Context部分

View的构造函数

View.java

        
                
  1. /**
    2.             
    3.             
  3. *Simpleconstructortousewhencreatingaviewfromcode.
    4.             
    5.             
  5. *
    6.             
    7.             
  7. *@paramcontextTheContexttheviewisrunningin,throughwhichitcan
    8.             
    9.             
  9. *accessthecurrenttheme,resources,etc.
    10.             
    11.             
  11. */
    12.             
    13.             
  13. publicView(Contextcontext){
    14.             
    15.             
  15. mContext=context;
    16.             
    17.             
  17. mResources=context!=null?context.getResources():null;
    18.             
    19.             
  19. mViewFlags=SOUND_EFFECTS_ENABLED|HAPTIC_FEEDBACK_ENABLED;
    20.             
    21.             
  21. //Usedfordebugonly
    22.             
    23.             
  23. //++sInstanceCount;
    24.             
    25.             
  25. mTouchSlop=ViewConfiguration.get(context).getScaledTouchSlop();
    26.             
    27.             
  27. }
    28.

View 保存了对context的引用，mContext = context;

1.我们知道结构其实和DOM差不多，都会保存其父容器、子容器的引用，因而context的使用需要注意，不要使用静态的子View。

2.来看View中的setBackgroundDrawable方法

View.java

        
                
  1. /**
    2.             
    3.             
  3. *SetthebackgroundtoagivenDrawable,orremovethebackground.Ifthe
    4.             
    5.             
  5. *backgroundhaspadding,thisView'spaddingissettothebackground's
    6.             
    7.             
  7. *padding.However,whenabackgroundisremoved,thisView'spaddingisn't
    8.             
    9.             
  9. *touched.Ifsettingthepaddingisdesired,pleaseuse
    10.             
    11.             
  11. *{@link#setPadding(int,int,int,int)}.
    12.             
    13.             
  13. *
    14.             
    15.             
  15. *@paramdTheDrawabletouseasthebackground,ornulltoremovethe
    16.             
    17.             
  17. *background
    18.             
    19.             
  19. */
    20.             
    21.             
  21. publicvoidsetBackgroundDrawable(Drawabled){
    22.             
    23.             
  23. booleanrequestLayout=false;
    24.             
    25.             
    26.             
    27.             
  27. mBackgroundResource=0;
    28.             
    29.             
    30.             
    31.             
  31. /*
    32.             
    33.             
  33. *Regardlessofwhetherwe'resettinganewbackgroundornot,wewant
    34.             
    35.             
  35. *toclearthepreviousdrawable.
    36.             
    37.             
  37. */
    38.             
    39.             
  39. if(mBGDrawable!=null){
    40.             
    41.             
  41. mBGDrawable.setCallback(null);
    42.             
    43.             
  43. unscheduleDrawable(mBGDrawable);
    44.             
    45.             
  45. }
    46.             
    47.             
    48.             
    49.             
  49. if(d!=null){
    50.             
    51.             
  51. Rectpadding=sThreadLocal.get();
    52.             
    53.             
  53. if(padding==null){
    54.             
    55.             
  55. padding=newRect();
    56.             
    57.             
  57. sThreadLocal.set(padding);
    58.             
    59.             
  59. }
    60.             
    61.             
  61. if(d.getPadding(padding)){
    62.             
    63.             
  63. setPadding(padding.left,padding.top,padding.right,padding.bottom);
    64.             
    65.             
  65. }
    66.             
    67.             
    68.             
    69.             
  69. //ComparetheminimumsizesoftheoldDrawableandthenew.Ifthereisn'tanoldor
