渲染篇
- 大多數(shù)手機(jī)的屏幕刷新頻率是60HZ，如果在1000/60=16.67ms內(nèi)沒有辦法把這一幀的任務(wù)執(zhí)行完畢，就會(huì)發(fā)生丟幀的現(xiàn)象。丟幀越多，用戶感受的卡頓情況就越嚴(yán)重。
- 渲染操作通常依賴于兩個(gè)核心組件：CPU與GPU。CPU負(fù)責(zé)包括Measure，Layout，Record，Execute的計(jì)算操作，GPU負(fù)責(zé)Rasterization(柵格化)操作。CPU通常存在的問題的原因是存在非必需的視圖組件，它不僅僅會(huì)帶來重復(fù)的計(jì)算操作，而且還會(huì)占用額外的GPU資源。
Resterization柵格化是繪制那些Button，Shape，Path，String，Bitmap等組件最基礎(chǔ)的操作。它把那些組件拆分到不同的像素上進(jìn)行顯示。這是一個(gè)很費(fèi)時(shí)的操作，GPU的引入就是為了加快柵格化的操作。
- CPU負(fù)責(zé)把UI組件計(jì)算成Polygons，Texture紋理，然后交給GPU進(jìn)行柵格化渲染。
- Overdraw(過度繪制)描述的是屏幕上的某個(gè)像素在同一幀的時(shí)間內(nèi)被繪制了多次。
- 減少過度繪制的步驟：
- 移除Window默認(rèn)的background
- XML布局中去除多余的background（一般父布局中的background可以不設(shè)置，具體視情況而定）
- 自定義view時(shí)，需注意有重疊部分時(shí)采用ClipRect方法，避免重疊部分被過度繪制
- 提升布局性能的關(guān)鍵點(diǎn)時(shí)盡量保持布局層級(jí)的扁平化，避免出現(xiàn)重復(fù)的嵌套布局。
運(yùn)算篇
- TraceView使用了解
- Batching and Caching
Batching是在真正執(zhí)行運(yùn)算操作之前對(duì)數(shù)據(jù)進(jìn)行批量預(yù)處理，例如你需要有這樣一個(gè)方法，它的作用是查找某個(gè)值是否存在于一堆數(shù)據(jù)中。
- 把可能有性能問題的代碼放到非主線程中。
- Vector，ArrayList,LinkedList,HashMap,SparseArray
內(nèi)存篇
- Android系統(tǒng)里面有一個(gè)Generatioal Heap Memory的模型，系統(tǒng)會(huì)根據(jù)不同的內(nèi)存數(shù)據(jù)類型分別執(zhí)行不同的GC操作。例如，最近剛分配的對(duì)象會(huì)放在Young Generation區(qū)域，這個(gè)區(qū)域的對(duì)象通常都是會(huì)快速被創(chuàng)建并且很快被銷毀回收的，同時(shí)這個(gè)區(qū)域的GC操作速度也是比Old Generation區(qū)域的GC操作速度更快的。除了速度差異之外，執(zhí)行GC操作的時(shí)候，所有線程的任何操作都會(huì)需要暫停，等待GC操作完成之后，其他操作才能夠繼續(xù)運(yùn)行。通常來說，單個(gè)的GC并不會(huì)占用太多時(shí)間，但是大量不停的GC操作則會(huì)顯著占用幀間隔時(shí)間(16ms)。如果在幀間隔時(shí)間里面做了過多的GC操作，那么自然其他類似計(jì)算，渲染等操作的可用時(shí)間就變得少了。
- memory monitor工具
- 發(fā)生內(nèi)存泄漏會(huì)導(dǎo)致Memory Generation中的剩余可用Heap Size越來越小，這樣會(huì)導(dǎo)致頻繁觸發(fā)GC，更進(jìn)一步引起性能問題。
內(nèi)存泄漏表示的是不再用到的對(duì)象因?yàn)楸诲e(cuò)誤引用而無法進(jìn)行回收。
- Memory Churn內(nèi)存抖動(dòng)，內(nèi)存抖動(dòng)是因?yàn)樵诙虝r(shí)間內(nèi)大量的對(duì)象被創(chuàng)建又馬上被釋放。瞬間產(chǎn)生大量的對(duì)象會(huì)嚴(yán)重占用Young Generation的內(nèi)存區(qū)域，當(dāng)達(dá)到閥值，剩余空間不夠的時(shí)候，會(huì)觸發(fā)GC從而導(dǎo)致剛產(chǎn)生的對(duì)象又很快被回收。即使每次分配的對(duì)象占用了很少的內(nèi)存，但是他們疊加在一起會(huì)增加Heap的壓力，從而觸發(fā)更多其他類型的GC。這個(gè)操作有可能會(huì)影響到幀率，并使得用戶感知到性能問題。
