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fmTimer.h

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+//
+//  fmTimer.h
+//
+//  Created by 柯浩然 on 7/24/18.
+//
+
+#import <Foundation/Foundation.h>
+
+@interface fmTimer : NSObject
+/// Initializes a timer using the specified object and selector. Upon firing, the timer sends the message aSelector to target
+/// - partmeter:  date  The time at which the timer should first fire.
+- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)ti target:(id)t selector:(SEL)s userInfo:(nullable id)ui repeats:(BOOL)rep runloopMode:(NSRunLoopMode)mode;
+
+///After ti seconds have elapsed, the timer fires, invoking invocation.
+/// - partmeter:  mode  Creates a new timer and schedules it on the mode.
++ (instancetype)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)target selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo runloopMode:(NSRunLoopMode)mode;
+
+- (void)fire;
+- (void)invalidate;
+@end

fmTimer.m

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+//
+//  fmTimer.m
+//
+//  Created by 柯浩然 on 7/24/18.
+//
+
+#import "fmTimer.h"
+
+@interface fmProxy:NSProxy
+
++ (instancetype)weakProxyWithTarget:(id)target;
+
+@end
+
+@interface fmProxy()
+
+@property(nonatomic, weak) id target;
+
+@end
+
+@implementation fmProxy
+
+// This is the designated creation method of an `fmProxy` and
+// as a subclass of `NSProxy` it doesn't respond to or need `-init`.
++ (instancetype)weakProxyWithTarget:(id)target {
+    fmProxy *p = [fmProxy alloc];
+    p.target = target;
+    return p;
+}
+
+#pragma mark Forwarding Messages
+- (id)forwardingTargetForSelector:(SEL)selector
+{
+    // Keep it lightweight: access the ivar directly
+    return _target;
+}
+
+#pragma mark Handling Unimplemented Methods
+
+- (void)forwardInvocation:(NSInvocation *)invocation
+{
+    // Fallback for when target is nil. Don't do anything, just return 0/NULL/nil.
+    // The method signature we've received to get here is just a dummy to keep `doesNotRecognizeSelector:` from firing.
+    // We can't really handle struct return types here because we don't know the length.
+    void *nullPointer = NULL;
+    [invocation setReturnValue:&nullPointer];
+}
+
+
+- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
+{
+    // We only get here if `forwardingTargetForSelector:` returns nil.
+    // In that case, our weak target has been reclaimed. Return a dummy method signature to keep `doesNotRecognizeSelector:` from firing.
+    // We'll emulate the Obj-c messaging nil behavior by setting the return value to nil in `forwardInvocation:`, but we'll assume that the return value is `sizeof(void *)`
+
+    return [NSObject instanceMethodSignatureForSelector:@selector(init)];
+}
+@end
+
+
+@interface fmTimer()
+{
+    NSTimer *_timer;
+}
+@end
+
+@implementation fmTimer
+
+- (void)dealloc {
+
+    [self invalidate];
+}
+
+- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)ti target:(id)t selector:(SEL)s userInfo:(nullable id)ui repeats:(BOOL)rep runloopMode:(NSRunLoopMode)mode{
+
+    self = [super init];
+    if (self) {
+
+        fmProxy *weakProxy = [fmProxy weakProxyWithTarget:t];
+        _timer = [[NSTimer alloc] initWithFireDate:date interval:ti target:weakProxy selector:s userInfo:ui repeats:rep];
+        [[NSRunLoop mainRunLoop] addTimer:_timer forMode:mode];
+    }
+    return self;
+}
+
++ (instancetype)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)target selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo runloopMode:(NSRunLoopMode)mode {
+
+    fmTimer *t = [[fmTimer alloc] initWithFireDate:[NSDate dateWithTimeIntervalSinceNow:ti] interval:ti target:target selector:aSelector userInfo:userInfo repeats:yesOrNo runloopMode:mode];
+    return t;
+}
+
+- (void)fire {
+    [_timer setFireDate:[NSDate distantPast]];
+}
+
+- (void)invalidate {
+    [_timer invalidate];
+    _timer = nil;
+}
+
+@end