    70.             
    71.             
  71. //ifithasadifferentminimumsize,weshouldlayoutagain
    72.             
    73.             
  73. if(mBGDrawable==null||mBGDrawable.getMinimumHeight()!=d.getMinimumHeight()||
    74.             
    75.             
  75. mBGDrawable.getMinimumWidth()!=d.getMinimumWidth()){
    76.             
    77.             
  77. requestLayout=true;
    78.             
    79.             
  79. }
    80.             
    81.             
    82.             
    83.             
  83. d.setCallback(this);
    84.             
    85.             
  85. if(d.isStateful()){
    86.             
    87.             
  87. d.setState(getDrawableState());
    88.             
    89.             
  89. }
    90.             
    91.             
  91. d.setVisible(getVisibility()==VISIBLE,false);
    92.             
    93.             
  93. mBGDrawable=d;
    94.             
    95.             
    96.             
    97.             
  97. if((mPrivateFlags&SKIP_DRAW)!=0){
    98.             
    99.             
  99. mPrivateFlags&=~SKIP_DRAW;
    100.             
    101.             
  101. mPrivateFlags|=ONLY_DRAWS_BACKGROUND;
    102.             
    103.             
  103. requestLayout=true;
    104.             
    105.             
  105. }
    106.             
    107.             
  107. }else{
    108.             
    109.             
  109. /*Removethebackground*/
    110.             
    111.             
  111. mBGDrawable=null;
    112.             
    113.             
    114.             
    115.             
  115. if((mPrivateFlags&ONLY_DRAWS_BACKGROUND)!=0){
    116.             
    117.             
  117. /*
    118.             
    119.             
  119. *ThisviewONLYdrewthebackgroundbeforeandwe'reremoving
    120.             
    121.             
  121. *thebackground,sonowitwon'tdrawanything
    122.             
    123.             
  123. *(henceweSKIP_DRAW)
    124.             
    125.             
  125. */
    126.             
    127.             
  127. mPrivateFlags&=~ONLY_DRAWS_BACKGROUND;
    128.             
    129.             
  129. mPrivateFlags|=SKIP_DRAW;
    130.             
    131.             
  131. }
    132.             
    133.             
    134.             
    135.             
  135. /*
    136.             
    137.             
  137. *Whenthebackgroundisset,wetrytoapplyitspaddingtothis
    138.             
    139.             
  139. *View.Whenthebackgroundisremoved,wedon'ttouchthisView's
    140.             
    141.             
  141. *padding.ThisisnotedintheJavadocs.Hence,wedon'tneedto
    142.             
    143.             
  143. *requestLayout(),theinvalidate()belowissufficient.
    144.             
    145.             
  145. */
    146.             
    147.             
    148.             
    149.             
  149. //Theoldbackground'sminimumsizecouldhaveaffectedthis
    150.             
    151.             
  151. //View'slayout,solet'srequestLayout
    152.             
    153.             
  153. requestLayout=true;
    154.             
    155.             
  155. }
    156.             
    157.             
    158.             
    159.             
  159. computeOpaqueFlags();
    160.             
    161.             
    162.             
    163.             
  163. if(requestLayout){
    164.             
    165.             
  165. requestLayout();
    166.             
    167.             
  167. }
    168.             
    169.             
    170.             
    171.             
  171. mBackgroundSizeChanged=true;
    172.             
    173.             
  173. invalidate();
    174.             
    175.             
  175. }
    176.