解決上面的問題有簡潔直觀方法，如果你在Memory Monitor里面查看到短時(shí)間發(fā)生了多次內(nèi)存的漲跌，這意味著很有可能發(fā)生了內(nèi)存抖動(dòng)。同時(shí)我們還可以通過Allocation Tracker來查看在短時(shí)間內(nèi)，同一個(gè)棧中不斷進(jìn)出的相同對(duì)象。這是內(nèi)存抖動(dòng)的典型信號(hào)之一。
當(dāng)你大致定位問題之后，接下去的問題修復(fù)也就顯得相對(duì)直接簡單了。例如，你需要避免在for循環(huán)里面分配對(duì)象占用內(nèi)存，需要嘗試把對(duì)象的創(chuàng)建移到循環(huán)體之外，自定義View中的onDraw方法也需要引起注意，每次屏幕發(fā)生繪制以及動(dòng)畫執(zhí)行過程中，onDraw方法都會(huì)被調(diào)用到，避免在onDraw方法里面執(zhí)行復(fù)雜的操作，避免創(chuàng)建對(duì)象。對(duì)于那些無法避免需要?jiǎng)?chuàng)建對(duì)象的情況，我們可以考慮對(duì)象池模型，通過對(duì)象池來解決頻繁創(chuàng)建與銷毀的問題，但是這里需要注意結(jié)束使用之后，需要手動(dòng)釋放對(duì)象池中的對(duì)象。
- 關(guān)于Allocation Tracker工具的使用，不展開了，參考下面的鏈接：
- http://developer.android.com/tools/debugging/ddms.html#alloc
- http://android-developers.blogspot.com/2009/02/track-memory-allocations.html
- 三種測量內(nèi)存工具，各自的特點(diǎn)：
- Memory Monitor:跟蹤整個(gè)APP內(nèi)存變化情況。
- Heap Viewer：查看當(dāng)前內(nèi)存快照，便于對(duì)比分析哪些對(duì)象有可能發(fā)生內(nèi)存泄露
- Alloction Tracker：追蹤內(nèi)存對(duì)象的來源
電量篇
- 使用WakeLock或者JobScheduler喚醒設(shè)備處理定時(shí)的任務(wù)之后，一定要及時(shí)讓設(shè)備回到初始狀態(tài)。每次喚醒蜂窩信號(hào)進(jìn)行數(shù)據(jù)傳遞，都會(huì)消耗很多電量，它比WiFi等操作更加的耗電。
- 我們可以通過下面的代碼來獲取手機(jī)的當(dāng)前充電狀態(tài)：
// It is very easy to subscribe to changes to the battery state, but you can get the current
// state by simply passing null in as your receiver. Nifty, isn't that?
IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
Intent batteryStatus = this.registerReceiver(null, filter);
int chargePlug = batteryStatus.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1);
boolean acCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_AC);
if (acCharge) {
Log.v(LOG_TAG,“The phone is charging!”);
}
在上面的例子演示了如何立即獲取到手機(jī)的充電狀態(tài)，得到充電狀態(tài)信息之后，我們可以有針對(duì)性的對(duì)部分代碼做優(yōu)化。比如我們可以判斷只有當(dāng)前手機(jī)為AC充電狀態(tài)時(shí) 才去執(zhí)行一些非常耗電的操作。
/**
* This method checks for power by comparing the current battery state against all possible
* plugged in states. In this case, a device may be considered plugged in either by USB, AC, or
* wireless charge. (Wireless charge was introduced in API Level 17.)
*/
private boolean checkForPower() {
// It is very easy to subscribe to changes to the battery state, but you can get the current
// state by simply passing null in as your receiver. Nifty, isn't that?
IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
Intent batteryStatus = this.registerReceiver(null, filter);
// There are currently three ways a device can be plugged in. We should check them all.
int chargePlug = batteryStatus.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1);
boolean usbCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_USB);
boolean acCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_AC);
boolean wirelessCharge = false;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
wirelessCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_WIRELESS);
}
return (usbCharge || acCharge || wirelessCharge);
}
- 一個(gè)最簡單的喚醒手機(jī)的方法是使用PowerManager.WakeLock的API來保持CPU工作并防止屏幕變暗關(guān)閉。這使得手機(jī)可以被喚醒，執(zhí)行工作，然后回到睡眠狀態(tài)。知道如何獲取WakeLock是簡單的，可是及時(shí)釋放WakeLock也是非常重要的，不恰當(dāng)?shù)氖褂肳akeLock會(huì)導(dǎo)致嚴(yán)重錯(cuò)誤。例如網(wǎng)絡(luò)請求的數(shù)據(jù)返回時(shí)間不確定，導(dǎo)致本來只需要10s的事情一直等待了1個(gè)小時(shí)，這樣會(huì)使得電量白白浪費(fèi)了。這也是為何使用帶超時(shí)參數(shù)的wakelock.acquice()方法是很關(guān)鍵的。
但是僅僅設(shè)置超時(shí)并不足夠解決問題，例如設(shè)置多長的超時(shí)比較合適？什么時(shí)候進(jìn)行重試等等？解決上面的問題，正確的方式可能是使用非精準(zhǔn)定時(shí)器。通常情況下，我們會(huì)設(shè)定一個(gè)時(shí)間進(jìn)行某個(gè)操作，但是動(dòng)態(tài)修改這個(gè)時(shí)間也許會(huì)更好。例如，如果有另外一個(gè)程序需要比你設(shè)定的時(shí)間晚5分鐘喚醒，最好能夠等到那個(gè)時(shí)候，兩個(gè)任務(wù)捆綁一起同時(shí)進(jìn)行，這就是非精確定時(shí)器的核心工作原理。我們可以定制計(jì)劃的任務(wù)，可是系統(tǒng)如果檢測到一個(gè)更好的時(shí)間，它可以推遲你的任務(wù)，以節(jié)省電量消耗。這正是JobScheduler API所做的事情
JobScheduler會(huì)根據(jù)當(dāng)前的情況與任務(wù)，組合出理想的喚醒時(shí)間，例如等到正在充電或者連接到WiFi的時(shí)候，或者集中任務(wù)一起執(zhí)行。我們可以通過這個(gè)API實(shí)現(xiàn)很多免費(fèi)的調(diào)度算法。
- 為了減少電量的消耗，在蜂窩移動(dòng)網(wǎng)絡(luò)下，最好做到批量執(zhí)行網(wǎng)絡(luò)請求，盡量避免頻繁的間隔網(wǎng)絡(luò)請求。WiFi連接下，網(wǎng)絡(luò)傳輸?shù)碾娏肯囊纫苿?dòng)網(wǎng)絡(luò)少很多，應(yīng)該盡量減少移動(dòng)網(wǎng)絡(luò)下的數(shù)據(jù)傳輸，多在WiFi環(huán)境下傳輸數(shù)據(jù)。
- 使用Job Scheduler，應(yīng)用需要做的事情就是判斷哪些任務(wù)是不緊急的，可以交給Job Scheduler來處理，Job Scheduler集中處理收到的任務(wù)，選擇合適的時(shí)間，合適的網(wǎng)絡(luò)，再一起進(jìn)行執(zhí)行。
下面是使用Job Scheduler的一段簡要示例，需要先有一個(gè)JobService
public class MyJobService extends JobService {
private static final String LOG_TAG = "MyJobService";
@Override
public void onCreate() {
super.onCreate();
Log.i(LOG_TAG, "MyJobService created");
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i(LOG_TAG, "MyJobService destroyed");
}
@Override
public boolean onStartJob(JobParameters params) {
// This is where you would implement all of the logic for your job. Note that this runs
// on the main thread, so you will want to use a separate thread for asynchronous work
// (as we demonstrate below to establish a network connection).
// If you use a separate thread, return true to indicate that you need a "reschedule" to
// return to the job at some point in the future to finish processing the work. Otherwise,
// return false when finished.
