardware(){
System.out.println("硬體檢查");
}
protected void loadOS(){
System.out.println("載入作業系統");
}
protected void login(){
System.out.println("小白的計算機無驗證，直接進入系統");
}
/***
*
*/
public final void startUp(){
System.out.println("------ 開機 START ------");
powerOn();
checkHardware();
loadOS();
login();
System.out.println("------ 關機 START ------");
}
}
public class CoderComputer extends AbstractComputer {
@Override
protected void login() {
System.out.println("程式設計師只需要進行使用者和密碼的驗證就可以了");
}
}
public class MilitaryComputer extends AbstractComputer{
@Override
protected void checkHardware() {
super.checkHardware();
System.out.println("檢查硬體和防火牆");
}
@Override
protected void login() {
super.login();
System.out.println("進行指紋和複雜的使用者驗證");
}
}
public static void main(String[] args){
AbstractComputer comp = new CoderComputer();
comp.startUp();
comp = new MilitaryComputer();
comp.startUp();
}
通過上面的例子可以看到，在startUp方法中有一些固定的步驟，依次為開啟電源，檢查硬體，載入系統，使用者登入4個步驟，這四個步驟是開機過程不會變動
image
但是，不同使用者的這幾個步驟的實現可能各不相同，因此，子類需要複寫相應的方法來進行自定義處理，這裡要注意startUp為final方法，這樣就保證了邏輯流程不能背子類修改，子類只能夠改變某一步驟中的具體實現，這樣保證了這個邏輯流程的穩定性。
Android中的模版方法模式
AsyncTask
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
execute是一個final方法，這樣防止了子類充血，在executeOnExecutor中判斷了狀態，如果不是PENDING狀態就會丟擲異常，這也是為什麼AsyncTask中execute只能呼叫一次的原因，繼續往下看，會呼叫到onPreExecute()，那隨後的mWorker和mFuture是什麼？在建構函式中初始化了這兩個物件
public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
mWorker其實是一個Callable，FutureTask對它進行了封裝,FutureTask的run函式又會呼叫call方法
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//呼叫call方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
在call方法中執行了AsyncTask的doInBackground方法，這裡是線上程池裡執行的.執行完過後呼叫到postResult(result)
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
postResult會發送一個訊息回撥到主執行緒
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
上面的getHandler()實際上是InternalHandler
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
InternalHandler,接收到MESSAGE_POST_RESULT訊息過後會呼叫result.mTask.finish(result.mData[0])，result是AsyncTaskResult型別
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
AsyncTaskResult裡面的mTask就是AsyncTask，通過呼叫result.mTask.finish(result.mData[0])把結果通過finish返回到了AsyncTask的onPostExecute中.
這個過程整體上是用到模版方法模式
Activity的啟動流程
Activity的啟動流程實際上也是使用了模版方法，例如它的onCreate,onStart,onResume等生命週期方法. 具體的執行流程不再描述.
【附錄】
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