我们注意到d.setCallback(this);

即Drawable保存了View的引用，处理了一些回调。因此，Drawable对象使用时需要注意了。不要作为静态对象使用。

Resource.getDrewable()方法是比较好的，Resource内部使用了一个Drawable.ConstantState的弱引用缓存，提高了效率。

来看代码：

Resource.java

        
                
  1. /*package*/DrawableloadDrawable(TypedValuevalue,intid)
    2.             
    3.             
  3. throwsNotFoundException{
    4.             
    5.             
    6.             
    7.             
  7. if(TRACE_FOR_PRELOAD){
    8.             
    9.             
  9. //Logonlyframeworkresources
    10.             
    11.             
  11. if((id>>>24)==0x1){
    12.             
    13.             
  13. finalStringname=getResourceName(id);
    14.             
    15.             
  15. if(name!=null)android.util.Log.d("PreloadDrawable",name);
    16.             
    17.             
  17. }
    18.             
    19.             
  19. }
    20.             
    21.             
    22.             
    23.             
  23. finallongkey=(((long)value.assetCookie)<<32)|value.data;
    24.             
    25.             
  25. Drawabledr=getCachedDrawable(key);
    26.             
    27.             
    28.             
    29.             
  29. if(dr!=null){
    30.             
    31.             
  31. returndr;
    32.             
    33.             
  33. }
    34.             
    35.             
    36.             
    37.             
  37. Drawable.ConstantStatecs=sPreloadedDrawables.get(key);
    38.             
    39.             
  39. if(cs!=null){
    40.             
    41.             
  41. dr=cs.newDrawable(this);
    42.             
    43.             
  43. }else{
    44.             
    45.             
  45. if(value.type>=TypedValue.TYPE_FIRST_COLOR_INT&&
    46.             
    47.             
  47. value.type<=TypedValue.TYPE_LAST_COLOR_INT){
    48.             
    49.             
  49. dr=newColorDrawable(value.data);
    50.             
    51.             
  51. }
    52.             
    53.             
    54.             
    55.             
  55. if(dr==null){
    56.             
    57.             
  57. if(value.string==null){
    58.             
    59.             
  59. thrownewNotFoundException(
    60.             
    61.             
  61. "ResourceisnotaDrawable(colororpath):"+value);
    62.             
    63.             
  63. }
    64.             
    65.             
    66.             
    67.             
  67. Stringfile=value.string.toString();
    68.             
    69.             
    70.             
    71.             
  71. if(DEBUG_LOAD)Log.v(TAG,"Loadingdrawableforcookie"
    72.             
    73.             
  73. +value.assetCookie+":"+file);
    74.             
    75.             
    76.             
    77.             
  77. if(file.endsWith(".xml")){
    78.             
    79.             
  79. try{
    80.             
    81.             
  81. XmlResourceParserrp=loadXmlResourceParser(
    82.             
    83.             
  83. file,id,value.assetCookie,"drawable");
    84.             
    85.             
  85. dr=Drawable.createFromXml(this,rp);
    86.             
    87.             
  87. rp.close();
    88.             
    89.             
  89. }catch(Exceptione){
    90.             
    91.             
  91. NotFoundExceptionrnf=newNotFoundException(
    92.             
    93.             
  93. "File"+file+"fromdrawableresourceID#0x"
    94.             
    95.             
  95. +Integer.toHexString(id));
    96.             
    97.             
  97. rnf.initCause(e);
    98.             
    99.             
  99. throwrnf;
    100.             
    101.             
  101. }
    102.             
    103.             
    104.             
    105.             
  105. }else{
    106.             
    107.             
  107. try{
    108.             
    109.             
  109. InputStreamis=mAssets.openNonAsset(
    110.             
    111.             
  111. value.assetCookie,file,AssetManager.ACCESS_STREAMING);
    112.             
    113.             
  113. //System.out.println("Openedfile"+file+":"+is);
    114.             
    115.             
  115. dr=Drawable.createFromResourceStream(this,value,is,
    116.             
    117.             
  117. file,null);
    118.             
    119.             
  119. is.close();
    120.             
    121.             
  121. //System.out.println("Createdstream:"+dr);
    122.             
    123.             
  123. }catch(Exceptione){
    124.             
    125.             
  125. NotFoundExceptionrnf=newNotFoundException(
    126.             
    127.             
  127. "File"+file+"fromdrawableresourceID#0x"
    128.             
    129.             
  129. +Integer.toHexString(id));
    130.             
    131.             
  131. rnf.initCause(e);
    132.             
    133.             
  133. throwrnf;
    134.             
    135.             
  135. }
    136.             
    137.             
  137. }
    138.             
    139.             
  139. }
    140.             
    141.             
  141. }
    142.             
    143.             
    144.             
    145.             
  145. if(dr!=null){
    146.             
    147.             
  147. dr.setChangingConfigurations(value.changingConfigurations);
    148.             
    149.             
  149. cs=dr.getConstantState();
    150.             
    151.             
  151. if(cs!=null){
    152.             
    153.             
  153. if(mPreloading){
    154.             
    155.             
  155. sPreloadedDrawables.put(key,cs);
    156.             
    157.             
  157. }else{
    158.             
    159.             
  159. synchronized(mTmpValue){
    160.             
    161.             
  161. //Log.i(TAG,"Savingcacheddrawable@#"+
    162.             
    163.             
  163. //Integer.toHexString(key.intValue())
    164.             
    165.             
  165. //+"in"+this+":"+cs);
    166.             
    167.             
  167. mDrawableCache.put(key,newWeakReference<Drawable.ConstantState>(cs));
    168.             
    169.             
  169. }
    170.             
    171.             
  171. }
    172.             
    173.             
  173. }
    174.             
    175.             
  175. }
    176.             
    177.             
    178.             
    179.             
  179. returndr;
    180.             
    181.             
  181. }
    182.