Log.i(LOG_TAG, "Totally and completely working on job " + params.getJobId());
// First, check the network, and then attempt to connect.
if (isNetworkConnected()) {
new SimpleDownloadTask() .execute(params);
return true;
} else {
Log.i(LOG_TAG, "No connection on job " + params.getJobId() + "; sad face");
}
return false;
}
@Override
public boolean onStopJob(JobParameters params) {
// Called if the job must be stopped before jobFinished() has been called. This may
// happen if the requirements are no longer being met, such as the user no longer
// connecting to WiFi, or the device no longer being idle. Use this callback to resolve
// anything that may cause your application to misbehave from the job being halted.
// Return true if the job should be rescheduled based on the retry criteria specified
// when the job was created or return false to drop the job. Regardless of the value
// returned, your job must stop executing.
Log.i(LOG_TAG, "Whelp, something changed, so I'm calling it on job " + params.getJobId());
return false;
}
/**
* Determines if the device is currently online.
*/
private boolean isNetworkConnected() {
ConnectivityManager connectivityManager =
(ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
return (networkInfo != null && networkInfo.isConnected());
}
/**
* Uses AsyncTask to create a task away from the main UI thread. This task creates a
* HTTPUrlConnection, and then downloads the contents of the webpage as an InputStream.
* The InputStream is then converted to a String, which is logged by the
* onPostExecute() method.
*/
private class SimpleDownloadTask extends AsyncTask<JobParameters, Void, String> {
protected JobParameters mJobParam;
@Override
protected String doInBackground(JobParameters... params) {
// cache system provided job requirements
mJobParam = params[0];
try {
InputStream is = null;
// Only display the first 50 characters of the retrieved web page content.
int len = 50;
URL url = new URL("https://www.google.com");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(10000); //10sec
conn.setConnectTimeout(15000); //15sec
conn.setRequestMethod("GET");
//Starts the query
conn.connect();
int response = conn.getResponseCode();
Log.d(LOG_TAG, "The response is: " + response);
is = conn.getInputStream();
// Convert the input stream to a string
Reader reader = null;
reader = new InputStreamReader(is, "UTF-8");
char[] buffer = new char[len];
reader.read(buffer);
return new String(buffer);
} catch (IOException e) {
return "Unable to retrieve web page.";
}
}
@Override
protected void onPostExecute(String result) {
jobFinished(mJobParam, false);
Log.i(LOG_TAG, result);
}
}
}
然后模擬通過點(diǎn)擊Button觸發(fā)N個(gè)任務(wù)，交給JobService來處理：
public class FreeTheWakelockActivity extends ActionBarActivity {
public static final String LOG_TAG = "FreeTheWakelockActivity";
TextView mWakeLockMsg;
ComponentName mServiceComponent;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_wakelock);
mWakeLockMsg = (TextView) findViewById(R.id.wakelock_txt);
mServiceComponent = new ComponentName(this, MyJobService.class);
Intent startServiceIntent = new Intent(this, MyJobService.class);
startService(startServiceIntent);
Button theButtonThatWakelocks = (Button) findViewById(R.id.wakelock_poll);
theButtonThatWakelocks.setText(R.string.poll_server_button);
theButtonThatWakelocks.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
pollServer();
}
});
}
/**
* This method polls the server via the JobScheduler API. By scheduling the job with this API,
* your app can be confident it will execute, but without the need for a wake lock. Rather, the
* API will take your network jobs and execute them in batch to best take advantage of the
* initial network connection cost.
*
* The JobScheduler API works through a background service. In this sample, we have
* a simple service in MyJobService to get you started. The job is scheduled here in
* the activity, but the job itself is executed in MyJobService in the startJob() method. For
* example, to poll your server, you would create the network connection, send your GET
* request, and then process the response all in MyJobService. This allows the JobScheduler API
* to invoke your logic without needed to restart your activity.
*
* For brevity in the sample, we are scheduling the same job several times in quick succession,
* but again, try to consider similar tasks occurring over time in your application that can
* afford to wait and may benefit from batching.
*/
public void pollServer() {
JobScheduler scheduler = (JobScheduler) getSystemService(Context.JOB_SCHEDULER_SERVICE);
for (int i=0; i<10; i++) {
JobInfo jobInfo = new JobInfo.Builder(i, mServiceComponent)
.setMinimumLatency(5000) // 5 seconds
.setOverrideDeadline(60000) // 60 seconds (for brevity in the sample)
.setRequiredNetworkType(JobInfo.NETWORK_TYPE_ANY) // WiFi or data connections
.build();
mWakeLockMsg.append("Scheduling job " + i + "!\n");
scheduler.schedule(jobInfo);
}
}
}