使用Drawable一般情况下效率较高，且不易发生内存泄露。

接下来我们来看下BitMap的使用

BitMapFactory.java

        
                
  1. /**
    2.             
    3.             
  3. *Decodeaninputstreamintoabitmap.Iftheinputstreamisnull,or
    4.             
    5.             
  5. *cannotbeusedtodecodeabitmap,thefunctionreturnsnull.
    6.             
    7.             
  7. *Thestream'spositionwillbewhereeveritwasaftertheencodeddata
    8.             
    9.             
  9. *wasread.
    10.             
    11.             
  11. *
    12.             
    13.             
  13. *@paramisTheinputstreamthatholdstherawdatatobedecodedintoa
    14.             
    15.             
  15. *bitmap.
    16.             
    17.             
  17. *@paramoutPaddingIfnotnull,returnthepaddingrectforthebitmapif
    18.             
    19.             
  19. *itexists,otherwisesetpaddingto[-1,-1,-1,-1].If
    20.             
    21.             
  21. *nobitmapisreturned(null)thenpaddingis
    22.             
    23.             
  23. *unchanged.
    24.             
    25.             
  25. *@paramoptsnull-ok;Optionsthatcontroldownsamplingandwhetherthe
    26.             
    27.             
  27. *imageshouldbecompletelydecoded,orjustissizereturned.
    28.             
    29.             
  29. *@returnThedecodedbitmap,ornulliftheimagedatacouldnotbe
    30.             
    31.             
  31. *decoded,or,ifoptsisnon-null,ifoptsrequestedonlythe
    32.             
    33.             
  33. *sizebereturned(inopts.outWidthandopts.outHeight)
    34.             
    35.             
  35. */
    36.             
    37.             
  37. publicstaticBitmapdecodeStream(InputStreamis,RectoutPadding,Optionsopts){
    38.             
    39.             
  39. //wedon'tthrowinthiscase,thusallowingthecallertoonlycheck
    40.             
    41.             
  41. //thecache,andnotforcetheimagetobedecoded.
    42.             
    43.             
  43. if(is==null){
    44.             
    45.             
  45. returnnull;
    46.             
    47.             
  47. }
    48.             
    49.             
    50.             
    51.             
  51. //weneedmark/resettoworkproperly
    52.             
    53.             
    54.             
    55.             
  55. if(!is.markSupported()){
    56.             
    57.             
  57. is=newBufferedInputStream(is,16*1024);
    58.             
    59.             
  59. }
    60.             
    61.             
    62.             
    63.             
  63. //sowecancallreset()ifagivencodecgivesupafterreadingupto
    64.             
    65.             
  65. //thismanybytes.FIXME:needtofindoutfromthecodecswhatthis
    66.             
    67.             
  67. //valueshouldbe.
    68.             
    69.             
  69. is.mark(1024);
    70.             
    71.             
    72.             
    73.             
  73. Bitmapbm;
    74.             
    75.             
    76.             
    77.             
  77. if(isinstanceofAssetManager.AssetInputStream){
    78.             
    79.             
  79. bm=nativeDecodeAsset(((AssetManager.AssetInputStream)is).getAssetInt(),
    80.             
    81.             
  81. outPadding,opts);
    82.             
    83.             
  83. }else{
    84.             
    85.             
  85. //passsometempstoragedowntothenativecode.1024ismadeup,
    86.             
    87.             
  87. //butshouldbelargeenoughtoavoidtoomanysmallcallsback
    88.             
    89.             
  89. //intois.read(...)Thisnumberisnotrelatedtothevaluepassed
    90.             
    91.             
  91. //tomark(...)above.
    92.             
    93.             
  93. byte[]tempStorage=null;
    94.             
    95.             
  95. if(opts!=null)
    96.             
    97.             
  97. tempStorage=opts.inTempStorage;
    98.             
    99.             
  99. if(tempStorage==null)
    100.             
    101.             
  101. tempStorage=newbyte[16*1024];
    102.             
    103.             
  103. bm=nativeDecodeStream(is,tempStorage,outPadding,opts);
    104.             
    105.             
  105. }
    106.             
    107.             
    108.             
    109.             
  109. returnfinishDecode(bm,outPadding,opts);
    110.             
    111.             
  111. }
    112.

我们每次调用BitMapFactory中的生产bitmap的方法的时候都会new一个bitmap出来，为了避免内存溢出，我们有两种主要的思路：

1. 使用缓存，避免重复new同一个图片

2. 销毁bitmap，使用完之后立刻销毁bitmap

缓存需要自己实现，如果是销毁的话，可以使用Bitmap中的recycle()方法。

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